Active Soft Matter
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View Article Online / Journal Homepage / Table of Contents for this issue Soft Matter Dynamic Article LinksC< Cite this: Soft Matter, 2011, 7, 3050 www.rsc.org/softmatter EDITORIAL Active soft matter DOI: 10.1039/c1sm90014e Nature provides us with many examples appear in this themed issue. Within it, particular viscoelastic properties, by of complex materials, ranging from rela- various aspects and manifestations of enzymatic activity. In this issue, the tively passive, near equilibrium structures activity are explored in biological or bio- effects of activity on viscoelasticity are such as wood and bone to highly active, logically inspired systems. These range explored in systems ranging from highly far from equilibrium systems such as from the nanometre or single-molecule simplified in vitro constructs to living cells migrating cells. Living cells are kept out of level to the near macroscopic level of and even tissues. In addition to mechan- equilibrium in large part by metabolic collective behavior, and represent many ical and viscoelastic effects, local motor processes and energy-consuming enzymes different avenues and approaches to the activity also leads to non-thermal fluctu- such as molecular motors that generate study of active soft matter. ations,9,10 which are also studied in this forces and drive the machinery behind cell Activity can manifest itself in dynamic issue. locomotion and many other cellular patterns, such as vortices or asters, in At a more discrete level, the motion of processes; molecular motors are also the which stiff filaments radiate out from individual active particles, or modest basis of muscle contraction at the a common center,4 in analogy with the numbers of these, continues to surprise. macroscopic scale. Increasingly, reduc- formation of mitotic spindles of stiff This is true particularly of hydrodynamic tionist efforts to create simplified model biopolymers in the case of dividing cells. interactions, which are capable of systems based on living cells have led to Even in the case of simplified in vitro completely altering the steady state a better understanding of their inner constructs such as are studied in this issue, behaviour of interacting motile particles workings. At the same time, such efforts however, such pattern formation is often in suspensions,11 in a fashion that is have inspired many researches to explore highly dynamic,5 and of a strongly non- forbidden for non-active particles such as more widely the physics and chemistry of equilibrium origin, although the resulting Brownian colloids (whose steady states ‘‘active states of matter’’, including those static structures can be reminiscent of are given by the Boltzmann distribution, of non-biological origin. liquid crystalline order. In addition to which is blind to those interactions). Published on 02 March 2011. Downloaded 3/29/2019 1:38:50 AM. Such work offers many promising non-equilibrium pattern formation, As described in two reviews in this avenues for creating novel materials with molecular motor activity in polymer issue, hydrodynamics also (i) strongly tunable properties. Many of the relevant solutions and gels can lead to striking constrains biological and material-design materials, just like the cell, are highly changes in their mechanical properties, strategies for creating locomotion in deformable soft solids or viscoelastic such as enhanced fluidization6,7 or its particles too small for their inertia to fluids. Some, such as dilute suspensions of opposite: active stiffening by orders of effectively compete against viscosity motile bacteria, at first might be mistaken magnitude.8 From a materials perspec- (i.e., at the micron scale and below in for simple fluids; but they are far from tive, this suggests novel mechanisms for water), and (ii) leads to unexpectedly simple. For instance, while adding active control of material behaviour, in subtle synchronization effects between conventional (inactive) colloids to a simple solvent can only increase its viscosity, the addition of active particles such as bacteria can reduce it.1 And when such particles interact in suspensions at higher density, remarkably complex behaviour can emerge, ranging from an almost complete loss of viscosity2 to spontaneous chaotic flow sometimes called ‘bacterial turbulence’.3 The growing interest of physicists, chemists and materials scientists (along- side biologists) in such systems has given rise to a growing sub-field of Active Soft Matter, whose breadth, dynamism and Michael Cates Fred MacKintosh variety is reflected in the papers that University of Edinburgh, UK Vrije Universiteit, The Netherlands 3050 | Soft Matter, 2011, 7, 3050–3051 This journal is ª The Royal Society of Chemistry 2011 View Article Online otherwise independent swimming objects. narrowly specified for equilibrium 2 M. E. Cates, S. M. Fielding, Furthermore, as also discussed in this systems such as molecular liquid crystals, D. Marenduzzo and J. M. Yeomans, Shearing active gels close to the isotropic- issue, hydrodynamics influences the but which are much less understood for nematic transition, Phys. Rev. Lett., 2008, rectifying effect of the ratchet-potentials some of the active examples of interest 101, 068102. utilized by molecular motors and other (bacterial suspensions, for instance). 3 L. H. Cisneros, R. Cortez, C. Dombrowski, directional propulsion systems. A third task is as follows. Having R. E. Goldstein and J. O. Kessler, Fluid-dynamics of self-propelled Thus, ideas delineated above—pattern extracted from biology some of the microorganisms, from individuals to formation and viscoelasticity at the thematic ideas of the subject, and having concentrated populations, Exp. Fluids, continuum level, and hydrodynamic developed these through ‘in vitro’ experi- 2007, 43, 737–753. in silico 4 F. J. Nedelec, T. Surrey, A. C. Maggs and interactions among discrete active ments, ‘ ’ simulations, and funda- S. Leibler, Self-organization of components—are recurrent concepts in mental theory, it is important before long microtubules and motors, Nature, 1997, active soft matter. Yet the inter- to carry these new insights back towards 389, 305–308. disciplinarity and breadth of the subject is the biological realm. Indeed, the full bio- 5 V. Schaller, C. Weber, C. Semmrich, E. Frey and A. R. Bausch, Polar patterns also reflected in the fact that other logical implications of the active soft of driven filaments, Nature, 2010, 467, contributions in this issue defy any simple matter paradigm—as developed by 73–77. classification. Rather than list them all physical scientists—are probably exten- 6 T. B. Liverpool, A. C. Maggs and A. Ajdari, here, we urge readers to explore for sive. These implications are now ripe for Viscoelasticity of solutions of motile polymers, Phys. Rev. Lett., 2001, 86, themselves the breadth of science covered. further exploration. 4171–4174. There remain many challenges ahead 7 D. Humphrey, C. Duggan, D. Saha, for this field. One of these is to create D. Smith and J. Kas, Active fluidization of Fred C. MacKintosh polymer networks through molecular much stronger conceptual bridges Michael E. Cates motors, Nature, 2002, 416, 413– between naturally occurring structures 416. and synthetic ones—for instance by 8 D. Mizuno, C. Tardin, C. F. Schmidt and engineering synthetic motors that can be F. C. MacKintosh, Nonequilibrium Acknowledgements mechanics of active cytoskeletal networks, designed, or instructed, to carry out tasks Science, 2007, 315, 370–373. different from those already achieved by MEC holds a Royal Society Research 9 A. W. C. Lau, B. D. Hoffman, naturally occurring examples. These tasks Professorship and is funded by EPSRC/ A. Davies, J. C. Crocker and T. C. Lubensky, Microrheology, stress might include the directed assembly of EO30173. FCM was funded in part by the Foundation for Fundamental Research fluctuations, and active behavior of new materials that nature has not yet living cells, Phys. Rev. Lett., 2003, 91, thought of. A second challenge is to tie on Matter (FOM), which is part of the 198101. more closely to experimental systems Netherlands Organisation for Scientific 10 F. C. MacKintosh and A. J. Levine, Research (NWO). Nonequilibrium mechanics and dynamics (biological or otherwise) the important of motor-activated gels, Phys. Rev. Lett., progress already made in the continuum 2008, 100, 018104. theoretical descriptions of active systems References 11 R. W. Nash, R. Adhikari, J. Tailleur Published on 02 March 2011. Downloaded 3/29/2019 1:38:50 AM. with nematic or polar order. Such andM.E.Cates,Run-and-tumble particles with hydrodynamics: descriptions generally contain various 1 Y. Hatwalne, S. Ramaswamy, M. Rao and R. A. Simha, Rheology of active-particle Sedimentation, trapping and upstream material parameters (such as bending suspensions, Phys. Rev. Lett., 2004, 92, swimming, Phys.Rev.Lett., 2010, 104, constants) whose values are quite 118101. 258101. This journal is ª The Royal Society of Chemistry 2011 Soft Matter, 2011, 7, 3050–3051 | 3051.