APS March Meeting 2012 Boston, Massachusetts http://www.aps.org/meetings/march/index.cfm i Monday, February 27, 2012 8:00AM - 11:00AM — Session A51 DCMP DFD: Colloids I: Beyond Hard Spheres Boston Convention Center 154 8:00AM A51.00001 Photonic Droplets Containing Transparent Aqueous Colloidal Suspensions with Optimal Scattering Properties JIN-GYU PARK, SOFIA MAGKIRIADOU, Department of Physics, Harvard University, YOUNG- SEOK KIM, Korea Electronics Technology Institute, VINOTHAN MANOHARAN, Department of Physics, Harvard University, HARVARD UNIVERSITY TEAM, KOREA ELECTRONICS TECHNOLOGY INSTITUTE COLLABORATION — In recent years, there has been a growing interest in quasi-ordered structures that generate non-iridescent colors. Such structures have only short-range order and are isotropic, making colors invariant with viewing angle under natural lighting conditions. Our recent simulation suggests that colloidal particles with independently controlled diameter and scattering cross section can realize the structural colors with angular independence. In this presentation, we are exploiting depletion-induced assembly of colloidal particles to create isotropic structures in a milimeter-scale droplet. As a model colloidal particle, we have designed and synthesized core-shell particles with a large, low refractive index shell and a small, high refractive index core. The remarkable feature of these particles is that the total cross section for the entire core-shell particle is nearly the same as that of the core particle alone. By varying the characteristic length scales of the sub-units of such ‘photonic’ droplet we aim to tune wavelength selectivity and enhance color contrast and viewing angle. 8:12AM A51.00002 Curvature-Induced Capillary Interaction between Spherical Particles at a Liquid Interface1 , NESRIN SENBIL, CHUAN ZENG, BENNY DAVIDOVITCH, ANTHONY D. DINSMORE, University of Massachusetts Amherst — Capillary interactions among particles adsorbed at a fluid interface are important in a variety of natural and technological systems but still pose many mysteries. Capillary interactions induced by buoyancy, referred to as the”Cheerios ”effect, have been studied for years. Here, we experimentally investigate how anisotropic interfacial shape affects capillary forces among millimeter-sized spheres. The Cheerios model predicts that particles with densities that are higher and lower compared to the fluids adsorbed at an initially flat interface will repel. Our experiments, however, clearly show that they can attract one another at the short range. We explain our results with a model, in which each sphere creates an anisotropic curvature at the position of the other sphere. To satisfy the constant contact-angle boundary condition, the interface is deformed with quadrupolar symmetry around each sphere. This quadrupolar deformation creates a short-ranged, attractive capillary force. The range of size and density ratios at which we observe a dominant short-range attraction is consistent with the model. Our results show how interfacial shape may be used to direct the assembly of interfacial particles. 1Funded by the NSF-supported MRSEC on Polymer at UMASS(DMR-0820506)and NSF CBET-0967620. 8:24AM A51.00003 Using Micron-Sized Ellipsoids as a New Tool for Microrheology , DAVID C. KILGORE, KENNETH W. DESMOND, ERIC R. WEEKS, Emory University — Microrheology is a well-established technique, and in its simplest form it allows you to measure the viscosity of a fluid by examining the diffusion of microspheres, provided the diameter of the microspheres is known. We are developing a similar technique using ellipsoids, where the viscosity can be calculated without prior knowledge of the length and width of the ellipsoid. The asymmetry of ellipsoids provides a distinct advantage, allowing for the diffusion to be decomposed into two translational motions and one rotational motion. For each of these diffusive motions, we can measure a diffusion constant and relate the constant to the three unknowns: the length and width of the ellipsoid, and the viscosity. By measuring the three diffusion constants, we can determine the three unknowns. To verify this technique, we produce ellipsoids in the lab and suspend them in a viscous solution for three-dimensional imaging of the diffusion with a confocal microscope. We are able to get good agreement between the microrheological measurements and macroscopic viscosity measurements. 8:36AM A51.00004 Non-capillary binding of colloidal particles to liquid interfaces , DAVID KAZ, UC Berkeley, RYAN MCGORTY, UC San Fransisco, VINOTHAN MANOHARAN, Harvard University — We observe colloidal polystyrene particles binding reversibly to an oil-water interface through the combination of a repulsive electrostatic force and an attractive van der Waals force. Previously studied interactions of an aqueous colloidal particle and a liquid interface have generally fallen into two categories: 1) electrostatic repulsion indicated by the dependence on salt and 2) capillary adsorption where surface tension brings the particle in contact with both phases and is indicated by practically irreversible binding. With our technique of pushing individual colloidal particles towards a planar oil-water interface and observing their motion in three-dimensions with holographic microscopy we have observed both interactions. However, our observations indicate that under certain conditions the electrostatic repulsion, which is due to repulsive image charges, is weak enough for a particle to experience a van der Waals attraction while strong enough to prevent a particle from penetrating the interface and becoming bound through capillary action. We observe individual particles transition between repulsive and attractive interactions with the interface suggesting that these colloidal particles have a heterogeneous surface charge. 8:48AM A51.00005 Memory effects in soap film arrangements , NICOLAS VANDEWALLE, STEPHANE DORBOLO, GEOFFROY LUMAY, JULIEN SCHOCKMEL, MARTIAL NOIRHOMME, GRASP, Institute of Physics B5a, University of Liege, B4000 Liege — We report experiments on soap film configurations in a triangular prism for which the shape factor can be changed continuously. Two stable configurations can be observed for a range of the shape factor h. A hysteretic behaviour is found, due to the occurence of another local minima in the free energy. Experiments demonstrate that soap films can be trapped in a particular configuration being different from a global surface minimization. This metastability can be evidenced from a geometrical model based on idealized structures. Depending on the configuration, providing clues on the structural relaxations taking place into 3D foams, such as T1 rearrangements. The composition of the liquid is also investigated leading to dynamical picture of the transition. (Phys. Rev. E 83, 021403 (2011)) 9:00AM A51.00006 Droplet-based microfluidics and the dynamics of emulsions , JEAN-CHRISTOPHE BARET, QUENTIN BROSSEAU, BENOIT SEMIN, XIAOPENG QU, Max-Planck Institute for Dynamics and Self-Organization, DROPLETS, MEMBRANES AND INTERFACES TEAM — Emulsions are complex fluids already involved for a long time in a wide-range of industrial processes, such as, for example, food, cosmetics or materials synthesis [1]. More recently, applications of emulsions have been extended to new fields like biotechnology or biochemistry where the compartmentalization of compounds in emulsion droplets is used to parallelise (bio-) chemical reactions [2]. Interestingly, these applications pinpoint to fundamental questions dealing with surfactant dynamics, dynamic surface tension, hydrodynamic interactions and electrohydrodynamics. Droplet-based microfluidics is a very powerful tool to quantitatively study the dynamics of emulsions at the single droplet level or even at the single interface level: well- controlled emulsions are produced and manipulated using hydrodynamics, electrical forces, optical actuation and combination of these effects. We will describe here how droplet-based microfluidics is used to extract quantitative informations on the physical-chemistry of emulsions for a better understanding and control of the dynamics of these systems [3]. [1] J. Bibette et al. Rep. Prog. Phys., 62, 969-1033 (1999) [2] A. Theberge et al., Angewandte Chemie Int. Ed. 49, 5846 (2010) [3] J.-C. Baret et al., Langmuir, 25, 6088 (2009) 9:12AM A51.00007 Pseudo-Steady Liquid Transport in Aqueous Foams during Filling of a Container , MICHAEL CONROY1, RAMAGOPAL ANANTH2, Naval Research Laboratory — Various applications of aqueous foams involve filling a container or a column (e.g., fractionation), where the foam is formed and processed. However, existing studies in the literature do not treat the filling stage and only describe liquid transport within a static foam bed. We developed a theory that predicts liquid loss from the foam and the liquid distribution within its interior during the filling and post-filling stages. During the filling stage, the theory predicts that the foam reaches a pseudo-steady state characterized by a time-independent drainage rate and liquid fraction. The pseudo-steady-state liquid fraction appears above a thin, liquid-saturated boundary layer that exists at the bottom of the foam bed. During the post-filling stage, the theory predicts that the drainage rate decreases with time, similar to static foams beds studied by others. The theory compares well with our previously reported volume-averaged macroscopic model and drainage measurements for dry (high-expansion) foams. We will