Poster Abstracts

Poster abstracts

P01 Penetration of a model membrane by a self-propelled active A. Daddi-Moussa-Ider particle P02 The squirmer model and beyond F. Fadda P03 Theoretical Investigation of Structure Formation by Magne- V. Telezki totactic Bacteria P04 Active Nematics Formed by Bacteria in Patterned R. Koizumi Chromonics P05 Self-propulsion of Camphor Symmetric Interfacial Swim- D. Boniface mers P06 Microswimmers self-propelled by Thermophoresis S. Roca-Bonet P07 Active Brownian filaments in dilute solution A. Mart´ın-Gomez´ P08 Scattering of E. coli at surfaces M. Mousavi P09 Light dependent motility of microalgae induces pattern for- A. Fragkopoulos mation in confinement P10 Longwave nonlinear theory for chemically active droplet di- M. Abu Hamed vision instability P11 Bead-spring modelling microswimmers S. Ziegler P12 Light-driven Janus microswimmers in dense colloidal ma- T. Huang trix P13 Active Brownian Particles in Crowded Media A. Liluashvili P14 Evolution in range expansions with competition at rough S. Chu boundaries P15 Mode-Coupling Theory for Active Brownian Particles J.Reichert P16 Structure and dynamics of a self-propelled semiflexible fil- S. P. Singh ament P17 IHRS Biosoft T. Auth P18 IHRS Biosoft T.Auth P19 Pairing, waltzing and scattering of chemotactic active col- S. Saha loids P20 Instability in settling array of discs R. Chajwa P21 Self-propelled particles in anisotropic environments A. R. Sprenger P22 A phase field crystal approach to active systems with inertia D. Arold P23 Ring polymers are much stronger depleting agents than lin- I. Chubak ear ones P24 Enhanced rotational diffusion of squirmers in viscoelastic K. Qi fluids P25 Dynamics of confined phoretic colloids K. R. Prathyusha P26 Enhanced dynamic heterogeneity in model active glass K. Paul forming liquids P27 Tracer Diffusion in a Dense Active Bath L. Abbaspour P28 pH dependence of Swimming Direction of Janus Micromo- F. Ruhle¨ tors P29 Role of pH in Micro-swimming N. Moller¨ P30 Chemical micromotors that self-assemble T. Yu P31 Active apolar doping determines routes to colloidal clusters H. Massana-Cid and gels P32 Efficient photocatalytic bismuth vanadate microparticles for S. Heckel active propulsion P33 Polarization of Brownian swimmers with spatiotemporally S. Auschra heterogeneous activity P34 Active Brownian heat engine S. Steffenoni P35 Dependence of Swimming Direction of Janus Micromotors N. Murty on pH P36 Photo-Induced Motion of Polymer Brush Coated Small Par- M. Sokolowski ticles P37 Linear rheology of reversibly cross-linked biopolymer net- H. E. Amuasi works P38 Brownian molecules formed by delayed harmonic interac- D. Geiß tions P39 Collective rotations of active particles interacting with ob- Z. Mokhtari stacles P40 Influence of catalysts on the propulsion path of micromotors P. Chattopadhyay P41 Active matter systems can exhibit coexisting patterns of T. Kruger¨ competing symmetries P42 Impact of Brush/Water Interface on the self-propulsion of M. Heidari Janus Particles P43 Tissue Mechanics: Stokes Flow in Confluent Cell Simula- C. P. Beatrici tions P44 Self-propelled rods with quorum sensing T. Auth P45 Shape and motility of composite active agents with internal C. Abaurrea Velasco degrees of freedom P46 A Model For The Possible Role Of Substrate Rigidity For A.N. Simsek Bacterial Migration And Colony Formation P47 Collective intercellular communication through ultra-fast A. J. T. M. Mathijssen hydrodynamic trigger waves P48 Collective motion in biological systems: Langevin Equa- L. Amallah tion investigation P49 The tortoise and the hare: how collective behaviours within O. J. Meacock bacterial biofilms select for cells that move more slowly P50 Filamentous Active Matter: Band Formation, Bending, G. A. Vliegenthart Buckling, and Defects P51 Sperm motility in modulated microchannels S. Rode P52 Spontaneous Spatiotemporal Ordering of Shape Oscilla- M. Campo tions Enhances Cell Migration P53 Actuation of particles in modulated Poiseuille flow W. Schmidt P54 Multiparticle Collision Dynamics Modeling of Nematic S. Mandal Liquid Crystal with Variable Order Parameter P55 A minimal model for fluid-like collective cell migration D. Sarkar P56 Hydrodynamic interactions of beating cilia A. Solovev P01 Penetration of a model membrane by a self-propelled active particle A. Daddi-Moussa-Ider1, S. Goh1, B. Liebchen1, C. Hoell1, A. J. T. M. Mathijssen2, F. Guzmán-Lastra1,3, C. Scholz1, A. M. Menzel1, and H. Löwen1. 1Institut für Theoretische Physik II: Weiche Materie, Heinrich-Heine-Universität Düsseldorf, Universitätsstraße 1, 40225 Düsseldorf, Germany 2Department of Bioengineering, Stanford University, 443 Via Ortega, Stanford, CA 94305, USA 3Facultad de Ciencias, Universidad Mayor, Av. Manuel Montt 367, Providencia, Santiago de Chile, Chile E-mail: [email protected]

We present a model to describe the interaction of a self-propelling active particle with a minimal membrane system, allowing for both penetration and trapping events. We numerically calculate the state diagram of this system, the membrane shape, and its dynamics, ﬁnding that the active particle may either get trapped near the membrane or penetrates through it. We show that the membrane can either be permanently damaged or recover its initial shape by self-healing. We then systematically derive a continuum theory allowing to accurately predict most of our results analytically. Our results might be useful to predict mechanical properties of synthetic membranes.

References [1] A. Daddi-Moussa-Ider, S. Goh, B. Liebchen, C. Hoell, A. J. T. M. Mathijssen, F. Guzmán- Lastra, C. Scholz, A. M. Menzel, and H. Löwen. Membrane penetration and trapping of an active particle, J. Chem. Phys. 150, 064906 (2019) P02

The squirmer model and beyond F.Fadda1, J.J.Molina1 and R.Yamamoto1,2 1Department of Chemical Engineering, Kyoto University, Kyoto, 615-8510, Japan 2Institute of Industrial Science, The University of Tokyo, Tokyo, 153-8505, Japan E-mail: [email protected]

The squirmer model introduced by Lighthill and later extended by Blake [1] allows the description of microorganisms such as algae and bacteria. It consists in a spherical particle with a prescribed tangential velocity field, neglecting the radial component, responsible for the self-propulsion. If the microorganism repels fluid along its axis and attracts it to the sides it is called pusher (like the bacterium Escherichia Coli); in the opposite case it is called puller (like the alga Chlamydomonas Reinardtii) [2]. In this study the squirmer model is incorporated into the Smoothed Profile Method, an efficient calculation scheme to simulate solid objects into a fluid taking fully into account the hydrodynamics [3], which has already been successfully used in the past to study collective motion and interactions of squirmers [4]. Now the traditional squirmer model, which considers only a purely tangential surface velocity neglecting the radial one, is updated introducing also an azimuthal component of velocity to give a more realistic description of the motion of microorganisms like bacteria whose flagellar filaments attached to the cell bodies rotate in a counterclockwise direction. This is also accompanied by a counter rotation of the cell body and the translation of the bacterium in a general motion resembling a corkscrew [2,5]. We start analyzing the dynamics of a single squirmer with the intention in the future to investigate deeper the mutual interactions of a couple of them and the collective dynamics.

References

[1] M.J.Lighthill, Commun. Pure Appl. Math. 5, 109 (1952); J.R.Blake, J. Fluid Mech. 46, 199 (1971). [2] E.Lauga and T.Powers, Rep. Prog. Phys. 72, 096601 (2009); J.Elgeti, R.G.Winkler and G.Gompper, Rep. Prog. Phys. 78, 056601 (2015); E.Lauga, Annu. Rev. Fluid Mech. 48, 105-130 (2016). [3] Y.Nakayama and R.Yamamoto, Phys. Rev. E 71, 036707 (2005); Y.Nakayama, K.Kim and R.Yamamoto, Eur. Phys. J. E 26, 361 (2008). [4] J.J.Molina, Y.Nakayama and R.Yamamoto, Soft Matter 9, 4923 (2013); N.Oyama, J.J.Molina and R.Yamamoto, Phys. Rev. E 93, 043114 (2016); N.Oyama, J.J.Molina and R.Yamamoto, Eur. Phys. J. E 40, 95 (2017). [5] O.S.Pak and E.Lauga, J. Eng. Math. 88, 1-28 (2014).

P03 Theoretical Investigation of Structure Formation by Magnetotactic Bacteria Vitali Telezki, Omar Muñoz and Stefan Klumpp Institute for Nonlinear Dynamics, University of Göttingen, Germany E-mail: [email protected] [email protected] [email protected]

Magnetotactic bacteria are bacteria that orient in magnetic fields with the help of a specific cell organelle, the magnetosome chain, a chain of membrane-enclosed magnetic iron-oxide nanocrystals. The resulting orientation allows the bacteria to navigate along magnetic field lines. Because of the interplay between different physical interactions such as steric, hydrodynamic and magnetic interactions, complex collective behaviour is expected to emerge in dense systems of these bacteria. We use Brownian dynamics simulations to investigate the collective behaviour of magnetotactic bacteria. In a first step, we focus on the structure formation of active Brownian particles with a magnetic dipole moment. We analyse what structures can emerge and how they depend on the self-propulsion velocity, the magnetic moment and other attributes of the swimmers.

References [1] S. Klumpp, C.T. Lefevre, M. Bennet, D. Faivre, Phys Rep. in press/available online, DOI: 10.1016/j.physrep.2018.10.007 P04

Active Nematics Formed by Bacteria in Patterned Chromonics Runa Koizumi1*, Taras Turiv1, Chenhui Peng1, Hao Yu1, Yubing Guo1, QihuoWei1,2 and Oleg D. Lavrentovich1,2

1Liquid Crystal Institute and Chemical Physics Interdisciplinary Program, Kent State University, Kent, OH, 44240, United States 2Department of Physics, Kent State University, Kent OH, 44240, United States E-mail: [email protected]

Micro-swimmers of biological nature such as motile bacteria exhibit highly correlated collective behavior. This collective behavior can be controlled by placing such microswimmers in a nematic liquid crystal (NLC) with long-range orientational order. It is known that the NLC environment can control the local concentration, trajectories, and net flow of active bacteria, thereby triggering a circular unidirectional motion of the bacteria around topological charges1. In this work, we explore the collective motion of motile Bacillus Subtilis dispersed in an aqueous solution of DSCG, a lyotropic chromonic NLC. The director field is set to be spatially varying, representing an alternating one-dimensional system of splay and bend regions imposed through photoalignment. The bacteria exhibit unidirectional collective motion along the splay regions of the director field. If the bacteria enter the patterned director field in the opposite direction of the collective motion, its motion is realigned such that it swims in the same direction as the other bacteria. The experiments present a clear evidence of a highly compressible nature of bacterial dispersions in liquid crystals. The demonstrated unidirectional linear motion can be applied to using such micro-swimmers as potential power materials. The work is supported by NSF DMREF grant DMS-1729509.

References

[1] C. Peng, T. Turiv, Y. Guo, Q.-H. Wei, O. D. Lavrentovich, Science, 354 (6314), 882-885 (2016).

P05 Self-propulsion of Camphor Symmetric Interfacial Swimmers D. Boniface, C. Cottin-Bizonne, F. Detcheverry and C. Ybert Institut Lumière Matière, CNRS, Université Lyon 1, France E-mail: [email protected]

Interfacial swimmers are self-propelled objects evolving at the interface between air and liquid. Motion originates in the asymmetry of surface tension around the swimmer, induced by a gradient in surfactant concentration or temperature. The phenomenon is known for at least three centuries [1] and recent work [2, 3] has well characterized the swimming of asymmetric swimmers. In contrast, the propulsion of symmetric swimmers, though observed, is not well understood.

We have carried out experiment with millimetric disks of agar gel loaded with precipitated camphor and evolving in a pool of millipore water. We observed the spontaneous swimming of this symmetric object and measured how the velocity is affected by the swimmer size and the presence of a small asymmetry. Besides, we have developed a minimal “point-source” model which is analytically tractable. We show that spontaneous motion of a symmetric swimmer is expected through a symmetry-breaking mechanism. Finally, we discuss the experimental results in light of the model predictions.

References [1] Charles Tomlinson F.R.S. (1869) XLVII. On the motions of camphor on the surface of water , Philosophical Magazine Series 4, 38:257, 409-424 [2] S. Nakata et al., Phys.Chem.Chem.Phys., 2015, 17,10326 [3] S. Nakata et al., Chem. Eur.J. 2018, 24,6308 –6324 P06 Microswimmers self-propelled by Thermophoresis S. Roca-Bonet and M. Ripoll Theoretical Soft Matter and Biophysics, Institute of Complex Physics, Forschungszentrum Jülich, Germany E-mail: [email protected]

Self-propelled phoretic colloids have recently emerged as a promising avenue for the design of artificial microswimmers. By studying the hydrodynamics of a single swimmer, we can predict the behaviour of large ensembles of colloids. A hydrodynamic fluctuating mesoscale simulation approach is here employed [1]. We present the hydrodynamic velocity fields of various swimmers with different structural shapes. Dimers, rod-like trimers, and v-shaped trimers are investigated for swimmers with monomers of equal or different sizes in quasi-two and three dimensional systems. The competition between attractive or repulsive hydrodynamic and phoretic interactions varies as a function of the oligomer geometry and phoretic affinity (philic or pho- bic), which eventually results in a rich phenomenology such as clustering, swarming, or rotational motions [2].

References [1] A. Malevanets and R. Kapral, J. Chem. Phys., 110, 8605 (1999). M. Yang and M. Ripoll, Phys. Rev. E, 84, 061401 (2011). D. Lüsebrink, M. Yang, and M. Ripoll, J. Phys.: Condens. Matter, 24, 284132 (2012). M. Yang, A. Wysocki and M. Ripoll, Soft Matter, 10, 6208 (2014). [2] M. Wagner and M. Ripoll, EPL, 119, 66007 (2017). P07 Active Brownian ﬁlaments in dilute solution A. Martín-Gómez1, G. Gompper 1 and R. G. Winkler1 1Theoretical Soft Matter and Biophysics, ICS-2/IAS-2 - Forschungszentrum JÃijlich E-mail: [email protected]

Active matter is comprised of agents which either convert internal energy or exploit energy from the environment to generate directed motion. The associated out-of-equilibrium character of active matter is the origin of a number of fascinating phenomena [1]. In particular, active systems with many internal degrees of freedom like filamentous, polymer-like structures are involved in various biological processes and exhibit novel conformational [2] and dynamical properties [3]. On the nano- and microscale, these active agents are typically dissolved in a fluid, and hydrodynamic interactions (HI) are essential for their behavior. Yet, such interactions are usually neglected. To shed light onto the effect of fluid mediate interactions on the conformational and dynamical properties of active polymers, we performed analytical calculations and computer simulations. The polymer is described as a linear bead-spring chain exposed to a non-Markovian exponen- tially correlated temporal noise (colored noise), describing activity, and HI are taken into account by the Oseen tensor. Without HI, flexible polymers swell with increasing activity, whereas stiff polymers shrink initially and swell again at even larger activities [2, 3]. The presence of HI leads to an even stronger shrinkage of the polymers, even flexible polymers exhibit shrinkage at moderate activities. Contrarily, self-avoidance seems to hinder such shrinkage for flexible polymers, but becomes irrelevant at higher stiffness. Dynamically, the polymers exhibit an enhanced activity-induced diffusive motion, which is further amplified by HI.

References [1] Elgeti, J.; Winkler, R. G.; Gompper, G. Rep. Prog. Phys. 78, 056601 (2015) [2] Eisenstecken, T.; Gompper, G.; Winkler R. G. 8, 304 (2016) [3] Eisenstecken, T.; Gompper, G.; Winkler R. G. J. Chem. Phys. 146, 154903 (2017) P08

Scattering of E. coli at surfaces M. Mousavi, G. Gompper, and R. G. Winkler Institute for Advanced Simulation and Institute of Complex Systems (ICS-2 / IAS-2) Forschungszentrum Jülich, Germany E-mail: [email protected]

Wall entrapment of swimming bacteria such as E. coli has been studied both experimentally and theoretically1,2,3. However, the underlying mechanisms of the cell-wall interaction is only partially resolved and the contradicting experimental and theoretical results have to be obtained. Here, we study the near-wall behaviour of E. coli by applying mesoscale hydrodynamic simulations. The bacterium cell is composed of a spherocylindrical body and several helical flagella constructed of point particles4 and the fluid is described by the multiparticle collision dynamics approach5. We identify three main stages of wall entrapment: approach, alignment, and surface swimming, as was resolved experimentally2. While the cell swims close to a surface, a fast oscillation around the swimming direction is observed (wobbling). Moreover, absorbed cells swim with a preferred orientation pointing toward the wall (Fig. 1, pitch angle > 0). Increasing the initial-angle of swimming does not change the collision angle, indicating that steric interactions are the primary driving force for cell reorientation. Hydrodynamic effects are visible in the velocity profile of the cell which slows down as the cell approaches the wall. Furthermore, due to hydrodynamic interactions and counter-rotation of the body and the flagellar bundle cells swim in clockwise circular trajectories at no-slip boundaries6.

Figure 1. Illustration of an E. coli cell touching a wall along with pitch and wobbling angles.

References

[1] H. Shum, E. A. Gaffney, D. J. Smith, Proc. R. Soc. Lond. A 466, 1725 (2010). [2] S. Bianchi, F. Saglimbeni, R. Di Leonardo, Phys. Rev. X, 7, 011010 (2017) [3] J. Elgeti, G. Gompper, Eur. Phys. J. Special Topics 225, 2333 (2016) [4] J. Hu, M. Yang, G. Gompper, R. G. Winkler, Soft Matter. 11, 7867 (2015) [5] G. Gompper, T. Ihle, D. M. Kroll, R. G. Winkler, Adv. Polym. Sci. 221, (2009) [6] J. Hu, A. Wyoscki, R. G. Winkler, G. Gompper, Sci. Rep. 5, 9586 (2015) P09

Light dependent motility of microalgae induces pattern formation in confinement

A. Fragkopoulos1, J. Vachier1, J. Frey1, F.-M. Le Menn1, M. Mazza1,2, O. Bäumchen1 1Max Planck Institute for Dynamics and Self-Organization, 37077 Göttingen, Germany 2Department of Mathematical Sciences, Loughborough University, Loughborough, Leicestershire LE11 3TU, United Kingdom E-mail: [email protected]

A collection of self-propelled particles can undergo complex dynamics due to hydrodynamic and steric interactions. In highly concentrated suspensions, it is possible for such particles to form large-scale concentration patterns, where the active suspension separates into regions of high and low particle concentrations. Here we present that suspensions of Chlamydomonas reinhardtii cells, a unicellular soil-dwelling microalgae and a model organism of puller-type microswimmers, may form patterns of high and low cell density regions under confinement in specific light conditions. We find that there are significant deviations in the motility of the cells under different light intensities and cell densities, which regulate patern formation in such active suspensions. Finally, by performing active Brownian simulations of such active particles with the observed motility characteristics, we show that we can re-create the pattern observed in our experiments. P10

Longwave nonlinear theory for chemically active droplet division

instability

Mohammad Abu Hamed1,2 and Alexander A. Nepomnyashchy1 1. Department of Mathematics, Technion-Israel Institute of Technology, Haifa 32000, Israel 2. Department of Mathematics, The College of Sakhnin - Academic College for Teacher Education, Sakhnin 30810, Israel E-mail: [email protected]

It has been suggested recently that growth and division of a protocell could be modeled by a chemically active droplet with simple chemical reactions driven by an external fuel supply. This model is called the continuum model. Indeed it's numerical simulation reveals a shape instability which results in droplet division into two smaller droplets of equal size resembling cell division [1]. In this project, we investigate the reduced version of the continuum model, which is called the effective model. This model is studied both in the linear and nonlinear regime. First, we perform a linear stability analysis for the flat interface, and then we develop a nonlinear theory using the longwave approach. We find that the interface at the leading order is governed by the modified Kuramoto-Sivashinsky equation.

Therefore the interface is subject to a logarithmic blow up after a finite time.

References [1] D. Zwicker, R. Seyboldt, C. A. Weber, A. A. Hyman, and F. Julicher, Nature Physics 13, 408 (2017). P11 Bead-spring modelling microswimmers Sebastian Ziegler1, Alexander Sukhov2, Jens Harting2,3, and Ana-Suncanaˇ Smith1,4 1PULS Group, Institute for Theoretical Physics, Department of Physics, Friedrich-Alexander Universität Erlangen-Nürnberg, Erlangen, Germany 2Helmholtz Institute Erlangen-Nürnberg for Renewable Energy, Germany 3Dep. of Applied Physics, Eindhoven University of Technology, The Netherlands 4Division of Physical Chemistry, Ruder¯ Boškovic´ Institute Zagreb, Croatia E-mail: [email protected]

Commonly, the propulsion of mechanical microdevices is ensured by prescribing the deformation cycle of the swimmer during a periodically repeated stroke. A major drawback of this approach is that it does not allow for a backlash of the environment onto the swimmer’s stroke. The alternative approach, in which this problem can be resolved, is to impose not the stroke it- self but the forces acting on the components of the otherwise unrestrained swimmer. The actual stroke then arises from the interplay between external and viscous forces. We use this approach to derive a calculation scheme for the trajectory of bead-spring swimmers dependent on the system’s parameters (fluid viscosity, force amplitude, spring constants, bead radius) applicable to arbitrary swimmer geometries. The precision of the calculation is quantified and the scheme is applied to a triangular swimmer in order to determine its behaviour under the influence of external driving.

References [1] R. Golestanian, and A. Ajdari, Phys. Rev. E 77, 036308, (2008) [2] U. Felderhof, Phys. Fluids 18, 063101, (2006) [3] J. Pande, L. Merchand et al., Phys. Rev. Lett. 87, 053024, (2017) [4] M. Rizvi, A. Farutin et al., Phys. Rev. E 97, 023102, (2018). P12

Light-driven Janus microswimmers in dense colloidal matrix

Tao Huang1, Sophie Gobeil1, Xu Wang2, Vyacheslav R. Misko3,4, Denys Makarov2, Gianaurelio Cuniberti1, Larysa Baraban1*

1Max Bergmann Center of Biomaterials and Institute for Materials Science, Technische Universität Dresden, 01062 Dresden, Germany 2Helmholtz-Zentrum Dresden-Rossendorf e.V., Institute of Ion Beam Physics and Materials Research, Bautzner Landstrasse 400, 01328 Dresden, Germany 3TQC, Universiteit Antwerpen, Universiteitsplein 1, B-2610 Antwerpen, Belgium 4Theoretical Quantum Physics Laboratory, RIKEN Cluster for Pioneering Research, Wako- shi, Saitama 351-0198, Japan E-mail: [email protected]; [email protected]

Collective behaviours is a common phenomenon in nature, ranging from microorganisms up to mammalians. The individual interacts with each other exhibiting complex behaviour (swarming or schooling etc.).[1] Here, we report on the collective behaviour of visible light driven Ag/AgCl-based Janus microswimmers surrounded by a dense matrix of passive SiO2 beads in pure water, and quantitative study the dynamics of the Janus and passive beads. The very recent reports have demonstrated, in pure H2O, Ag/AgCl-based Janus microswimmers is of high-mobility [2] and present an efficient exclusive effect to the nearby light polystyrene beads under visible light illumination.[3] These properties benefit from the efficient anisotropic photo-chemical production of the ions and generate a local chemical gradient around the Janus particle,[4] results in exclusion behaviour of dense matrix passive beads. The exclusion effect to the passive beads was found to be much stronger in front of the Ag/AgCl cap than behind the AgCl cap, due to strong ﬂows of the products of the chemical reaction from large clusters. The analysis of this radius of exclusion allows concluding about the asymmetric interactions of a swimmer with passive beads, in front of the AgCl cap and behind it. The observed light-driven exclusion phenomenon could provide a novel insight into the interactive effects between active-passive particles, and offer a better understanding for further biological studies.

Fig1. A sequence of images shows the interaction between stuck Janus particles and

nearby dense passive matrix (SiO2), under blue light illumination.

References [1] M. E, Ibele. et al., ACS NANO, 4, 4845-4851 (2010). P13 Active Brownian Particles in Crowded Media A. Liluashvili1, J. Ónody1 and Th. Voigtmann1,2 1Deutsches Zentrum für Luft- und Raumfahrt e.V., Linder Höhe, 51147 Köln, Deutschland 2Fachgruppe Physik, Heinrich-Heine Universität Düsseldorf, Universitätsstrae 1, 40225 Düsseldorf, Deutschland E-mail: [email protected]

We investigate the dynamics of model microswimmers (active Brownian particles) evolving at high densities and in the presence of crowding, i.e., in model porous media, making use of the mode-coupling theory of the glass transition (MCT). The microswimmers are modeled by hard disks in two dimensions undergoing both, translational and rotational diffusion. In addition they posses a constant self-propulsion velocity in their direction of orientation. MCT predicts an idealized active-glass transition, and we discuss the features of the slow dynamics emerging close to that transition. The porous background is treated as a frozen disordered density ﬁeld. We discuss the structure of the resulting theory, distinguishing between connected and disconnected parts of the correlation functions.

References [1] A. Liluashvili, J. Ónody, and Th. Voigtmann, Phys. Rev. E 96, 062608, (2017) [2] V. Krakoviack, Phys. Rev. E 75, 031503, (2007) P14 Evolution in range expansions with competition at rough boundaries S. Chu1, M. Kardar1, D.R. Nelson2, and D.A. Beller3 1Department of Physics, Massachusetts Institute of Technology, Cambridge, MA 02139, USA 2Department of Physics, Department of Molecular and Cellular Biology and School of Engineering and Applied Sciences, Harvard University, Cambridge, MA 02138, USA 3Physics Department, University of California, Merced, CA 95343, USA E-mail: [email protected]

When a biological population expands into new territory, genetic drift develops an enormous influence on evolution at the propagating front. In such range expansion processes, fluctuations in allele frequencies occur through stochastic spatial wandering of both genetic lineages and the boundaries between genetically segregated sectors. Recent laboratory experiments on microbial range expansions have shown that this stochastic wandering, transverse to the front, is superdiffusive due to the front’s growing roughness, implying much faster loss of genetic diversity than predicted by simple flat front diffusive models. We study the evolutionary conse- quences of this superdiffusive wandering using two complementary numerical models of range expansions: the stepping-stone model, and a new interpretation of the model of directed paths in random media, in the context of a roughening population front. Through these approaches we compute statistics for the times since common ancestry for pairs of individuals with a given spatial separation at the front, and we explore how environmental heterogeneities can locally suppress these superdiffusive fluctuations.

References [1] K. Korolev, M. Avlund, O. Hallatschek, and D. R. Nelson, Rev. Mod. Phys 82, 1691 (2010). [2] M. Kimura and G. H. Weiss, Genetics 49, 561 (1964). [3] J.F. Wilkins and J. Wakeley, Genetics 161, 873 (2002). [4] M. Kardar and Y.-C. Zhang, Phys. Rev. Lett. 58, 2087 (1987). [5] B. Derrida and R. Dickman, J. Phys. A: Math. Gen. 24, L191 (1991). P15 Mode-Coupling Theory for Active Brownian Particles J.Reichert1 and T.Voigtmann1,2

1Deutsches Zentrum für Luft- und Raumfahrt, Cologne, Germany 2Heinrich-Heine Universität, Düsseldorf, Germany E-mail: [email protected]

We present a mode-coupling theory (MCT) for the slow dynamics of two-dimensional spherical active Brownian particles (ABPs). The key quantity of our theory is the dynamical density- density correlator for which the Mori-Zwanzig projection formalism provides an exact equation of motion containing a convolution with a memory-kernel. MCT is a widely used tool to approximate the memory-kernel [1] which we extend to the case of active Brownian hard disks predicting a glass-transition diagram in the three-dimensional parameter space of density, self-propulsion velocity, and rotational diffusivity [2]. Further applications are provided by the integration through transients formalism which allows to derive expressions for macroscopic quantities by integrating over the dynamical density-density correlator. By combining our theory with preliminary work on MCT for passive systems in shear flow [3] we derive a Green-Kubo formula for the shear stress. We also derive a theory for the dynamical density correlator of a tagged particle, which can be exploited to get an equation of motion for the mean squared displacement. This allows us to study different scenarios: a single ABP in a dense sea of passive or active particles or otherwise a passive particle driven by a crowd of active particles. The mean squared displacement shows regimes of sub- and superdiffusion caused by the caging of neighbouring particles on the one hand and activity on the other hand. We also observe a transition in the long-time diffusion coefficient from a finite to a vanishing value at the glass transition point.

References [1] Götze, W., Complex Dynamics of Glass-Forming Liquids - A Mode-Coupling Theory (2008) [2] Liluashvili, A., Mode Coupling Theory for Active Brownian Particles, Phys. Rev. E 96, 062608 (2017) [3] Fuchs, M., A mode coupling theory for Brownian particles in homogeneous steady shear flow, Journal of Rheology 53, 957 (2009) P16 Structure and dynamics of a self-propelled semiflexible filament Shalabh K. Anand1 and Sunil P. Singh1 1Department of Physics, Indian Institute Of Science Education and Research, Bhopal 462 066, Madhya Pradesh, India E-mail: [email protected]

We study the structure and dynamics of a semi-flexible polymer chain subjected to tangential active force. The idea is to get a better insight into the behaviour of active polymer that can aid understanding various cytoskeleton systems. We perform Langevin dynamic simulation under varying conditions of propulsion strength and bending stiffness. A self-propulsion force is applied along the bond vectors, i.e., tangent to the filament and their locations are considered in two different manners. In case one, force is applied to all beads of the filament, which is termed as homogeneous self-propulsion. Here we obtain a monotonic decrease in the stiffness of the filament with Peclet´ number, hence, the radius of gyration also displays the same trend. The effective diffusivity of the filament shows enhancement with the active force, and it increases linearly with force and bending rigidity. In case two, self-propulsion force is applied only to a few bond vectors. The location of active forces is chosen in a periodic manner starting from the tail of the filament and leaving the front end without force. In this case, the filament acquires various structures such as the rodlike, helical, circular, and folded states. The transition from several states is understood in terms of tangent-tangent correlation, bending energy, and torsional order parameter. The helical state is identified through a crossover from exponential to oscillatory behaviour of the tangent-tangent correlation. A sudden increase in the bending energy separates a helical state to a folded state of the filament.

References [1] Shalabh K. Anand, Phys. Rev. E 98, 042501, (2018) P17 & P18

International Helmholtz Research School BioSoft T. Auth 1Theoretical Soft Matter and Biophysics, Institute of Complex Systems and Institute for Advanced Simulation, Forschungszentrum Jülich, 52425 Jülich, Germany E-mail: [email protected]

The International Helmholtz Research School of Biophysics and Soft Matter (IHRS BioSoft) provides excellent research opportunities for PhD projects. The aim of our complementary research training is to offer a curriculum at the interfaces between biology, chemistry, and physics.

Students benefit not only from lectures, seminars, and lab courses given by experts in the field, but also from courses in transferable skills. In addition, they experience the environment provided by a large, multidisciplinary research center. http://www.ihrs-biosoft.de/ihrs-biosoft/EN/Home/home_node.html P19 Pairing, waltzing and scattering of chemotactic active colloids Suropriya Saha1,2, Sriram Ramaswamy2 and Ramin Golestanian3 1Max Planck Institute for Dynamics and Self-Organization, Am Fassberg 17, 37077, Göttingen. Germany 2Indian Institute of Science, Bengaluru, Karnataka 560012, India. 3Max Planck Institute for Physics of Complex systems, Nöthnitzer Strasse 38, 01187, Dresden, Germany. E-mail: [email protected]

Two chemotactic active colloids, which can rotate their polar axis to align with an imposed chemical gradient, form bound states by cancellation of velocities. Their interactions are dynamical in origin, with contributions from self-propulsion and phoretic response to chemical field generated by each other, are thus non-central and non-reciprocal. Two swimmers remain bound at long times when the chemotactic response of at least one of the swimmers is positive, i.e. it rotates its polar axis to point up a linear gradient. These bound states fall in two broad categories (i) active dimers, separation fixed and polar axes orient along a line (ii) periodic orbits, relative inclination of the polar axes fixed, while the centre of mass executes cyclic motion. Chemotactic swimmers unbind and scatter away depending on initial conditions or with an increase of self-propulsion; while mutually anti-chemotactic swimmers always scatter away. These findings are summarized in state diagrams and representative trajectories are calculated to illustrate the rich dynamics. For the special case of a swimmer moving in a localised source of fuel, the fixed points underlying the bound states and the bifurcations that lead to transition between from one type of final state to another are classified. P20

Instability in settling array of discs

Chajwa, R 1, Govindarajan, R1, Menon, N 2 and Ramaswamy, S 3

1. International Centre for Theoretical Sciences, TIFR Survey no. 151 Bangalore 560089. 2. University of Massachusetts Amherst MA 01003. 3. Indian Institute of Science, Bangalore 560012

Collective stokesian sedimentation is a challenging problem in the physics of many- body system with long range interactions. It offers rich dynamical behaviour when particles have nontrivial shapes, due to coupling of geometry and flow. Our experiments demonstrate the effect of shape anisotropy at the level of two particles, finding a variety of bound and scattering orbits for settling pair of discs. These orbits can be understood by a mapping to Kepler problem of planetary motion, through the emergence of an effective Hamiltonian for this wholly dissipative system. This pair interaction serves as a building block for understanding collective sedimentation of disc shaped objects. In this poster I will summarize our experiments and theory on the collective settling of an array of disc shaped particles. P21 Self-propelled particles in anisotropic environments 1 1 1 Alexander R. Sprenger ∗, Christian Scholz , Anton Ldov , Raphael Wittkowski2,3, Hartmut Löwen1 1 Institut für Theoretische Physik II: Weiche Materie, Heinrich-Heine-Universität Düsseldorf, D-40225 Düsseldorf, Germany 2 Institut für Theoretische Physik, Westfälische Wilhelms-Universität Münster, D-48149 Münster, Germany 3 Center for Nonlinear Science (CeNoS), Westfälische Wilhelms-Universität Münster, D-48149 Münster, Germany ∗E-mail: [email protected]

Self-propelled particles in an anisotropic environment typically exhibit a motility that depends on their orientation relative to the prescribed external anisotropy. This situation is relevant for a plethora of living organisms and unanimous synthetic particles in external ﬁelds and anisotropic media. Here we present a minimalistic realization of orientation-dependent motility by explor- ing the motion of a self-propelled vibrated granular particles on an striated substrate. We also propose an extended model of active Brownian motion involving orientation-dependent motility and inertial effects which we solve analytically and numerically, in good agreement with our experimental data. The resulting anisotropic diffusional behavior can be exploited to optimize search strategies in anisotropic environments.

References [1] Christian Scholz, Nature Communications 9, 5156, (2018) P22 A phase ﬁeld crystal approach to active systems with inertia D. Arold1 and M.Schmiedeberg1 1Institut für Theoretische Physik 1, Friedrich-Alexander-Universität Erlangen-Nürnberg, Staudtstrasse 7, 91058 Erlangen, Germany E-mail: [email protected]

A phase field crystal approach for active systems consisting of particles with inertia is investigated. In our model the direction of the inertia given by the velocity can be different from the direction of the self-propulsion due to the activity. The implementation of the inertia is moti- vated by the derivation of an phase field crystal model of underdamped passive particle from a dynamical density functional theory [1], while the activity is modelled as in a phase field crystal model of active systems without inertia [2, 3]. In the overdamped regime the results of the latter model can be reproduced including the formation of stable resting or migrating crystals. In the opposite underdamped regime where inertial effects become relevant the migrating crystalline order is destroyed due to the high self-propulsion strength and a chaotic behaviour is observed instead.

References [1] A. J. Archer, J. Chem. Phys. 130, 014509 (2009). [2] A. Menzel and H. Löwen, Phys. Rev. Lett. 110, 055702 (2013). [3] A. Menzel, T. Ohta and H. Löwen, Phys. Rev. E 89, 022301 (2014). P23 Ring polymers are much stronger depleting agents than linear ones I. Chubak, E. Locatelli and C. N. Likos Faculty of Physics, University of Vienna, Vienna, Austria E-mail: [email protected]

We investigate the conformations and shapes of circular polymers close to planar, hard walls as well as the ensuing ringwall and polymer-induced wallwall interactions in ring polymer solutions. We derive, by means of Monte Carlo simulations, the effective interaction potential between the centres of mass of flexible, unknotted ring polymers and a hard wall for different polymerisa- tion degrees N. Adopting the coarse-grained description of ring polymers as ultra- soft, penetrable spheres, mean-field density functional theory is then employed in order to ex- amine ring polymer solutions under confinement. We demonstrate that, below the semi-dilute regime, ring polymers structure close to walls much more strongly than their linear counter- parts, their density profiles fea- turing pronounced oscillations. Moreover, the polymer-induced depletion potential between the two walls exhibits an oscillatory profile reminiscent of hard- sphere systems, with oscillations that intensify upon increasing polymer concentration. The obtained form of the depletion interaction is shown to be qualitatively different in comparison to the case of linear polymer solutions at the corresponding densities.

References [1] I. Chubak, E. Locatelli and C. N. Likos, Mol. Phys. 116, 2911-2926 (2018). P24 Enhanced rotational diffusion of squirmers in viscoelastic ﬂuids K. Qi1, E. Westphal2, G. Gompper1, and R. G. Winkler1 1Theoretical Soft Matter and Biophysics, Institute for Advanced Simulation and Institute of Complex Systems, Forschungszentrum Jülich, D-52425 Jülich, Germany 2Peter Grünberg Institute and Jülich Centre for Neutron Science, Forschungszentrum Jülich, D-52425 Jülich, Germany E-mail: [email protected], [email protected], [email protected], [email protected]

Squirmers are generic models for biological microswimmers and synthetic self-propelled particles. Fluid-mediated interactions are essential for their swimming behavior, which can be strongly affected by the fluid viscoelasticity [1]. Here, we perform mesoscale hydrodynamic simulations via the multiparticle collision dynamics (MPC) method [2] for a spherical squirmer in a viscoelastic fluid, which is composed of MPC fluid particles and polymers. Polymers are either of phantom nature or self-avoiding. The concentration of monomers on the squirmer surface is enhanced by introducing a short-range attraction between the squirmer and polymers. This leads to a decrease of the rotational diffusion for a passive colloid in the presence of polymers. Self-propulsion reduces the monomer concentration on the surface and the squirmer’s rotational diffusion is enhanced considerably, up to a factor 20 for phantom polymers. The actual change of the rotational diffusion Dr depends on the polymer length. An increasing polymer 0 length reduces Dr of the passive colloid, but Dr of the squirmer is enhanced. Both effects 0 contribute to the obtained substantial increase of the ratio Dr/Dr . For the squirmer embedded in solutions of self-avoiding polymers, a stronger effect is expected due to polymer-polymer interactions, which dramatically increase the fluid viscoelasticity.

References [1] J. R. Gomez-Solano, A. Blokhuis, and C. Bechinger, Phys. Rev. Lett. 116, 138301 (2016). [2] G. Gompper, T. Ihle, D. M. Kroll, and R. G. Winkler, Adv. Polym. Sci. 221, 1 (2009). [3] J. Elgeti, R. G. Winkler, and G. Gompper, Rep. Prog. Phys. 78, 056601 (2015). P25 Dynamics of conﬁned phoretic colloids K. R. Prathyusha1, Suropriya Saha1 and Ramin Golestanian1 1Max Planck Institute for Dynamics and Self-Organization, Am Fassberg 17, 37077, Göttingen. Germany E-mail: [email protected]

Phoretic colloids are known to form clusters by mechanisms similar to those in black holes formed by long ranged gravitational forces. The long ranged interactions are driven by phoretic response of one colloid to the chemical field generated by others. We investigate the dynamics of uniformly coated colloidal assembly in a confined geometry using Brownian dynamics simulation. In steady state, the colloids self assemble to form a cluster and exhibits slow spontaneous fluctuations. The interesting cluster dynamics is studied as a function of number of particles and strength of phoretic interaction. The mean square displacement shows distinct plateaus indicating cage breaking dynamics. P26

Enhanced dynamic heterogeneity in model active glass forming liquids Kallol Paul1 , Indrajit Tah1 and Smarajit Karmakar1 1Tata Institute of Fundamental Research Hyderabad, India E-mail: [email protected]

As many recent experiments suggesting the near-glassy nature of the cell interior, there are many theoretical works on modelling the glass transition in presence of activity. An important question in this context is how does activity play a role in the non-equilibrium glass transition. How does different dynamical features of typical glassy systems get modified in the presence of activity. For example, does activity change the static or dynamic heterogeneity length scale in the system? We used a recently proposed method, block analysis, to compute both the dynamic and static length scales. The dynamic length scale is obtained from the finite size scaling analysis of the four-point dynamic susceptibility on the block size and static length scale is obtained from block size dependence of the variance of alpha-relaxation time. Our findings show that dynamic heterogeneity length scale is increasing rapidly with activity whereas the temperature dependence of static length scale becomes weak with increasing activity. Activity also reduces the fragility of the glassy system, thus the origin of fragility in this model system can be related to the static length scale.

References

[1] R. Mandal et al, Soft matter 12 (29), 6268-6276 (2016) [2] SK Nandi et al, PNAS 115 (30), 7688-7693 (2018) [3] S Chakrabarty et al, Phys. Rev. Lett. 119 (20), 205502 (2017) [4] KE Avila et al, Phys. Rev. Lett. 113, 025701 (2014)

P27 Tracer Diffusion in a Dense Active Bath L.Abbaspour1 and S.Klumpp2 1,2Institute for Nonlinear Dynamics, University of Göttingen, Germany E-mail: 1 [email protected] 2 [email protected]

Many Biological systems are densely packed, from the molecular crowding in the cytoplasm to colonies of bacteria and bioﬁlm. Many features of the crowded system can be understood based on excluded volume effects. Here we investigate diffusion of a passive tracer in a bath consisting of a mixture of self-propelled active particles and passive particles. We study how density and activity of the bath particles affect the diffusion of the tracer particle. Our simulation reveals striking features such as anomalous diffusion coefﬁcient. We explore the different regimes that are observed for the diffusion of the tracer particle. P28 pH dependence of Swimming Direction of Janus Micromotors F. Rühle

The self-propulsion of biological or synthetic microswimmers is often influenced by a gravitational field [1,2], where a density mismatch leads to sedimentation and an offset center of mass triggers reorientation along the direction of gravity so that they swim upwards [2]. Combining these passive effects with the non-equilibrium properties of active motion creates novel and interesting dynamics, both in dense and dilute suspensions [3]. In particular, a large variety of dynamical behaviours has been observed for the squirmer microswimmer model [4,5]. In this contribution we focus on bottom-heavy squirmers and determine their state diagram, depending on the gravitational force and acting torque. For strong gravitational forces we observe conventional sedimentation, whereas the density profile is inverted for weaker forces. Addi- tionally, we find stable convective plumes for neutral squirmers that become metastable as the torque increases. We also observe spawning clusters at the bottom if the sedimentation velocity almost equalizes the swimming speed. Spawning clusters and continuous plumes do not occur for pusher and puller type swimmers.

References [1] J. Palacci, et al., Phys. Rev. Lett. 105, 088304 (2010). [2] K. Drescher et al., Phys. Rev. Lett. 102, 168101 (2009). [3] K. Wolff, A. M. Hahn and H. Stark, EPJE 36, 1 (2013). [4] J.-T. Kuhr et al., Soft Matter 13, 7548 (2017). [5] F. Rühle et al., New J. Phys. 20, 025003 (2018). P29

Role of pH in Micro-swimming N.Möller1,T.Palberg1 1Institut für Physik, Johannes-Gutenberg Universität, Staudingerweg 7, Mainz, Germany E-mail: [email protected]

In our modular micro-swimmer, a settled ion exchange (IEX) sphere exchanges residual Na+ ions for h+ and thus generates a pH-gradient. This gradient in turn induces a diffusion-electric field which drives an electro-osmotic solvent flow along the substrate [1]. The flow assembles dispersed cargo spheres, which, once assembled break the flow symmetry and set the formed complex in motion [2]. While a comprehensive characterisation of swimmer trajectories, speeds and stability as a function of experimental boundary conditions has been performed, and a theoretical model has been developed, the key input of modelling, i.e. the pH gradients, are available only in approximations. To gain insight of the influence the pH has on the particles motions, we utilize a home-built setup for pH-micro-photometry [3]. In principle, a solution of two indicators are used to visualize the pH of the particle neighbourhood. Applying the Lambert-Beer law with different colour channels of the used consumer DSLR camera, the pH can be deduced at a high spatiotemporal resolution with a resolution of a few microns. Our setup aids in analysing the correlation of pH with a local electric field E via the particle velocity v and electrophoretic analysis. We demonstrate performance and scope of this new technique on systems involving cationic ion-exchange resins and polystyrene (PS) micro-particles or platinum-PS Janus particles. Flow characteristics of the pH-field are further compared between a free-moving particle and a static IEX in the presence of an external flow. This method can be employed in a variety of other systems for characterization of microfluidic and chemical attributes of electro-osmotic-driven particles.

References

[1] R. Niu et al., Physical Chemistry Chemical Physics. 19.4, 3104-3114 (2017) [2] R. Niu, T. Palberg, T. Speck., Physical Review Letters. 119.2, 028001 (2017) [3] R. Niu et al., New Journal of Physics. 19.11, 115014 (2017)

P30

Chemical micromotors that self-assemble Tingting Yu†1,2, Prabha Chuphal†3, Snigdha Thakur3, Shang Yik Reigh1, Dhruv P. Singh1, and Peer Fischer1,2 1 Max-Planck-Institut für Intelligent Systeme, Stuttgart, Germany 2Institute for Physical Chemistry, University of Stuttgart, Stuttgart, Germany 3 Department of Physics, Indian Institute of Science Education and Research Bhopal, India E-mail: [email protected]

Self-propelling chemical motors rely on an asymmetric distribution of substrate of product molecules around the motor. The chemical fields around a catalytically active particle can give rise to phoretic interactions with other motors or colloids. Under the right conditions an attractive potential generated by a spherical catalytically-active colloid can attract nearby passive colloids. The active-passive dimer is then naturally self-assembled and symmetry broken chemical motor, even though the particles in isolation show no active propulsion. The poster describes the conditions under which isotropic titanium dioxide microspheres show attractive interactions with isotropic passive silica or polystyrene spheres in a 2% hydrogen peroxide solution. Because the TiO2 is semi-conduction, light can be used to switch the interactions on and off. The assembly and the active motion of the dimer motor can then be remotely controlled by the illumination conditions. Computer simulations show good agreement with experimental results and observations [1].

Figure 1. Schematic view for fabrication of self-propelling motors self-assembled from non-motile colloid particles by light control. The chemically active (black) and passive (white) particles are non- motile when light is off but they form a self-propelling dimer when the light is on.

Reference: P31 Active apolar doping determines routes to colloidal clusters and gels H. Massana-Cid1, J. Codina1,2, I. Pagonabarraga2,3 and P. Tierno1,2,4 1Departament de Física de la Matèria Condensada, Universitat de Barcelona, 08028 Barcelona, Spain 2Universitat de Barcelona Institute of Complex Systems (UBICS), Universitat de Barcelona, Barcelona, Spain 3CECAM, Centre Européen de Calcul Atomique et Moléculaire, École Polytechnique Fédérale de Lasuanne, Batochime, Avenue Forel 2, 1015 Lausanne, Switzerland 4Institut de Nanociència i Nanotecnologia, Universitat de Barcelona, Barcelona, Spain E-mail: [email protected]

We realize solid and stable structures from passive elements that are assembled by a few active dopants [1]. We use synthetic active but not self-propelling units. Blue light rapidly assembles 2D colloidal clusters and gels via nonequilibrium diffusiophoresis, where microscopic hematite dockers form long-living interstitial bonds that strongly glue passive silica microspheres. By varying the relative fraction of doping, we uncover a rich phase diagram including ordered and disordered clusters, space-filling gels, and bicontinuous structures formed by filamentary dockers percolating through a solid network of silica spheres. We characterize the slow relaxation and dynamic arrest of the different phases via correlation and scattering functions. Our findings provide a pathway toward the rapid engineering of mesoscopic gels and clusters via active colloidal doping.

References [1] H. Massana-Cid, J. Codina, I. Pagonabarraga, P. Tierno, Proceedings of the National Academy of Sciences 115, 10618-10623, (2018) P32

Efficient photocatalytic bismuth vanadate microparticles for active propulsion S. Heckel and J. Simmchen Technische Universität Dresden, Physical Chemistry, Dresden, Germany

E-mail: [email protected]

Photocatalytic microswimmers have many advantages. They are activated by light, which is a renewable, non-invasive energy source that can be applied remotely. It triggers the photocatalytic degradation of a certain fuel, which propels the microswimmers by self-electrophoretic and/or diffusiophoretic mechanisms. To further exploit the potential applications of photocatalytic microswimmers into more sensitive, biological setups, harmless fuels like pure H2O need to replace hydrogen peroxide, in combination with non-toxic material combinations. Bismuth vanadate is a very promising material to fulfil these requirements. With its narrow bandgap, it shows the potential to split pure water into hydrogen and oxygen (without depending on UV light).1 Additionally, it is completely nontoxic.2

Until now, no BiVO4 microswimmer has been proposed, but many differently shaped microstructures have been developed and compared concerning their photocatalytic performance. Especially hollow structures show great potential as they inhibit electron-hole recombination in the material.3

In this poster, we present BiVO4 microstructures with strong photocatalytic activity that show active propulsion under visible light and have therefore a great potential to become the powerful next generation of photocatalytic microswimmers.

Figure 1 SEM image of hollow BiVO4 microparticles. Scale bar is 2 µm.

References

[1] Li, R., Han, H., Zhang, F., Wang, D. & Li, C. Energy Environ. Sci. 7, 1369–1376 (2014) [2] Telpande, H.D. & Parwate, D. V., IJIRSET 4, 5350-5354 (2015) [3] Zong, L. et al, Mater. Res. Bull. 86, 44-50 (2017) P33

Polarization of Brownian swimmers with spatiotemporally heterogeneous activity S. Auschra1, N. Söker2, P. Cervenak1, V. Holubec1,3, K. Kroy1, F. Cichos2 1Institut für Theoretische Physik, Universität Leipzig, 04009 Leipzig, Germany 2 Peter Debye Institute for Soft Matter Physics, Universität Leipzig, 04103 Leipzig, Germany 3Charles University, Faculty of Mathematics and Physics, Department of Macromolecular Physics, V Holešovičkách 2, CZ-180 00 Praha, Czech Republic E-mail: [email protected]

Janus particles fuelled by laser heating are paradigmatic autophoretic microswimmers. Their dynamics under constant driving has been well characterized [1-3]. We consider situations in which the particles' propulsion strength fluctuates in space and time, due to a variable fuel supply. Specifically, we analyze their spatial and orientational distribution experimentally, realizing prescribed spatial and temporal activity variations via the laser heating. We find depletion in regions of higher activity and polarization in activity gradients. Using Brownian dynamics simulations and a powerful numerical solver for Fokker-Planck equations [4], we can reproduce the experimental observations. A simple run-and-tumble process captures the observed features, qualitatively, and provides some analytical insights

P34

Active Brownian heat engine S. Steffenoni1,2, V. Holubec2,3, K. Kroy2 1Max Planck Institute for Mathematics in the Sciences, Inselstr. 22, 04103 Leipzig, Germany 2Institut für Theoretische Physik, Universität Leipzig, 04009 Leipzig, Germany 3Charles University, Faculty of Mathematics and Physics, Department of Macromolecular Physics, V Holešovičkách 2, CZ-180 00 Praha, Czech Republic E-mail: [email protected]

We investigate a heat engine based on an active Brownian particle confined in a parabolic potential and immersed in a thermal bath. The average energetics of the engine is determined by the second moment of the particle position. We map the model to a passive engine in a bath with a suitable time-dependent effective temperature. The performance of both the active and the passive engine including maximum efficiency, efficiency at maximum power and maximum efficiency at a fixed power thus obeys the same formal limitations. The two models differ in observables sensitive to higher moments of the particle position. An important example is the entropy of the system, which is sensitive to the whole position distribution. We present a detailed analytical and numerical analysis of the dynamics and thermodynamics of the active engine both in the quasi-static regime and for finite-time cycles.

P35 Dependence of Swimming Direction of Janus Micromotors on pH Nikhilesh Murty1, Dhruv Singh1 and Peer Fischer1,2 1 Max-Planck-Institut fÃijr Intelligente Systeme, HeisenbergstraÃ§e 3, 70569, Stuttgart, Germany. 2 Institut fÃijr Physikalische Chemie, UniversitÃd’t Stuttgart, Pfaffenwaldring 55, 70569, Stuttgart, Germany. E-mail: [email protected]

Micro-organisms in the nature are capable of overcoming the Brownian randomness in their motion by using chemical energy whilst moving in a low Reynolds number ﬂuid. Synthetic colloidal swimmers have been designed to achieve the same albeit with a different propulsion mechanism [1]. Since these artiﬁcial micro-swimmers are highly dependent on their environment for propulsion, it becomes important to understand the effect that certain changes in their surroundings have on their motion. Colloidal particles partially coated with TiO2 and dispersed in H2O2 have been known to display directional motion under illumination of a UV-light source[2]. In a system where the pH of the medium is approximately 6.5 and not altered, these particles move away from the TiO2 coating owing to neutral self-diffusiophoresis[3]. However, the directionality of the motion of these particles is reversed when the pH of the medium is changed[4]. Further, the activity of particle also depends on the pH of the medium. It was observed that by controlling the pH of the medium and the intensity of light, the directionality and the speed of the Janus swimmers can be well controlled. The study provides a better understanding of the swimming behaviour of the colloids under different pH conditions and it also provides a simple technique for manipulation of the motion of the colloids which can be interesting for several applications. Additional effects like dynamic clustering and time-varying activity can be exploited for further control of the system.

References [1] Gomez-Solano, J. R. et al. Tuning the motility and directionality of self-propelled colloids. Sci. Rep. 7, 1âA¸S12(2017).˘ [2] Singh, D. P., Choudhury, U., Fischer, P. & Mark, A. G. Non-Equilibrium Assembly of Light-Activated Colloidal Mixtures. Adv. Mater. 29, 1âA¸S7(2017).˘ [3] Howse, J. R. et al. Self-Motile Colloidal Particles: From Directed Propulsion to Random Walk. Phys. Rev. Lett. 99, 8âA¸S11(2007).˘ [4] Brown, A. & Poon, W. Ionic effects in self-propelled Pt-coated Janus swimmers. Soft Matter 10, 4016âA¸S4027(2014).˘ P36

Photo-Induced Motion of Polymer Brush Coated Small Particles M. Sokolowski and S. Santer Institute of Physics and Astronomy, Potsdam, Germany E-mail: [email protected]

For manipulating nano- and micro sized objects at interfaces there is no immediate concept to gently move an ensemble of objects within a selected, possibly very small area. Atomic Force Microscope (AFM) has been the only method allowing a controlled motion of micro/nano- objects by mechanical pushing of single particles adsorbed at a surface.1 With that approach the reordering and manipulation is only possible on few particles, where on a macroscopic scale this approach is very time consuming and thus not beneficial. Recently Feldmann et al. reported a method of particle manipulation at liquid/solid interface by using light as external stimuli. The mechanism of particle manipulation is based on light induced generation of local hydrodynamic flow in the solution containing photosensitive molecules. These molecules are cationic surfactant in hydrophobic tail of which azobenzene group is incorporated to. Under illumination the photosensitive surfactant photo-isomerizes from trans- to cis- state resulting in changing of the hydrophobicity of the whole molecule.3 By local illumination with light of appropriate wavelength, the particles trapped at the solid interface can be collected or removed at the desired area of choice.2 So far, another attractive application is a near perfect particle separation at interface. Here we report on light induced manipulation and active motion of particles decorated by polymer brushes. The driving force of motion is a reversible release of brush interior surfactant molecules into the bulk solution by a controlled illumination of the macroscopic area. The release of surfactant molecules creates a hydrodynamic flow around each particle. On a macroscopic scale the lateral hydrodynamic flow around each particle separates all brush modified particles uniformly at the interface. In this study we systematically studied how the extend of particle motion depends on different parameters such as brush thickness, the surfactant concentration, the degree of ionization of polyelectrolyte brush.

References [1] Y. Sun, X. Liu. Micro- and Nanomanipulation Tools (Wiley-VCH: Weinheim, 2015). [2] D. Feldmann, S. R Maduar, M. Santer, N. Lomadze, O. I. Vinogradova, S. Santer, Scientific Reports, 6, 36443 (2016) [3] S. Santer, «Remote control of soft nano-objects by light using azobenzene containing surfactants» Journal of Physics D: Applied Physics, 51, 013002 (2017)

P37

Linear rheology of reversibly cross-linked biopolymer networks H. E. Amuasi1 , A. Fischer1 , A. Zippelius1 and C. Heussinger1 1 Institute of Theoretical Physics, Georg-August University of Göttingen, 37073 Göttingen, Germany E-mail: [email protected]

We suggest a simple model for reversible crosslinks, binding/unbinding to/from a network of semiflexible polymers. The resulting frequency dependent response of the network to an applied shear is calculated via Brownian dynamics simulations. It is shown to be rather complex with the time-scale of the linkers competing with the excitations of the network. If the lifetime of the linkers is the longest time scale, as is indeed the case in most biological networks, then a distinct low frequency peak of the loss modulus develops. The storage- modulus shows a corresponding decay from its plateau-value, which for irreversible cross- linkers extends all the way to the static limit. This additional relaxation mechanism can be controlled by the relative weight of reversible and irreversible linkers.

References

[1] H. E. Amuasi, A. Fischer, A. Zippelius, and C. Heussinger, J. Chem. Phys. 149, 084902 (2018). [2] J. Plagge, A. Fischer, and C. Heussinger, Phys. Rev. E 93, 062502 (2016).

P38

Brownian molecules formed by delayed harmonic interactions

Daniel Geiß and Viktor Holubec

A time-delayed response of individual living organisms to information exchange within groups or swarms leads to the formation of complicated collective behavior. A recent experimental setup where Brownian particles interact via time-delayed forces aims to mimic this behaviour [1]. We study a system of N Brownian particles interacting via a harmonic, time-delayed potential. For N < 4, we model the problem analytically by linear stochastic delay differential equations, and for N > 3 we use Brownian dynamics simulations. The particles form molecular-like structures which become increasingly unstable with the number of particles and the length of the delay. We evaluate the entropy fluxes in the system and develop an appropriate time-dependent transition state theory to characterize transitions between different isomers of the molecules

References

[1] U. Khadka, V. Holubec, H. Yang, and F. Cichos: Nat. Commun. 9, 3864 (2018)

P39 Collective rotations of active particles interacting with obstacles Z.Mokhtari1, T.Aspelmeier, and A.Zippelius1 1Institute for Theoretical Physics, Friedrich-Hund-Platz 1, 37077 Goettingen, Germany E-mail: [email protected]

We study the motion of active particles in the presence of static obstacles. We observe accu- mulation and crystallization of active particles around the obstacles which serve as nucleation sites, a phenomenon that is expected due to the known absorption of active particles at solid boundaries. In the limit of high activity, the crystals start to rotate spontaneously around the obstacle, resembling a rotating rigid body. We explain the occurrence of such rotations through the enhanced attraction of particles to the cluster whose orientation points along its rotational velocity as compared to those whose orientation points in the opposite direction. P40

Influence of catalysts on the propulsion path of micromotors.

Purnesh Chattopadhyay1, Veronika Magdanz2, Juliane Simmchen1

1Chair of Physical Chemistry, TU Dresden 2Chair of Applied Zoology, TU Dresden.

E.mail: [email protected]

Over the last decade there have been exciting developments in the field of self-propelling micromotors1 with a strong emphasis on catalytic micromotion2 often based on metal catalysts3. So far, the attention had mostly focused on the development of different catalytic systems for self- propulsion rather than understanding the concrete influences. Some recent studies discuss the influence of ionic species on the swimming speed of the Pt- catalytic micromotors4. The decomposition of peroxide in presence of a catalyst is much more complex than the generally assumed overall reaction5:

Pt + H2O2 Intermediate complexes Pt + H2O +O2 The formation of different intermediate complexes as well as the association-dissociation of fuel on the catalyst can influence the propulsion speed6. Here, we study the influence of the type of metal catalyst and fuel concentration on the trajectories of motion in order to evaluate which role the catalysts play in the formation of different intermediate complexes which tend to govern the motion path. References: 1. Purcell, E. M. Life at low Reynolds number. Am. J. Phys. (1977). doi:10.1119/1.10903 2. Safdar, M., Simmchen, J. & Jänis, J. Environmental Science Nano TUTORIAL REVIEW Light-driven micro-and nanomotors for environmental remediation. Environ. Sci 4, 1602 (2017). 3. Simmchen, J. et al. Topographical pathways guide chemical microswimmers. Nat. Commun. 7, 10598 (2016). 4. Brown, A. & Poon, W. Ionic effects in self-propelled Pt-coated Janus swimmers. Soft Matter (2014). doi:10.1039/c4sm00340c 5. Ibrahim, Y., Golestanian, R. & Liverpool, T. B. Multiple phoretic mechanisms in the self- propulsion of a Pt-insulator Janus swimmer. J. Fluid Mech. (2017). doi:10.1017/jfm.2017.502 6. Brown, A. T., Poon, W. C. K., Holm, C. & De Graaf, J. Ionic screening and dissociation are crucial for understanding chemical self-propulsion in polar solvents. Soft Matter (2017). doi:10.1039/c6sm01867j

P41

Active matter systems can exhibit coexisting patterns of competing symmetries T.Krüger1, L.Huber1, R.Suzuki2,3, A. R. Bausch2 and E.Frey1 1Arnold Sommerfeld Center for Theoretical Physics and Center for NanoScience, Department of Physics, Ludwig-Maximilians-Universität München, D-80333 Munich, Theresienstrasse 37, Germany. 2Lehrstuhl für Biophysik (E27), Technische Universität München, James-Franck-Strasse 1, D-85748 Garching, Germany. 3Center for Integrative Medicine and Physics, Institute for Advanced Study, Kyoto University, 606-8501 Kyoto, Japan. E-mail: [email protected]

References

Impact of Brush/Water Interface on the self-propulsion of Janus Particles M. Heidari, F. Jakob and R. von Klitzing1 1Soft Matter at Interfaces, Department of Physics, TU Darmstadt, Darmstadt, Germany E-mail: [email protected]

We explore the 2D self-propulsion of Au-PS Janus particles between two brush functionalized glass substrates. The autonomous motion of the particle is achieved under wide parallel laser beam illumination (λ=532 nm) with various intensities. We vary the chain length of the brush layer, hence the slip boundary condition at the brush/water interface alters which subsequently influences the active motion of particles. Our results demonstrate an enhanced self-propulsion of particles near brush coated substrate as well as a significant dependency on the brush chain length.

P43 Tissue Mechanics: Stokes Flow in Conﬂuent Cell Simulations C. P. Beatrici1 and F. Graner1 1 Laboratoire Matière et Systèmes Complexes, Université Denis Diderot - Paris 7, CNRS UMR 7057, Condorcet building, 10 rue Alice Domon et Léonie Duquet, F-75205 Paris Cedex 13, France E-mail: [email protected]

Cell migration plays an important role in embryogenesis, wound healing and tumor metastasis. Cell monolayer migration experiments help to understand what determines the movement of cells. However the usual monolayer set up can not discriminate the differences in migration created by each cell ingredient. That is why we find useful to observe cell monolayers dynamics in a discriminant benchmark: a heterogenous flow, i. e. a cells flow around a circular obstacle (known as Stokes geometry)[1, 2]. We work with experiments in vivo (cell monolayer migration in culture) and in vitro (in drosophila thorax during metamorphosis). We simulate cell monolayer evolution using different computation models of active agents. We compare several computation cell models among themselves and with in vitro experiments. To compare the data from different sources we take the same measurements in each one: velocity field, density field, deformation field and others. The aim is to list the minimum cellular ingredients necessary to correctly simulate real cell migration. We plan to compare the measurements in the several active matter models[3, 4, 5] (including the so-called Vicsek and Potts ones) which all have their own specific advantages. Altogether, the dialog between in vivo, in vitro and in silico assays, and analytical calculations will enable to evidence specific cell-level contributions to tissue mechanics and collective active migration.

References [1] F. Graner, B. Dollet, C. Raufaste and P. Marmottant, Eur. Phys. J. E 25, 349-369 (2008) [2] I. Cheddadi, P. Saramito, B. Dollet, C. Raufaste, and F. Graner, Eur. Phys. J. E 34, 1, (2011) [3] F. Graner and J. A. Glazier, Phys. Rev. Lett. 69, 2013 (1992). [4] T. Vicsek, A. Cziròk, E. Ben-Jacob, I. Cohen, and O. Shochet, Phys. Rev. Lett. 75, 1226 (1995). [5] Daniel L. Barton, Silke Henkes, Cornelis J. Weijer, and Rastko Sknepnek, PLoS Comput Biol. 13(6): e1005569 (2017 Jun) P44

Self-propelled rods with quorum sensing C. Abaurrea Velasco, M. Abkenar, G. Gompper, and T. Auth 1Theoretical Soft Matter and Biophysics, Institute of Complex Systems and Institute for Advanced Simulation, Forschungszentrum Jülich, 52425 Jülich, Germany E-mail: [email protected], [email protected], [email protected]

Active agents show a large variety of motility-induced collective behaviors, such as the formation of static and motile clusters. The behavior of dense suspensions of engineered phoretic particles and of bacteria during biofilm formation is determined by two qualitatively different physical mechanisms: (i) volume exclusion (short-range steric repulsion) [1] and (ii) quorum sensing (longer-range reduced propulsion due to alteration of the local chemical environment) [2]. Both mechanisms lead to phase separation; however, the structure of the systems and the rod dynamics vastly differ. To systematically characterize such systems, we study semi-penetrable self-propelled rods in two dimensions with a propulsion force that decreases with increasing local rod density, by employing Brownian dynamics simulations [3].

We find that quorum sensing enhances cluster formation and the polarity of the clusters, and induces perpendicularity of rods at the cluster borders. For systems where the rods essentially become passive at high densities, formation of asters and stripes is observed. Systems of rods with larger aspect ratios show more ordered structures compared to those with smaller aspect ratios, due to their stronger alignment, with almost circular asters for strongly density-dependent propulsion force. With increasing range of the quorum sensing, the asters start to rotate and to intermittently eject streams, and finally form clusters with domains for a high sensitivity to the number of neighbors and polar hedgehog clusters for a low sensitivity to the number of neighbors. For Bacillus subtilis populations also increased propulsion with increased density has been observed. We therefore plan to employ our model also to study the collective behavior of self-propelled rods with density-dependent enhanced propulsion force.

References

[1] M. Abkenar, K. Marx, T. Auth, G. Gompper, Phys. Rev. E 88, 062314 (2013) [2] M. E. Cates, J. Tailleur, Annu. Rev. Condens. Matter Phys. 6, 219 (2015) [3] C. Abaurrea Velasco, M. Abkenar, G. Gompper, T. Auth, Phys. Rev. E 98, 022605 (2018) P45 Shape and motility of composite active agents with internal degrees of freedom C. Abaurrea Velasco, T. Auth and G. Gompper Forschungszentrum Jülich, Institute of Complex Systems and Institute for Advanced Simulation, Theoretical Soft Matter and Biophysics, 52425 Jülich, Germany E-mail: [email protected]

The cytoskeleton is a highly dynamic three-dimensional network of polar filamentous proteins and molecular motors. It provides structural stability for biological cells and generates and transmits mechanical forces. For example, in mesenchymal cell motility actin filaments poly- merize at their plus ends, which exerts pushing forces on the cell membrane. Here, we present a generic two-dimensional model of a flexocyte, where self-propelled filaments attached to semiflexible polymer rings form a mechanosensitive self-propelled agent. We find universal correlations between shape and motility. To probe the internal dynamics of flexocytes, we study the effect of the substrate patterning on their mechanical response.

The flexocytes reproduce experimentally observed shapes and motility patterns of biological cells. They assume circular, keratocyte-like, and neutrophil-like shapes and show both persistent random and circling motion. Interestingly, explicit pulling forces only are sufficient to reproduce this cell-like behavior. Also for the reflection of flexocytes at walls and the deflection of their trajectories at friction interfaces we find parallels to the behavior of biological cells. Our model may thus serve as a filament-based, minimal model for cell motility.

30 m 20 y/R

10 -18 -12 -6 x/Rm

Figure 1: Self-propelled flexocytes with a keratocyte-like shape. Left: A flexocyte with explicit pulling forces only. Right: Trajectory of the center of mass of a flexocyte. The black point represents the position for the last frame. The color wheel indicates the rod orientation.

References [1] C. Abaurrea Velasco, S. Dehghani Ghahnaviyeh, H. Nejat Pishkenari, T. Auth, and G. Gompper., Soft Matter 13, 5865 (2017) [2] M. Paoluzzi, R. Di Leonardo, M. C. Marchetti, L. Angelani, Sci. Rep. 6, 34146 (2016) P46

A Model For The Possible Role Of Substrate Rigidity For Bacterial Migration And Colony Formation A.N. Simsek1, M.D. Koch2, Z. Gitai2, J.W. Shaevitz2, B. Sabass1, G. Gompper1 1ICS-2/IAS-2 Forschungszentrum Jülich, Jülich, Germany 2Lewis-Sigler Institute for Integrative Genomics, Princeton University, NJ, USA

E-mail: [email protected]

Pseudomonas aeruginosa is an ubiquitous pathogen responsible for severe and chronic infections. The bacterium employs retractable type-IV pili for migration and colonizes a broad variety of biotic and abiotic surfaces. How surface properties affect migration and colony formation of P. aeruginosa is a potentially important factor for bacterial surface contamination and infections alike. Here, we theoretically and experimentally study the effect of surface rigidity on the migration of P. aeruginosa. In our model, we assume a rod-like bacterium and treat each pilus, as well as the substrate as elastic springs. Pilus assembly and retraction are modeled as stochastic, force-dependent processes. To generate experimental data, we perform extensive image analysis to record the migration of different strains of P. aeruginosa on polyacrylamide substrates. Experimental data shows a non-linear dependence of migration speed and mean square displacement on substrate rigidity, which is in accordance with simulations. Finally, we present an analytical theory, that catches the trends in experiments, and insights we gained from theoretical approach. P47

Collective intercellular communication through ultra-fast hydrodynamic trigger waves

Arnold JTM Mathijssen, Josh Culver, Saad Bhamla, Manu Prakash Stanford University, Dept. of Bioengineering, 443 Via Ortega, Stanford, California, USA E-mail: [email protected]

The biophysical relationships between sensors and actuators have been fundamental to the development of complex life forms; Abundant flows are generated and persist in aquatic environments by swimming organisms, while responding promptly to external stimuli is key to survival. Here, akin to a chain reaction, we present the discovery of hydrodynamic trigger waves in cellular communities of the protist Spirostomum ambiguum, propagating hundreds of times faster than their swimming speed. Coiling its cytoskeleton, Spirostomum can contract its long body by 50% within milliseconds, with accelerations reaching 14g-forces. Surprisingly, a single cellular contraction (transmitter) is shown to generate long-ranged vortex flows at intermediate Reynolds numbers, which can trigger neighbouring cells, in turn. To measure the sensitivity to hydrodynamic signals (receiver), we further present a high- throughput suction-flow device to probe mechanosensitive ion channel gating by back- calculating the microscopic forces on the cell membrane. These ultra-fast hydrodynamic trigger waves are analysed and modelled quantitatively in a universal framework of antenna and percolation theory. A phase transition is revealed, requiring a critical colony density to sustain collective communication. Our results suggest that this signalling could help organise cohabiting communities over large distances, influencing long-term behaviour through gene expression, comparable to quorum sensing. More immediately, as contractions release toxins, synchronised discharges could also facilitate the repulsion of large predators, or conversely immobilise large prey. We postulate that beyond protists numerous other freshwater and marine organisms could coordinate with variations of hydrodynamic trigger waves.

P48

Collective motion in biological systems: Langevin Equation investigation

L. Amallah1, A. Hader1,d, H. Sbiaai1, I. Achik2 and Y.Boughaleb1,3

1LBGIM, Ecole Normale Supérieure, University Hassan II. Casablanca, Morocco 2LPMC , Faculty of sciences Ben M’sik . University Hasan II. Casablanca, Morocco 3LPMC , University Chouaib Doccali. El Jadida, Morocco 4 Centre régional des métiers d’éducation et de formation Casablanca- Settat/ établissement Settat, Morocco E-mail: [email protected]

Most of us must have been flabbergasted by an elegant collective movement show. Many animal groups as fishing schools and bird flocks are definitely constitutional, with the literal of organisms so coordinated. They may change form and direction; they appear to be a coherent single entity. Many of the collective behaviors presented by such groups can be understood only by considering the very large number of interactions between the group components. Individual computer simulations are adequate methods to study movement groups. That’s why physicists have made a great effort to study widely the motion of these biological systems, in order to better understand and describe the physical mechanisms that govern the out-of-equilibrium dynamics of these systems. In our work, attention is drawn to propose a new approach to explicitly identify dynamic properties and scaling behavior of self- propelled particles for a given system in collective motion rather than Vicsek as mostly used in the literal. To address our problems, we will be based on choosing the Langevin equation as a stochastic differential equation. Our investigation involves velocity as an important parameter to discuss the different dynamic properties of our system. In order to affirm the validity of our approach, the results show that the system velocity increases with time and reaches to finite value at the equilibrium phase. This result is more consisting with the one of the Vicsek model.

P49

The tortoise and the hare: how collective behaviours within bacterial biofilms select for cells that move more slowly Oliver J. Meacock1,2, Amin Doostmohammadi3, Kevin R. Foster1, Julia M. Yeomans3, William M. Durham1,2 1 Department of Zoology, South Parks Road, University of Oxford, Oxford, UK 2 Department of Physics and Astronomy, University of Sheffield, Sheffield, UK 3 Rudolf Peierls Centre for Theoretical Physics, University of Oxford, Oxford, UK E-mail: [email protected]

Collective movements are a ubiquitous feature of motile living systems, being observed in systems as diverse as wildebeest herds to thin films of microtubules. Typically, organisms are thought to benefit from coordinating their movement, for example to enhance propagation of information through a herd in response to attack by a predator. However, motile bacteria living in biofilms inevitably generate collective behaviours as a consequence of the extremely high density of the communities that form on surfaces. In this context, collective behaviours may create problems that evolution must overcome, rather than opportunities that evolution can exploit. We study the collective movements of the opportunistic pathogen Pseudomonas aeruginosa, which utilises grappling hook-like structures known as Type IV Pili to move within biofilms. Remarkably, we find a disconnect between individual and collective behaviours in this system. We have characterised the motility of a mutant strain in low-density communities, demonstrating that it moves more quickly than the wild-type as solitary cells. However, in high-density biofilms, where cells move as a collective, wild-type cells are able to acquire new territory four times more quickly than the mutant. The mutant consequently suffers from a dramatic reduction in fitness in the high-density system. Through a combination of single- cell tracking and individual-based modelling, we show that populations at the expanding edge of biofilms can be modelled as an active nematic, spontaneously generating +½ and -½ topological defects. In biofilms formed of the mutant, collisions between +½ defects cause cells to reorient perpendicular to the surface. These cells become trapped at the collision site and are no longer able to contribute to the expansion of the colony. In contrast, the lower activity of the wild-type allows it to resolve these defect collisions without inducing vertical reorientation. Our results suggest that the physics of collective movement set a fundamental evolutionary constraint on the motion of individual cells within biofilms. P50 Filamentous Active Matter: Band Formation, Bending, Buckling, and Defects G. A. Vliegenthart, A. Ravichandran, M. Ripoll, T. Auth and G. Gompper Theoretical Soft Matter and Biophysics, Institute for Advanced Simulation and Institute of Complex Systems, Forschungszentrum Jülich, D-52425 Jülich, Germany E-mail: [email protected]

Motor proteins drive the persistent motion of cytoskeletal filaments in vivo as well as in vitro. We perform component-based Brownian dynamics simulations of polar semiflexible filaments and molecular motors. This allows for linking the microscopic interactions and the filament activity to self-organisation and dynamics from the fundamental two-filament level all the way up to the of mesoscopic domain level. Dynamic filament crosslinking and sliding, and excluded- volume interactions promote formation of motor-bound bundles at small filament densities, and of active polar nematics at high densities. An Euler buckling-type instability sets the size of the polar domains and the density of topological defects. We predict a universal scaling of the active diffusion coefficient and the domain size with the active force, and its dependence on parameters like motor concentration, filament concentration and persistence length.

References [1] G. A. Vliegenthart, A. Ravichandran, M. Ripoll, T. Auth and G. Gompper, Filamentous Active Matter: Band Formation, Bending, Buckling, and Defects eprint arXiv:1902.07904 P51

Sperm motility in modulated microchannels

S. Rode, J. Elgeti and G. Gompper Forschungszentrum Jülich, Germany Theoretical Soft Matter and Biophysics

Sperm cells swim through the fluid by a periodic wave-like beating of their flagellum [1-3]. At low Reynolds numbers and in confinement, the directed motion of sperm is strongly influenced by steric and hydrodynamic surface interactions. We model sperm motility in mesoscale hydrodynamics simulations by imposing a planar traveling bending wave along the flagellum [2]. Thus, the flagellum beats in a periodic pattern within a defined beat plane. Sperm are simulated swimming in curved, straight, shallow and zigzag-shaped microchannels [4]. Changes in the sidewall modulations and the imposed beat pattern allow the identification of a strong dependence of the surface attraction on the beat-shape envelope of the sperm cell. The simulations reveal a strong dependence of the deflection angle on the orientation of the beat plane with respect to the channel sidewall, and thus deepen the understanding of sperm navigation under strong confinement. Detachment of sperm, while swimming along curved walls, is dominated by the change of beat-plane orientation. In particular, we will discuss how strong confinement in shallow channels drastically increases wall attraction. Our simulation results reveal a consistent picture of passive sperm guidance that is dominated by the steric interactions of the beat pattern with the nearby surfaces.

References

[1] J. Elgeti et al., Rep. Prog. Phys. 78, 056601 (2015) [2] J. Elgeti et al., Biophys. J. 99, 1018 (2010) [3] G. Saggiorato et al., Nat. Commun. 8, 1415 (2017) [4] S. Rode, J. Elgeti, G. Gompper, NJP 21, 013016 (2019)

P52

Spontaneous Spatiotemporal Ordering of Shape Oscillations Enhances Cell Migration Matteo Campo1,2, Simon K. Schnyder3, John J. Molina4, Thomas Speck1, Ryoichi Yamamoto4,5 1Institut für Physik, Johannes Gutenberg-Universität Mainz, Staudingerweg 7-9, 55128 Mainz, Germany, 2Graduate School Materials Science in Mainz, Staudinger Weg 9, 55128 Mainz, Germany, 3Fukui Institute for Fundamental Chemistry, Kyoto University, Kyoto, 606-8103, Japan, 4Department of Chemical Engineering, Kyoto University, Kyoto, 615-8510, Japan, 5Institute of Industrial Science, The University of Tokyo, Tokyo 153-8505, Japan

E-mail: [email protected]

The movement of large aggregates of cells is relevant for many biological phenomena including wound healing, invasion of cancer cells, and morphogenesis. So far, the influence of the cells’ cyclic shape changes on their collective behavior has not been investigated systematically. Here we demonstrate, using numerical simulations, that the interplay of directed motion, shape fluctuations, and excluded volume enables cells to locally synchronize their motion and thus enhance collective migration. Our model captures elongation and contraction of crawling ameboid cells controlled by an internal clock, mimicking the internal cycle of biological cells. We show that shape oscillations are crucial for local rearrangements that induce synchronization of internal cycles between neighboring cells even in the absence of signaling and regularization. Our findings reveal a novel, purely physical mechanism through which the internal dynamics of cells influences the collective behavior, which is distinct from well known mechanisms like chemotaxis, cell division, and cell-cell adhesion.

References

Campo, M., Schnyder, S. K., Molina, J. J., Speck, T., & Yamamoto, R. arXiv preprint arXiv:1901.06707 (2019)

P53 Actuation of particles in modulated Poiseuille ﬂow W. Schmidt1,2, M. Laumann1, E. Kanso2, and W. Zimmermann1 1Physikalisches Institut, Universität Bayreuth, Germany 2Aerospace and Mechanical Engineering, University of Southern California, USA E-mail: [email protected]

What is the dynamical behavior of micro-particles in Poiseuille flow with oscillating flow direction at low Reynolds number? We investigate the overdamped motion of bead-spring models, e.g., capsules and red blood cells. We predict net motion of the particles, despite vanishing mean flow. This effect is generic as it does not depend on the model and is explained by a broken symmetry. The mean actuation velocity of passive particles is caused by their varying shape in both half periods. Since the net velocity depends also on the size and the elasticity of the particles, this novel actuation mechanism is appropriate for particle sorting. The system is also explored for active particles. P54

Multiparticle Collision Dynamics Modeling of Nematic Liquid Crystal with Variable Order Parameter S. Mandal1 and M. G. Mazza1,2 1Max Planck Institute for Dynamics and Self-Organization, Göttingen, Germany 2Loughborough University, Loughborough, United Kingdom E-mail: [email protected]

We have generalized the particle-based multiparticle collision dynamics (MPCD) method to model the hydrodynamics of nematic liquid crystals. Following Qian-Sheng theory [Phys. Rev. E 58, 7475 (1998)] of nematic liquid crystals, the spatial and temporal variations of the nematic director field and the nematic degree of order are described by a tensor order parameter. The principle idea is to assign a tensor quantity to each MPCD particle, whose average resembles the macroscopic tensor order parameter. The applicability of this new method is verified by performing several physical and numerical tests. We have tested: (a) the isotropic-nematic phase transition, (b) the annihilation dynamics of a pair of point defects, (c) the flow alignment of the nematic director in shear and Poiseuille flows, and (d) the velocity profile in shear and Poiseuille flows. We have found excellent agreement with existing literature. Additionally, we study the decay of force-dipole flow field in nematic liquid crystals. The present method can have far-reaching implications not only in modeling of nematic flows, but also to study the motion of colloids and microswimmers immersed in an anisotropic medium.

P55 A minimal model for ﬂuid-like collective cell migration D. Sarkar, G. Gompper and J. Elgeti Institute of Complex Systems and Institute for Advanced Simulation Forschungszentrum Jülich, 52425 Juelich, Germany E-mail: [email protected]

The collective dynamics of cell plays the key role in many fundamental biological processes like morphogenesis, tissue repair and tumour metastasis etc. Madin-Darby canine kidney (MDCK) cells have been established as one model system to study collective cell migration. On adhesive substrates, cells grow in roughly circular colonies, expanding with time and displaying fascinating motile behavior. Two aspects of their motion lie at the heart of this study: (a) Cells move throughout the colony, forming large scale patterns like swirls or fingers at the edge; reflecting a fluid like behaviour of the cell colony. At the same time, (b) the colonies are extremely cohesive. The colony is not maintained by a constant flux of cells leaving and entering from the surrounding. Instead, the surrounding is devoid of cells, one might say these colonies display liquid-vacuum coexistence. The active Brownian particle (ABP) model has been used intensively to model such motile cell colonies. However, âA˘ Inormalâ˙ A˘ I˙ ABPâA˘ Zs´ show either liquid-gas coexistence - with a finite density of cells away from the colony, or crystallize, if the adhesion is strong enough to prevent particles from escaping. We propose a novel particle-particle interaction potential that allows for cells to move, despite strong adhesion. We show that this model results in colonies with fluid like properties while remaining cohesive in nature at the same time. Furthermore, these colonies can be under tensile stress, as reported for growing MDCK colonies [1]. In combination with velocity alignment, cells can escape the mother colony collectively. A group of cells can form a finger that eventually pinches of.

References [1] X. Trepat et al., Nat. Phys. 5(6), 426âA¸S430(2009).˘ [2] A. Puliaﬁto, et al., Proc. Natl. Acad. Sci. USA 109(3), 739âA¸S744(2012).˘ [3] M. Basan et al., Proc. Natl. Acad. Sci. USA 110(7), 2452âA¸S2459(2013).˘ [4] Y. Fily, and M. C. Marchetti, Phys. Rev. Lett. 108, 235702 (2012). [5] G. S. Redner, A. Baskaran, and M. F. Hagan, Phys. Rev. E 88, 012305 (2013). P56

Hydrodynamic interactions of beating cilia A. Solovev1 and B.M. Friedrich2 1Center for Advanced Electronics Dresden (cfaed) 1,2Institute for Theoretical Physics, TU Dresden, Germany E-mail: [email protected], [email protected]

Cilia are thin elastic filaments grown on epithelia surfaces that exhibit active regular bending waves; collections of cilia can synchronize their beat into mesoscale metachronal waves.

We perform hydrodynamic simulations of beating cilia using experimental beat patterns. From pairwise hydrodynamic interactions, we deduce synchronization strength as a function of cilia orientation. The aim of this project is to understand how disorder in cilia orientation affects emergent metachronal waves in collections of beating cilia.

We use a previously developed description of the cilia beat as limit-cycle oscillator with known load response to external hydrodynamic forces [1,2], using the framework of generalized forces and a fast boundary-element method to solve the three-dimensional Stokes equation.

References

[1] Klindt et al. Load response of the flagellar beat. Physical Review Letters 117(25) (2016) [2] B.M. Friedrich, Hydrodynamic synchronization of flagellar oscillators. The European Physical Journal Special Topics 225.11-12 (2016)