Bulletin of the American Physical Society
APS March Meeting 2013
Volume 58, Number 1
Monday–Friday, March 18–22, 2013; Baltimore, Maryland
Session B30: Colloids: Transitions and Structures |
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Sponsoring Units: DCMP Chair: Piotr Habdas, St. Joseph's University Room: 338 |
Monday, March 18, 2013 11:15AM - 11:27AM |
B30.00001: Coarsening of firefighting foams containing fluorinated hydrocarbon surfactants Matthew J. Kennedy, John A. Dougherty, Nicholas Otto, Michael W. Conroy, Bradley A. Williams, Ramagopal Ananth, James W. Fleming Diffusion of gas between bubbles in foam causes growth of large bubbles at the expense of small bubbles and leads to increasing mean bubble size with time thereby affecting drainage. Experimental data shows that the effective diffusivity of nitrogen gas in aqueous film forming foam (AFFF), which is widely used in firefighting against burning liquids, is several times smaller than in 1{\%} sodium dodecyl sulfate (SDS) foam based on time-series photographs of bubble size and weighing scale recordings of liquid drainage. Differences in foam structure arising from foam production might contribute to the apparent difference in the rates of coarsening. AFFF solution produces wetter foam with initially smaller bubbles than SDS solution due in part to the lower gas-liquid surface tension provided by the fluorosurfactants present in AFFF. Present method of foam production generates microbubble foam by high-speed co-injection of surfactant solution and gas into a tube of 3-mm diameter. These results contribute to our growing understanding of the coupling between foam liquid fraction, bubble size, surfactant chemistry, and coarsening. [Preview Abstract] |
Monday, March 18, 2013 11:27AM - 11:39AM |
B30.00002: Competition between phase separation and crystallization in attractive colloids Barbara Frisken, Arthur Bailey, Juan Sabin, Gabriel Espinosa We will present results from recent experiments on Earth and on the International Space Station investigating the interplay between phase separation and crystallization in samples prepared in the three-phase region (gas-liquid-crystal) of the phase diagram of a colloid-polymer mixture. On Earth, our samples first separate into a colloid-rich phase and a colloid-poor phase, with crystals forming in the colloid-rich phase. The denser phases sediment as expected. In microgravity, photographic images obtained in the BCAT-5 experiment reveal phase separation with crystal formation in the denser phase, where the phase separation continues normally until the dominant length scale is about 25\% of the cell thickness, at which point both phase separation and crystal growth are arrested before macroscopic phase separation can occur. We propose that this arrest occurs because a crystalline network forms in the liquid phase and that the gas-liquid surface tension is not sufficient to overcome the stiffness of this network. [Preview Abstract] |
Monday, March 18, 2013 11:39AM - 11:51AM |
B30.00003: Colloidal Wigner Crystals Near the Melting Transition Emily Russell, David Weitz We demonstrate the formation of colloidal ``Wigner'' crystals at low particle volume fraction. Particles are suspended in a nonpolar solvent and charged by the addition of a small amount of surfactant, generating a long-range interparticle repulsion which induces crystallization above a critical volume fraction of order 10\%. Confocal microscopy allows us to study in detail the three-dimensional structure and dynamics of these colloidal crystals as we vary the volume fraction, and we find a growing population of especially mobile particles with large local Lindemann parameter as we approach the critical volume fraction. We discuss our results and the implications of our findings to competing ideas of the mechanism of bulk crystal melting. [Preview Abstract] |
Monday, March 18, 2013 11:51AM - 12:03PM |
B30.00004: Visualization of colloidal liquid nucleation induced by Critical Casimir forces Duc Nguyen, Peter Schall We show that with precise temperature control of critical Casimir forces we achieve reversible control of colloidal gas-liquid. The exquisite temperature control of the potential allows us to even tune the degree of supersaturation of the liquid phase. We use a confocal microscopy to elucidate the nucleation process on the single particle level: We determine the Gibbs free energy, interfacial tension and chemical potential of the liquid aggregates directly from their size distribution. We estimate the interfacial tension of the aggregates at different degree of supersaturation directly from the particle potential and pair correlation function using Kirkwood and Buff theory. A good agreement between the two methods provides new insight into the gas-liquid transition. [Preview Abstract] |
Monday, March 18, 2013 12:03PM - 12:15PM |
B30.00005: Shear Driven Aggregation in Latex Colloids Suresh Ahuja Reynolds number is small in colloidal flow and therefore, colloidal volume fraction and Peclet number are important. AS the volume fraction and attractive coupling between particles increase, relaxation time and Weisenberg number become significant. Shear-induced aggregation of latex colloids is due to the interplay between the shear-induced formation and breakage of latex .particles. While particle size is limited by breakage, their number density increases with the shearing-time. Upon cessation of shear, the particles interconnect into an assembly held by grainy bonds. It results in increase in yield stress and dynamic modulus. A contact model enables aggregates maintaining their structures under low stress while being restructured under high stress. Modeling involves solution of Navier- Stokes equation with moving particles as boundary condition for the flow like using the Lattice Boltzmann approach or by using (accelerated) Stokesian Dynamics. Alternate approach is to model the fluid phase by soft repulsive particles with pair-wise noise and friction, known as dissipative particle dynamics (DPD). This method by construction produces full inertial hydrodynamics, but applying the correct fluid-particle boundary condition is non-trivial. Both particle to particle and particle to wall collisions can be considered using Johnson-Kendall- Roberts (JKR) analysis of collision dynamics of dissipative forces using a soft-sphere modeling technique. Our experimental work used emulsion polymerized latex that was subjected to steady and dynamic shear. Yield stress, dynamic modulus and relaxation time increased on shearing in conjunction with changes in aggregate size. [Preview Abstract] |
Monday, March 18, 2013 12:15PM - 12:27PM |
B30.00006: Domain, Stripe, and Pattern Formation for Colloids on Optical Trap Arrays Danielle McDermott, Jeffery Amelang, Lena Lopatina, Cynthia Reichhardt, Charles Reichhardt We examine pattern formation of colloids atop a square periodic substrate using large scale numerical simulations. The pins forming the substrate are modeled with a muffin-tin potential which is flat with localized traps. We show that with 4 colloids per pinning site the system has triangular ordering and with 5 colloids per site it has square ordering. We study intermediate fillings and identify a rich variety of distinct ordering regimes including disordered grain boundaries, crystalline stripe structures, superlattice orderings, and disordered patches of multiple phases. These different regimes are characterized with a Voronoi analysis, energy dispersion plots, and ordination of domains. We extend our studies to a wide range of other fillings which feature similar boundary formation patterns. Our results show that periodic substrates of muffin-tin potentials can be used to tailor grain boundary formation. [Preview Abstract] |
Monday, March 18, 2013 12:27PM - 12:39PM |
B30.00007: Anisotropic colloids for building complex molecular structures using critical Casimir effect Truc Anh Nguyen, Daniela Kraft, Sandra Veen, Peter Schall Here, we present a new way to build complex colloidal scale structures using critical Casimir forces on anisotropic colloids. These forces arise from the confinement of critical solvent fluctuations between the particle surfaces and allow temperature-control over the particle interactions. We use doublet particles made of polymethyl-methacrylate (PMMA) and exhibiting anisotropic surface charge densities, suspended in a binary liquid mixture. By controlling the applied temperatures of the system, we can tune the particle interactions of the two ends of the particles to observe different superstructures formed in time and space: at low temperature, the particles are randomly distributed and represent a gas phase; however, at higher temperatures, the particles form long chain-like structures and cubic crystal structures depending on the temperature difference to the solvent phase separation. This opens new opportunities to assemble complex building blocks for nano- and micro-devices. [Preview Abstract] |
Monday, March 18, 2013 12:39PM - 12:51PM |
B30.00008: Direct observation of the nucleation in colloidal solid-solid transitions Yi Peng, Feng Wang, Ziren Wang, Yilong Han Solid-solid phase transitions are ubiquitous in nature, but their microscopic mechanisms remain poorly understood. We employed thermally sensitive microgels to study the solid-solid transitions between square and triangular lattices in colloidal thin films. Two types of nucleation processes were directly observed by video microscopy and studied at the single-particle level. Under low flow rates, the nucleation is a two-step process: square lattice $\to$ liquid nucleus $\to$ triangle nucleus and its precursor is a local particle-exchange loop, whereas under high flow rates the nucleus of the triangle lattice forms directly from a dislocation pair by a martensitic mechanism. We measured the critical nucleus size, the energy barrier height and the hysteresis loop of the solid-solid transitions. Our results cast new light to solid-solid transitions in carbon systems, nano-crystals and geophysics. [Preview Abstract] |
Monday, March 18, 2013 12:51PM - 1:03PM |
B30.00009: Frustrated Ordering of Colloidal Crystals in Spatially Varying Potentials Vishal Soni, William T.M. Irvine Frustrated ordering processes are of wide interest in condensed matter systems. Experiments on interfacial colloidal systems have resulted in several recent insights into the two dimensional ordering of crystalline lattices frustrated by Gaussian curvature. We study the ordering of two-dimensional lattices of colloids frustrated by spatially varying dielectrophoretic forces. In particular, we investigate the role of topological defects in organizing the conformal-crystal like ground state and the defect dynamics that lead to equilibration as the applied dielectrophoretic force is increased. [Preview Abstract] |
Monday, March 18, 2013 1:03PM - 1:15PM |
B30.00010: Preparation of monodisperse microspheres from the Laplace pressure induced droplet formation in micromolds Chang-Hyung Choi, Jongmin Kim, Sung-Min Kang, Jinkee Lee, Chang-Soo Lee Monodisperse microspheres play critical roles in many applications such as micro-electromechanical systems (MEMS), chemical release systems, optical materials and various biological applications. Although microfluidic systems have been developed for producing monodisperse microspheres, it still definitely requires pressure driven flow for continuous fluid injection as well as use of surfactant to achieve their uniformity. Here, we present a novel molding method that generates monodisperse microspheres through surface-tension-induced flow. Two immiscible fluids that consist of photocurable monomer and hydrophobic oil are sequentially applied onto the mold. The mold geometry results in Laplace pressure induced droplet formation, and these droplets formed are individually localized into each micromold. Photopolymerization of the droplets allow for the formation of polymer microspheres with narrow size distribution (CV$=$1.9{\%}). We obtain the microspheres with diameter ranging from 20 to 300 $\mu $m by modulating mold dimensions. We provide a synthesis method to produce microspheres in micromolds for various reaction schemes: UV-polymerization, sol-gel reactions and colloidal assemblies. [Preview Abstract] |
Monday, March 18, 2013 1:15PM - 1:27PM |
B30.00011: Formation of Uniform Hollow Silica microcapsules Huan Yan, Chanjoong Kim Microcapsules are small containers with diameters in the range of 0.1 -- 100 $\mu$m. Mesoporous microcapsules with hollow morphologies possess unique properties such as low-density and high encapsulation capacity, while allowing controlled release by permeating substances with a specific size and chemistry. Our process is a one-step fabrication of monodisperse hollow silica capsules with a hierarchical pore structure and high size uniformity using double emulsion templates obtained by the glass-capillary microfluidic technique to encapsulate various active ingredients. These hollow silica microcapsules can be used as biomedical applications such as drug delivery and controlled release. [Preview Abstract] |
Monday, March 18, 2013 1:27PM - 1:39PM |
B30.00012: Interstitials in 2D colloidal crystals Lichao Yu, Sungcheol Kim, Alexandros Pertsinidis, Xinsheng Ling Point defects in crystalline solids are important in many areas of condensed matter physics, ranging from the mechanical properties of metals, to supersolidity in quantum solids, and most recently the magnetic properties of graphene. A key question to point defects is how they diffuse in the crystalline lattice. Colloidal crystals provide a perfect model system for studying the dynamics of point defects, since the kinetic pathways of diffusion can be identified in direct real-time video imaging experiments. Here we report an experimental study of another type of point defects: interstitials. We found that interstitial diffusion in a 2D colloidal crystal is also dominated by a dislocation pair unbinding-binding process. Similar to vacancies, interstitial diffusion exhibits strong memory effects. However, the contrast lies in the observation that the interstitials, as quasi-particles, diffuse faster than vacancies. We propose that higher diffusion constant of the interstitials is a result of the suppression of the Peierls barrier for the edge dislocations by the excess strain created by the extra particle(s). This work was supported by NSF-DMR. [Preview Abstract] |
Monday, March 18, 2013 1:39PM - 1:51PM |
B30.00013: Effects of Particle Shape on Growth Dynamics at Edges of Evaporating Drops of Colloidal Suspensions Peter J. Yunker, Matthew A. Lohr, Tim Still, Alexei Borodin, D.J. Durian, A.G. Yodh We study the influence of particle shape on growth processes at the edges of evaporating drops. Aqueous suspensions of colloidal particles evaporate on glass slides, and convective flows during evaporation carry particles from drop center to drop edge, where they accumulate. The resulting particle deposits grow inhomogeneously from the edge on the air-water interface in two-dimensions. The deposition front, or growth line, varies in space and time. Measurements of the fluctuations of the deposition front during evaporation enable us to identify distinct growth processes. Interestingly, three distinct growth processes were discovered in the evaporating colloidal suspensions by tuning particle shape-dependent capillary interactions and thus varying the microscopic rules of deposition. Sphere deposition exhibits a classic Poisson like growth process; deposition of slightly anisotropic particles, however, appears to belong to the Kardar-Parisi-Zhang (KPZ) universality class, and deposition of highly anisotropic ellipsoids appears to belong to a third universality class, characterized by KPZ fluctuations in the presence of quenched disorder. [Preview Abstract] |
Monday, March 18, 2013 1:51PM - 2:03PM |
B30.00014: Non-equilibrium Ionic Assemblies of Oppositely Charged Colloids Rui Zhang, Prateek Jha, Monica Olvera de la Cruz The structure and evolution kinetics of non-equilibrium clusters formed in a solution of oppositely charged colloids are analyzed by a kinetic Monte Carlo simulation scheme. A wide range of dynamic cluster configurations are obtained by varying the various external parameters controlling the interaction strength between colloids, screening length, and packing density of colloids. At low-salt concentrations, clusters with structures ranging from NaCl-type cubic aggregates to fibril-like chains are observed, while at high-salt concentrations, disordered compact clusters are observed. A chain-folding barrier model is proposed to explain the kinetically trapped fibril-like assemblies. In higher-density solutions, ionic clusters of bigger size and percolated gel structures are observed. Our work demonstrates the structural richness of non-equilibrium ionic assemblies of oppositely charged colloids and elucidates the effect of ionic correlations, not captured by mean field models such as the modified Poisson-Boltzmann approaches, in determining the structure of assemblies of oppositely charged colloids. These ``ionic composites" hold great promise in a variety of emerging applications such as templated polymerization of charged molecules and assembly of charged particles. [Preview Abstract] |
Monday, March 18, 2013 2:03PM - 2:15PM |
B30.00015: Directional Entropic Forces in Hard Colloids Greg van Anders, Khalid Ahmed, Ross Smith, Michael Engel, Sharon Glotzer Based on known results from the literature of hard particles we introduce the concept of entropically patchy particles -- particles that bind with angular specificity entirely due to their geometry via directional entropic forces or ``bonds''. Unlike ordinary patchy particles, in which ``valence'' vis-a-vis angular specificity is dictated by microscopic energetic considerations (sticky patches), entropic forces causing the binding of particles at entropic patch sites are emergent. Using basic examples we show both theoretically and computationally that we can alter the geometry of a particle to create an entropic patch and tune the resulting effective pair potential in such a way that it can lead to angularly specific binding, even in the absence of depletants. [Preview Abstract] |
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