Bulletin of the American Physical Society
APS March Meeting 2014
Volume 59, Number 1
Monday–Friday, March 3–7, 2014; Denver, Colorado
Session Z18: Non-spherical Colloids and Complex Fluids |
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Chair: Ye Xu, University of Pennsylvania Room: 403 |
Friday, March 7, 2014 11:15AM - 11:27AM |
Z18.00001: Colloidal samaras Daniela Kraft, Raphael Wittkowski, Oliver Dlugosch, Hartmut L\"owen, David Pine We present a combined experimental and computational study of the sedimentation of colloidal particles composed of materials of different densities. We follow the sedimentation experimentally by confocal microscopy and compare the observed trajectories to simulation results. We find that particular mass distributions and shapes lead to a helical motion during sedimentation, similar to that of samaras. [Preview Abstract] |
Friday, March 7, 2014 11:27AM - 11:39AM |
Z18.00002: Does mass play a role in partition functions even in low Reynolds number systems? Rebecca W. Perry, Nica Franklin, Vinothan N. Manoharan Classical statistical mechanics predicts that heavy components of a reconfigurable object will preferentially occupy positions at the edges of the object while lighter components will most often reside near the object's center of mass. This predicted influence of mass comes in through the rotational component of the partition function, which favors configurations with larger moments of inertia. It is tempting to apply these findings of statistical mechanics directly to colloidal systems, but is this appropriate when colloidal systems are immersed in liquid rather than surrounded by vaccuum? Does mass have a place in the partition function of colloidal clusters at low Reynolds numbers where we are accustomed to ignoring inertia? Here, we measure how silica microspheres distribute themselves when mixed with identically-sized polystyrene microspheres to form weakly-bound clusters of up to ten spheres. Using an array of microwells, we observe thousands of two-dimensional clusters to answer these fundamental questions. [Preview Abstract] |
Friday, March 7, 2014 11:39AM - 11:51AM |
Z18.00003: Dynamics of non-spherical colloidal particles near and at oil-water interfaces Anna Wang, Thomas G. Dimiduk, Jerome Fung, Kundan Chaudhary, Jennifer A. Lewis, Sepideh Razavi, Ilona Kretzschmar, Vinothan N. Manoharan Whereas much is known about how spherical colloidal particles interact with and at oil-water interfaces, not much is known about their non-spherical counterparts. The rotation of non-spherically symmetric particles adds extra degrees of freedom to how such particles interact with each other and the interface, so to study their three-dimensional dynamics we must first be able to image the rotation which has so far only been possible in viscous fluids or for particles with large aspect ratios. Here we track both the three-dimensional translation and the rotation of non-spherical colloidal particles at high speeds using the discrete dipole approximation in conjunction with digital holographic microscopy. We study the dynamics of such particles at an oil-water interface to determine interactions and dynamics prior to or after attachment. We aim to connect these measurements to the formation and stability of Pickering emulsions. [Preview Abstract] |
Friday, March 7, 2014 11:51AM - 12:03PM |
Z18.00004: Interaction of a colloidal sphere near a flat boundary Bhaskar Jyoti Krishnatreya, David G. Grier A colloidal sphere's diffusion is hindered near a surface due to hydrodynamic interactions . We study the hindered diffusion of a colloidal sphere near a glass surface using Digital Holographic Microscopy (DHM). Analysis of in-line holographic images of a diffusing colloidal sphere provides its three dimensional positions with nanometer resolution. We propose a general technique to determine the forces acting on the colloidal sphere near a flat boundary using Kernel Density Estimates (KDE), as a function of distance from the boundary. The results will help in understanding interactions between micron-sized colloidal particles near a boundary. [Preview Abstract] |
Friday, March 7, 2014 12:03PM - 12:15PM |
Z18.00005: Brownian Motion of Boomerang Colloidal Particles Qi-Huo Wei, Andrew KOnya, Feng Wang, Jonathan V. Selinger, Kai Sun, Ayan Chakrabarty We present experimental and theoretical studies on the Brownian motion of boomerang colloidal particles confined between two glass plates. Our experimental observations show that the mean displacements are biased towards the center of hydrodynamic stress (CoH), and that the mean-square displacements exhibit a crossover from short-time faster to long-time slower diffusion with the short-time diffusion coefficients dependent on the points used for tracking. A model based on Langevin theory elucidates that these behaviors are ascribed to the superposition of two diffusive modes: the ellipsoidal motion of the CoH and the rotational motion of the tracking point with respect to the CoH. [Preview Abstract] |
Friday, March 7, 2014 12:15PM - 12:27PM |
Z18.00006: Brownian Motion of Asymmetric Boomerang Colloidal Particles Ayan Chakrabarty, Andrew Konya, Feng Wang, Jonathan Selinger, Kai Sun, Qi-Huo Wei We used video microscopy and single particle tracking to study the diffusion and local behaviors of asymmetric boomerang particles in a quasi-two dimensional geometry. The motion is biased towards the center of hydrodynamic stress (CoH) and the mean square displacements of the particles are linear at short and long times with different diffusion coefficients and in the crossover regime it is sub-diffusive. Our model based on Langevin theory shows that these behaviors arise from the non-coincidence of the CoH with the center of the body. Since asymmetric boomerangs represent a class of rigid bodies of more generals shape, therefore our findings are generic and true for any non-skewed particle in two dimensions. Both experimental and theoretical results will be discussed. [Preview Abstract] |
Friday, March 7, 2014 12:27PM - 12:39PM |
Z18.00007: A theory for depletion-induced colloidal membranes Louis Kang, Tom C. Lubensky Depletion-induced formation of colloidal membranes has been recently observed in suspensions of hard rods [E. Barry and Z. Dogic, Proc. Natl. Acad. Sci. U.S.A. {\bf 107}, 10348 (2010); T. Gibaud {\it et al.}, Nature {\bf 481}, 348 (2012)]. These membranes exhibit a variety of rich behaviors that must ultimately be driven by entropy alone. We propose an entropic model that can capture certain features of these membranes, including their curved edge shape and the presence of twist even with achiral rods. We calculate phenomenological parameters, such as the Frank twist constant and the Helfrich bending modulus, from physical quantities. Finally, we describe novel behaviors predicted by our model. [Preview Abstract] |
Friday, March 7, 2014 12:39PM - 12:51PM |
Z18.00008: Improving the accuracy of DLVO theory for dense systems of macroions Niels Boon, Guillermo Ivan Guerrero, Rene van Roij, Monica Olvera de la Cruz The widely used DLVO pair potential was originally derived for a pair of interacting macroions in a dilute colloidal suspension. Here, we present a modified effective pair potential that is also accurate for non-dilute systems. Our new theory significantly deviates from the classical DLVO theory in dense systems. In essence, we propose a modification of Alexander's prescription for the charge renormalization procedure that is used to derive effective charges from highly charged macroions. By comparing pair correlation functions and pressures from computationally expensive primitive-model simulations, our new method demonstrates an improved accuracy w.r.t. Alexander's DLVO-based approach. We show that our method is also suitable to describe salt-free suspensions. [Preview Abstract] |
Friday, March 7, 2014 12:51PM - 1:03PM |
Z18.00009: A model for restricted diffusion in complex fluids John de Bruyn, Jonathan Wylie We use a model originally due to Tanner [1] to study the diffusion of tracer particles in complex fluids both analytically and through Monte-Carlo simulations. The model consists of regions through which the particles diffuse freely, separated by membranes with a specified low permeability. The mean squared displacement of the particles calculated from the model agrees well with experimental data on the diffusion of particles in a concentrated colloidal suspension [2] when the membrane permeability is used as an adjustable parameter. Data on a micro-phase-separated polymer system [3] can be well modeled by considering two populations of particles constrained by membranes with different permeabilites. \\[4pt] [1] J. E. Tanner, J. Chem. Phys. 69, 1748 (1978).\\[0pt] [2] E. R. Weeks and D. A. Weitz, Chem. Phys. 284, 361 (2002).\\[0pt] [3] N. Yang, J. L. Hutter, and J. R. de Bruyn, J. Rheol. 56, 797 (2012). [Preview Abstract] |
Friday, March 7, 2014 1:03PM - 1:15PM |
Z18.00010: Theory of the dynamics of evaporation-driven colloidal patterning C. Nadir Kaplan, Ning Wu, Shreyas Mandre, Joanna Aizenberg, L. Mahadevan In the suspensions of colloidal particles in a volatile liquid film, deposits of the solute form near the contact line due to the flow generated by evaporation. An enticingly simple and experimentally realizable model system of this mechanism is the drying of a spilled drop of coffee on the countertop. Similarly, patterns of periodic bands or continuous solid films are commonly observed on a substrate suspended vertically in a container of the colloidal solution. In order to characterize these patterns, we develop a multiphase model that couples both the liquid and solid flows, local variation of the particle concentration, the propagation dynamics of the solid front, and the liquid-air interface deformation. For vertical liquid films, we further determine the nature of the filming-banding transition and the phase boundary in terms of the volume fraction of the colloids. The results of our theory are in good agreement with direct observations of these patterns. [Preview Abstract] |
Friday, March 7, 2014 1:15PM - 1:27PM |
Z18.00011: Towards a computational modeling of structure formation in colloidal drying Alexander Wagner We present lattice Boltzmann models at different scales for the simulation of colloidal drying in the presence of polymers and structure formation in resulting phase-separation fronts. When a drop of colloid polymer mixture is exposed to an environment in which the solvent in which these particles are suspended evaporates an accumulation of non-volotile material at the rim of the drop is observed (coffee ring effect). When the solvent concentration is reduced bejond a certain threshold, the colloid polymer mixture undergoes phase separation. The structures formed by this phase-separation is observed to depend on the processing conditions. In this presentation we will briefly present the experimental observations and our numerical approach to address the observed phenomena. [Preview Abstract] |
Friday, March 7, 2014 1:27PM - 1:39PM |
Z18.00012: Rediscovering Red: Full-Spectrum Structural Color in Colloidal Glasses Sofia Magkiriadou, Jin-Gyu Park, Young-Seok Kim, Gi-Ra Yi, Vinothan N. Manoharan We use colloidal glasses to develop pigments with structural color: color that arises from interference rather than absorption. This pigmentation mechanism is common in blue birds, whose feather barbs often contain glassy microstructures. When a glass is illuminated, the spatial correlations between neighboring particles can give rise to constructive interference for a small range of wavelengths. Unlike the colors arising from Bragg diffraction in crystals, the colors of these ``photonic glasses'' are independent of angle due to the disordered, isotropic structure. However, there are no known examples of photonic glasses with pure structural red color, either in nature or in the lab. We present both experimental evidence and a model showing that the absence of red is due to the wavelength-dependence of the single-particle scattering cross-section. We show that this problem can be solved in ``inverse glasses,'' namely glasses composed of particles with refractive index lower than that of their medium. Although these systems are similar to those in birds, no known species uses this mechanism to create red. We use inverse glasses to make full-spectrum, angle-independent structural colors. This will enable the use of colloidal glasses as a new type of long-lasting, non-bleaching pigment. [Preview Abstract] |
Friday, March 7, 2014 1:39PM - 1:51PM |
Z18.00013: Gravitational Drainage of Superspreader Films Stabilized by Disjoining Pressure Soumyadip Sett, Rakesh Sahu, Suman Sinha-Ray, Alexander Yarin Gravitational drainage from plane vertical films of two superspreaders SILWET L-77 and BREAK-THRU S278 and their respective ``cousin'' non-superspreaders SILWET L-7607 and BREAK-THRU S233 is studied experimentally and theoretically. The non-superspreader films showed ordered interferometric color bands, similar to those of ordinary surfactants and the film thickness decreased linearly in time. Their counterpart superspreaders showed complicated dynamic turbulent-like interferometric patterns and had an order of magnitude longer life time before bursting compared to that of the ``cousin'' non-superspreaders. The stabilization of the superspreader films and the nonlinear decrease of the film thickness with time are attributed to significant disjoining pressure associated with the van der Waals repulsion of the fluffy surfaces of the film formed by long superspreader bilayers hanging from the free surfaces. The non-superspreaders do not possess any significant disjoining pressure even in the film with thicknesses in the 30-50 nm range. The results show that gravitational drainage of vertical films is a useful simple tool for measuring disjoining pressure. [Preview Abstract] |
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