Bulletin of the American Physical Society
2009 APS March Meeting
Volume 54, Number 1
Monday–Friday, March 16–20, 2009; Pittsburgh, Pennsylvania
Session B14: Focus Session: Colloids I: Physical Behavior and Mechanisms |
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Sponsoring Units: DFD Chair: Eric Weeks, Emory University Room: 315 |
Monday, March 16, 2009 11:15AM - 11:51AM |
B14.00001: Restricted dislocation mobility in crystals of peanut-shaped colloidal particles Invited Speaker: Recent advances in colloidal particle synthesis techniques have enabled the production of a variety of anisotropic yet monodisperse particles, including colloidal ``peanuts,'' which consist of two connected spherical lobes. Since their shape crudely approximates a dimer, colloidal peanut particles constitute a simple but fundamental extension of the classic system of colloidal spheres. Experimental investigations as well as simulations of colloidal peanut monolayers have shown that at high area fractions the particles form a degenerate crystal (DC). In this structure, the peanut particle lobes order into a triangular lattice, much like close-packed spheres, while the connections between lobe pairs are randomly oriented, uniformly populating the three crystalline directions of the underlying lattice. Comparative studies of crystal formation in rapidly compressed monolayers of peanut-shaped versus spherical particles show that DCs harbor many more defects than equivalent crystals of spheres. This suggests that defect annealing may be frustrated by the constraining rigid connections between particle lobes. To elucidate the interactions between these geometric constraints and defect mobility, we directly examine the mechanisms for dislocation nucleation and propagation in DCs. In particular, we show that obstacles formed by certain particle orientations severely limit the range over which dislocations can glide. Furthermore, we observe that transport over longer distances can proceed through dislocation reactions, which switch the direction of propagation and allow dislocations to bypass such obstacles. In this talk I will discuss the impact that these restricted mechanisms have on the macroscopic properties of DCs. [Preview Abstract] |
Monday, March 16, 2009 11:51AM - 12:03PM |
B14.00002: Experimental Study of Brownian Dynamics of Bent-core Colloidal Particles Chun-zhen Fan, Bhuwan Joshi, Ji-ping Huang, Qi-huo Wei Bent-core or banana-shaped molecules exhibit a variety of intriguing liquid crystalline mesophases including nematics and smectic phases. We try to develop suspensions of bent-core shaped colloidal particles to mimic the bent-core liquid crystals. This report will focus on the fabrication of bent-core colloidal particle suspension, and optical microscopic studies of the Brownian dynamics of individual bent-core colloidal particles. The bent-core colloidal particles confined between two glass substrates are observed through dark-field optical microscopy, and their orientation and position are obtained through imaging processing. Results on the translational and rotational Brownian dynamics of these type of particles will be reported. [Preview Abstract] |
Monday, March 16, 2009 12:03PM - 12:15PM |
B14.00003: Pinch-off Dynamics of Non-Newtonian Fluids F. M. Huisman, P. Taborek The pinch-off dynamics of a variety of shear-thinning fluids (foams, concentrated emulsions, and slurries) were studied using high speed videography. The pinch was characterized by the variation of the minimum neck radius rmin as a function of the time to pinch $\tau$. For inviscid fluids, rmin scales as $\tau$ to the 2/3 power. We found that for all the shear-thinning fluids rmin scales with $\tau$ to a power in the range 0.2 to 0.5. To study the transition from conventional inviscid pinch, we systematically varied the concentration of a water-bentonite mixture. As the concentration increased the pinch event transitioned from a needle shape resulting in a satellite drop to a symmetric hyperbolic shape with no satellite drop. These results will be compared with the simulations of Suryo and Basaran (J. Non-Newtonian Fluid Mech. 138 (2006) 134-160). [Preview Abstract] |
Monday, March 16, 2009 12:15PM - 12:27PM |
B14.00004: Melting Dynamics of 3D Hard Sphere Colloidal Crystals Deniz Kaya, N. L. Green, C. E. Maloney, M. Widom, M. F. Islam We use thermally responsive monodisperse micron sized colloidal particles with hard-sphere interactions to study the melting mechanisms in colloidal crystals. As we increase the temperature, these spherical microgel particles decrease in volume, inducing melting in the colloidal crystals. We use video microscopy and image analysis to determine the dispersion relations and the local elasticity near the melting transition. We compare our findings with existing melting and freezing theories. This work has been partially supported by the NSF through Grants DMR-0619424 and DMR-0645596, and by ACS-PRF. [Preview Abstract] |
Monday, March 16, 2009 12:27PM - 12:39PM |
B14.00005: Multiple-Stage Melting and Freezing of Colloidal Crystallites with Short-range Attraction Liquan Pei, J.R. Savage, A.D. Dinsmore We study the dynamics of melting and freezing in a model colloidal system with short-range, temperature tunable attraction. In particular, we mix micron-sized, charge stabilized polystyrene spheres with salt and the surfactant pluronic P103. The pluronic micelles induce depletion attraction whose range is approximately 1.5$\%$ of the sphere diameter and whose magnitude changes strongly with temperature. We use optical microscopy to record the dynamics of freezing and melting following temperature changes. We use particle tracking algorithms to identify the particles with sub-pixel resolution and measure metastable cluster sizes, order parameters, and bond lengths. We have observed that melting and freezing occur in multiple stages, with a metastable liquid phase appearing in both processes. Our results are relevant to protein crystallization where the interactions are also of short range, and to other systems where non-equilibrium states may play a role in phase separation. We thank the NSF for support through grant DMR-0605839. [Preview Abstract] |
Monday, March 16, 2009 12:39PM - 12:51PM |
B14.00006: Melting and Freezing of Colloidal Crystals on Strained Substrates John Savage, Rajesh Ganapathy, Itai Cohen We present results of experiments studying the effect of strain on the dynamics of melting and freezing in single-layer colloidal crystals with a short-range attractive interaction. Our system consists of micron sized colloidal particles and a tunable depletant allowing reversible control of the interaction \textit{via} small temperature changes. We explore the role of strain in the dynamics of melting and freezing by investigating crystallization on a flat patterned substrate. We find that in comparison with previously performed experiments on flat unpatterned substrates, the dynamics of melting and freezing on such surfaces alter dramatically. For example whereas melting of such crystals on a flat substrate was shown to proceed through an intermediary metastable liquid phase, we find that for surfaces templated with a lattice that is commensurate with that of the melting crystal, this intermediary step is suppressed. [Preview Abstract] |
Monday, March 16, 2009 12:51PM - 1:03PM |
B14.00007: Transport and Sedimentation of Suspended Particles in Fracture Channels Tak Shing Lo, Joel Koplik Particulate suspensions are ubiquitous in nature and in many artificial situations, and their transport and deposition dynamics are of importance in many chemical, petroleum and environmental processes. While most of the studies in particle transport in confined geometry were done with smooth surfaces in the past, realistic geological fractures usually have irregular rough surfaces that have self-affine structures. We consider the combined effects of sedimentation and inertial transport of particles suspended in a Newtonian fluid in a pressure-driven flow in channels with self-affine surfaces, which is especially relevant to clogging phenomena where sediments may block continuous fluid flows in channels that may occur in geological or industrial processes. We perform a systematic study using the lattice Boltzmann method, which is flexible and particularly suitable for handling irregular geometry. Our results cover a board range in Reynolds and buoyancy numbers, and in particle concentrations. [Preview Abstract] |
Monday, March 16, 2009 1:03PM - 1:15PM |
B14.00008: Particle Organization by Absorbing State Dynamics Laurent Cort\'{e}, David Pine, P.M. Chaikin In a recent study we have found that irreversible collisions can lead to a dynamical phase transition between a constantly evolving state and an absorbing, quiescent state where particles self organize to avoid further collisions. Here we investigate the organization and order in the absorbing state in a model where active, overlapping particles are given random displacements. We contrast the order to what is obtained thermodynamically for hard spheres. We also show that correlated displacements between colliding particles can lead to crystallization and suggest that irreversible flows are a different yet effective tool for ordering particles in desired motifs. [Preview Abstract] |
Monday, March 16, 2009 1:15PM - 1:27PM |
B14.00009: Diffusion through Colloidal Shells under Stress J. Guery, J. Baudry, D.A. Weitz, P.M. Chaikin, J. Bibette The permeability of solids has long been associated with a diffusive process involving activated hopping. Tensile stress can affect the activation energy as originally envisioned by Eyring. Here we use liquid core - solid shell, core-shell, solid colloidal particles that are sensitive to osmotic pressure, to follow the permeation of encapsulated probes at various stresses. We unambiguously show that the tensile stress applied on colloidal shells linearly reduces the local energy barrier for diffusion. [Preview Abstract] |
Monday, March 16, 2009 1:27PM - 1:39PM |
B14.00010: Dispersion relation and density of states of coupled plasmon modes in periodic chains of metallic nanoparticles C.W. Ling, M.J. Zheng, K.W. Yu Energy transmission through one-dimensional chains of equally spaced metallic nanoparticles has been studied via the propagation of coupled surface-plasmon modes. These modes are characterized by well-defined dispersion relation $\omega(k)$ and group velocity $v_g=d\omega/dk$ in a band. The nanoparticles are routinely modelled by Drude metallic spheres and the coupled plasmon modes are calculated in the point-dipole approximation. When the particles approach and finally touch, these bands can differ significantly from those obtained by the point-dipole approximation due to strong multipolar interaction among the particles. In this regard, we have calculated the coupled plasmon modes by a tight-binding approach, taking fully multipolar interactions into account. For approaching particles, the dipolar bands move from the visible down to the infrared region and $\omega(k)$ becomes almost independent of $k$. Concomitantly, the group velocity $v_g$ showed an intriguing non-monotonic behavior versus the particle spacing. When the spacing decreases, $v_g$ increases initially but decreases when the particles approach and touch. For moderate spacing, $v_g$ can be reduced drastically to $0.01 c$, except at $kd=0$ and $kd=\pi$, resulting in a slow propagation. Thus one can tune the propagation of plasmon modes by simply varying the spacing between the particles. [Preview Abstract] |
Monday, March 16, 2009 1:39PM - 1:51PM |
B14.00011: Examining dynamic length scales in a two-dimensional colloidal system Zach Nadler, Cara Hageman, Vikram Prasad, Eric R. Weeks We study polystyrene colloids placed at an oil-water interface as a quasi-two-dimensional colloidal system. As the area fraction of the colloidal particles is increased, we see liquid, hexatic, and crystalline phases. The liquid phase is structurally disordered; the hexatic phase has long range orientational order but poor translational order; and the crystalline phase has long range orientational and translational order. We classify these different phases using structural and dynamic parameters from prior work. Using a laser tweezer we trap and drag a particle along the interface and observe its effect on the surrounding colloids. Our interest is in how the response changes near phase transition boundaries, where the ordering of particles can qualitatively change. We characterize the response by the structural defects induced by the dragged particle, as well as the perturbed motion of the surrounding particles. These responses are localized around the dragged particle, and we study how the localization length scale changes with the area fraction of the colloids. [Preview Abstract] |
Monday, March 16, 2009 1:51PM - 2:03PM |
B14.00012: State Diagram for Optical Tweezers Induced Brownian Motors Bo Sun, David Grier State Diagram for Optical Tweezers Induced Brownian Motors Bo Sun and David G. Grier Center for Soft Matter Research Department of Physics New York University Optical tweezers are extensively used in physics and biology, most study in literatures assume a colloidal particle trapped in optical tweezers relaxes to equilibrium state. To the contrary, we have found experimentally the particle became a Brownian motor. Further more, this Brownian motor showed reversible behavior: given input power, working direction changes when particle size grows; given a particle bigger than wave length of light, changing input power can also change the working direction. Thus we need a state diagram to describe the motor behavior of a colloidal particle in optical tweezers, rather than a potential landscape as most previous study uses. [Preview Abstract] |
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