Bulletin of the American Physical Society
2005 APS March Meeting
Monday–Friday, March 21–25, 2005; Los Angeles, CA
Session S37: Colloids III |
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Sponsoring Units: DFD Chair: Noel Clark, University of Colorado Room: LACC 512 |
Wednesday, March 23, 2005 2:30PM - 2:42PM |
S37.00001: Melting of 2D Colloidal Crystals J.R. Savage, A.D. Dinsmore We study the kinetics of melting of colloidal crystals formed by a short-range attractive potential. We use aqueous suspensions of micron-sized latex spheres mixed with surfactant (SDS) micelles, which create a depletion attraction among the spheres. Single-layer crystals appear on the glass surface. Upon uniformly heating the sample to 60 deg., the micelles shrink. The depletion attraction weakens by a factor of approximately 2.25, and the crystals melt. Optical microscopy is used to track the motions of hundreds of colloidal spheres for up to 2 hours, until crystals have melted. We initially observe a steady decrease in the size of the crystallites, limited by diffusion. When the size reaches approximately 15, however, crystallites rapidly shrink. The kinetics of individual bond-breaking events and the evolution of the crystalline order parameter will be presented. This work is supported by the Research Corporation and by NSF-DMR 0305395. [Preview Abstract] |
Wednesday, March 23, 2005 2:42PM - 2:54PM |
S37.00002: Melting Mechanisms of 3D Colloidal Crystals A.M. Alsayed, M.F. Islam, J. Zhang, A.G. Yodh We study the melting mechanisms of 3D colloidal crystals using aqueous suspensions of thermally responsive NIPA microgel colloidal particles. Below 32 $^{o}$C, the particle radius decreases approximately linearly with increasing temperature. We use this effect to tune the volume fraction of nearly hard-sphere aqueous NIPA colloidal suspensions from 0.74 to 0.54. Using video tracking microscopy, we measured the Lindemann parameter of particles within the crystal as a function of temperature. Interestingly, we find that melting of the 3D colloidal crystals starts at grain boundaries and free surfaces, rather than isolated vacancies or dislocations. Very near the melting temperature, the Lindemann parameter for particles near the grain boundaries and free surfaces was $\sim $0.16; the parameter decreased approximately exponentially with distance into the bulk crystal. These works has been partially supported by NSF through MRSEC DMR-0203378 and DMR-079909 and by NASA grant NAG8- 2172. [Preview Abstract] |
Wednesday, March 23, 2005 2:54PM - 3:06PM |
S37.00003: Electrokinetic measurements of a model colloidal system in low polar solvents Andrew Hollingsworth, William Russel, Paul Chaikin, Mirjam Leunissen, Alfons van Blaaderen In a low polar environment, sterically stabilized poly(methyl methacrylate) spheres become positively charged and exhibit significant long-range repulsive interactions. Particles were fluorescently labeled for confocal microscopy and suspended in near index and density matching solvents. Calculations show that small differences in the dielectric constant, which ranged from 6 to 8 in our experiments, can dramatically affect electrolyte dissociation. We have computed ionic strengths from conductivity measurements using Fuoss' theory of ionic association, and, in turn, interpreted electrophoretic mobility and dielectric response using the standard electrokinetic model. We hypothesize that the dehydrohalogenation of cyclohexyl bromide (CHB) produces HBr, which weakly dissociates in these organic solvents. To establish the acidic nature of the media, we dissolved CHB in water and measured the conductivity and pH as a function of CHB concentration and time. [Preview Abstract] |
Wednesday, March 23, 2005 3:06PM - 3:18PM |
S37.00004: Alloys of Suspended Colloidal Donuts and Spheres T.G. Mason, C. Hernandez, C.K. Harrison, P.M. Chaikin We present a study of the structure of mixed dispersions, or ``alloys'', of colloidal donuts and spheres. By contrast to previous studies of colloidal alloys, this study allows for the investigation of how the possible penetration of one species through a hole in another can alter the structures. Three different regimes have been investigated: when the diameter of the donut's hole is much larger than, equal to, and significantly smaller than the sphere's diameter. In the case when the spheres are slightly smaller than the holes, interpenetrating particle configurations are possible, and the spheres can explore a larger positional phase space. In this case, we report the variation of these structures with respect to the two independent volume fractions of spheres and of donuts. [Preview Abstract] |
Wednesday, March 23, 2005 3:18PM - 3:30PM |
S37.00005: A Microrheological Study of the Time Dependent Gelation of Single Wall Carbon Nanotube Suspensions. L. A. Hough, M. F. Islam, A. G. Yodh Single wall carbon nanotubes (SWNTs) dispersed in water using an anionic surfactant, sodium dodecylbenzene sulfonate (NaDDBS) form reversible gels because of the bonding between the individual nanotubes (L.A. Hough, M.F. Islam, P.A. Janmey and A. G. Yodh Phys. Rev. Lett. \textbf{93}, 168102 (2004)). In this talk, we present a microrheology study of the time dependence of this reversible gelation. We embed fluorescent tracer particles in SWNT suspensions and use optical microscopy tracking techniques to measure the mean-squared displacement during gelation. We then apply a time-cure superposition to obtain a master curve for the viscoelasticity that extends over several decades in frequency. We compare high frequency dynamics of the SWNTs solutions to those expected for semiflexible and rigid rod polymer systems. This work has been partially supported by the NSF through Grants DMR 00-79909 (MRSEC) and DMR-0203378, and by NASA Grant NAG8-2172. [Preview Abstract] |
Wednesday, March 23, 2005 3:30PM - 3:42PM |
S37.00006: Creaming of Emulsion aggregations and gels Chanjoong Kim, David Weitz Even though creaming of emulsion has been considered as a simple phenomenon due to the hydrodynamics interaction and the density mismatch between dispersed phase and continuous phase, actual creaming behavior is not so simple once emulsion droplets interact with each other. Interaction between droplets was controlled by depletion force, which depends on the concentration of smaller micelles. Creaming behaviors can be categorized to three different groups; 1) The boundary moves up fast at a constant speed with low surfactant concentrations, 2) it moves slowly in the beginning but suddenly collapses up at intermediated concentrations, and 3) it moves up slowly without collapsing at high concentrations. These behaviors are interpreted in terms of poroelatic model. [Preview Abstract] |
Wednesday, March 23, 2005 3:42PM - 3:54PM |
S37.00007: Internal dynamics of a model concentrated emulsion Vinothan Manoharan, John C. Crocker Foams and concentrated emulsions---glassy systems in which the volume fraction $\phi$ of the bubbles or droplets exceeds the close-packed threshold---display unique relaxation behavior due to intermittent and inhomogeneous structural rearrangements. The rearrangements are caused by diffusion of fluid from small to large droplets, which leads to variations in the local stress. Unfortunately in most of these systems the large refractive index mismatch prevents direct microscopic observation of the internal dynamics of the rearrangements. Furthermore, the concomitantly large density mismatch leads to changes in $\phi$ over time, thus limiting the timescale of dynamical measurements. We report the results of real-space, microscopic experiments on model concentrated emulsions ($\phi > 0.7$) in which the continuous and dispersed phases are both index- and density-matched. We characterize the internal dynamics by measuring the spatial and temporal correlations between the motion of embedded tracer particles. [Preview Abstract] |
Wednesday, March 23, 2005 3:54PM - 4:06PM |
S37.00008: Phonon spectrum measured in a 1D Yukawa chain J. Goree, B. Liu An experiment is reported for a 1D chain of charged microspheres with a Yukawa pair potential. This work is motivated by chains of colloids confined in laser beams and Xe atoms confined in carbon nanotubes. Polymer microspheres are dispersed in a weakly- ionized rarefied gas. The resulting suspension, termed a dusty plasma, is a colloidal crystal. Depending on the confinement geometry, these suspensions can be 1D, 2D, or 3D. Dusty plasma suspensions resemble charge-stabilized colloidal suspensions: they both allow precise tracking of particles with digital video microscopy and manipulation of particles with lasers. The suspension in a dusty plasma is distinguished, however, by its extreme parameters: it is vastly softer, more weakly damped, and more dilute than colloids in water. In this talk, we describe oscillations of a non-bifurcated 1D chain. Using particle tracking to compute velocities of individual microspheres, we find the phonon spectrum. This is done both with and without time-modulated laser manipulation to excite phonons at a specific frequency. The measured spectrum is compared to a theoretical dispersion relation. Work supported by NASA and DOE. [Preview Abstract] |
Wednesday, March 23, 2005 4:06PM - 4:18PM |
S37.00009: Shear viscosity measurements in a 2D Yukawa liquid Volodymyr Nosenko, John Goree Shear viscosity was measured for a 2D strongly-coupled Yukawa liquid. First, we formed a dilute monolayer suspension of microspheres in a partially-ionized rarefied gas, i.e., a dusty plasma. In the absence of manipulation, the suspension forms a 2D triangular lattice. We used a new in-situ method of applying a shear stress using the scattering forces applied by counter-propagating laser beams. The lattice melted and a shear flow formed. Using digital video microscopy for direct imaging and particle tracking, the microscopic dynamics of the shear flow are observed. Averaging the velocities of individual microspheres, a velocity flow profile was calculated. Using the Navier-Stokes equation with an additional frictional term to account for gas drag, we fit the velocity profile. The fit yielded the value of the shear viscosity. The kinematic viscosity of our particle suspension is of order 1 mm$^{2}$s$^{-1}$, which is comparable to that for liquid water. We believe this is the first report of a rheological measurement in a 2D dusty plasma. This talk is based on V. Nosenko and J. Goree, PRL 93, 155004 (2004). [Preview Abstract] |
Wednesday, March 23, 2005 4:18PM - 4:30PM |
S37.00010: The Interplay of Osmotic Transport and Coalescence on Stability of Double Emulsions Wang Yafei, Zhang Tao, Hu Gang The long-term stability and controlled release of encapsulated active materials are major concerns of important applications of double emulsions. Only recently can stable monodisperse W/O/W double emulsions be made with a controlled size and internal volume fractions of encapsulated aqueous phase. The size of uniform oil droplets can be varied from sub-micron to tens of microns. Conventional dynamic light scattering encounters significant difficulties to probe the internal microstructure of liquid droplets. The droplets-inside-droplets structure of double emulsions complicates the interpretation of scattering data. To probe the internal microstructure of liquid droplets, diffusing-wave spectroscopy shows a unique advantage to measure the restricted motion of internal aqueous droplets. The destabilization of double emulsions can be initiated by introducing osmotic unbalance between the inner and outer aqueous phases and the process of coalescence transfers a double emulsion into a simple emulsion. Light scattering is a powerful technique to study the kinetic process by probing the structure migration. Diffusing-wave spectroscopy is also used to monitor the aging of double emulsions on a time scale of one year. [Preview Abstract] |
Wednesday, March 23, 2005 4:30PM - 4:42PM |
S37.00011: Dynamic electrorheological effects of rotating spheres: a self-consistent theory Lei Shen, Kin Wah Yu, Guo Qing Gu, Jun Jun Xiao We have corrected and extended the previous work done by T. K. Wan[1]. By solving self-consistent relaxation equations under coupled dipole approximation, we got the exact analysis solution for two rotating particles. Next, we derived three typical rotating configurations. Several comparisons have been done for interacting forces as well as the angular velocity dependence. Further more, we extended this two-particle model to a lattice model which contains infinite periodically arranged rotating particles. It has been examined by employing Ewald summation method. The rotational motion leads to a shift of the ground state of this electrorheological solid. The original bct(body-centered tetragonal) structure is no longer stable. By noticing that the behavior of our model is extremely similar to the electrorotation case [2], we studied the relations between these two models. The explicit associating expression has been found. \\[4pt] [1] Jones T. K. Wan, K. W. Yu, and G. Q. Gu. Phys. Rev. E, 6846 (2002). [2] C. K. Lo and K. W. Yu. Phys. Rev. E, 031501 (2001). [Preview Abstract] |
Wednesday, March 23, 2005 4:42PM - 4:54PM |
S37.00012: A Coarse-Grained Simulation of Rheology of Colloidal Suspensions and Polymer Nano-Composites Victor Pryamitsyn, Venkat Ganesan We extend DPD model to address dynamical properties of suspensions of solid particles in complex fluids. In this approach, the solvent particles (polymer segments) are represented as DPD particles. In contrast, the solute particles are represented as spherical hard particles of appropriate size. To provide proper shear friction and grip of the colloids and solvent we utilize Espanol's extensions over standard DPD model by adding rotational degree of freedom and rotational friction and \textbf{non-central} dissipative and random forces. For non-polymeric fluids, our results focus on the equilibrium dynamics and the steady state shear rheological behavior for a range of volume fractions of the suspension, and demonstrate excellent agreement with many published experimental and theoretical results. Moreover, we are also able to track the glass transition of our suspension and associated dynamical signatures in both the diffusivities and the rheological properties of our suspension. For polymeric fluid, we have studied influence of polymer-particle friction and particle concentration on polymer matrix relaxation dynamics, particle diffusion and rheology of the composite. Our results suggest that the simulation approach can be used as a mesoscale model to examine quantitatively the rheological properties of colloidal suspensions in complex fluid solvents such as polymeric melts and solutions, as well as allied dynamical phenomena such as phase ordering in mixtures of block copolymers and particles. [Preview Abstract] |
Wednesday, March 23, 2005 4:54PM - 5:06PM |
S37.00013: Liquid-liquid phase transition and fragile-to-strong transition Limei Xu, Sergey Buldyrev, H. Eugene Stanley Using molecular dynamics simulations we study the dynamic properties for a model of liquid consisting of particles interacting via a spherically symmetric potential with repulsive and attractive ramps, first introduced by Jagla. The model displays anomalies similar to those found in liquid water, namely, expansion upon cooling and an increase of diffusivity upon compression as well as the liquid- liquid phase transition in the region of pase diagram easily accessible by simulations. At lower temperatures the low and high density liquids undergo glass transitions into correspondent glassy phases. We find that similarly to the behavior of water and silicon,the glass transition in the high density phase is fragile while in the low density phase it is strong. The difference is that in the present model, the co-existence line between low-density and high-density liquids as well as its Widom extension above the critical point have positive slopes. Thus, at constant pressure the behavior of diffusion coefficient changes upong cooling from strong to fragile, i.e. in the opposite way than in water and silicon. [Preview Abstract] |
Wednesday, March 23, 2005 5:06PM - 5:18PM |
S37.00014: Relaxation in a glassy binary mixture: A comparison of a Brownian Dynamics simulation and the mode-coupling theory Elijah Flenner, Grzegorz Szamel We compare results of Brownian Dynamics simulations of a glassy binary mixture with predictions of the mode-coupling theory for the same mixture. The mode-coupling equations for the time evolution of the coherent and the incoherent scattering functions were solved for a number of temperatures using as input the structure factors determined from the simulations. As found in earlier studies, the mode-coupling theory predicts an ergodicity breaking transition at a temperature that is approximately twice higher than the \textit{crossover} temperature inferred from Brownian Dynamics simulation results. However, when compared at the same $T-T_c$, time-dependent quantities predicted by the mode-coupling theory agree reasonably well with those obtained from simulations except at the lowest temperatures. [Preview Abstract] |
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