Bulletin of the American Physical Society
APS March Meeting 2013
Volume 58, Number 1
Monday–Friday, March 18–22, 2013; Baltimore, Maryland
Session F34: Charged Colloids with Short-Range Attractions I |
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Sponsoring Units: DPOLY DCMP DBIO Chair: Yun Liu, University of Delaware/NIST Room: 342 |
Tuesday, March 19, 2013 8:00AM - 8:36AM |
F34.00001: POLYMER PHYSICS PRIZE BREAK
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Tuesday, March 19, 2013 8:36AM - 9:12AM |
F34.00002: A colloidal perspective of protein solutions manipulated by multivalent ions: Phase behavior and associated dynamics Invited Speaker: Frank Schreiber After a brief overview of interactions in aqueous protein solutions, we will discuss how ions can be used to manipulate these interactions and the associated phase behavior as well as the diffusion dynamics. We show that multivalent ions do not only influence the ionic strength and the resulting interactions including effective attraction, but lead to qualitatively new effects. Particular attention will be given to the reentrant condensation of proteins (F. Zhang et al, PRL 101 (2008) 148101; F. Zhang et al, Soft Matter 8 (2012) 1313) and its relationship with liquid-liquid phase separation and protein crystallization. In particular, we attempt to rationalize crystallization controlled by trivalent ions and discuss the role of specific ions and their impact on the effective interaction potential. These results are compared to the diffusion dynamics in these systems studied using neutron spectroscopy and light scattering (F. Roosen-Runge et al, PNAS 108 (2011) 11815; Heinen et al, Soft Matter 8 (2012) 1404) and the question of transient clusters is discussed. Finally, we critically discuss to which extent proteins can be described by colloidal concepts. The work was performed in collaboration with F. Zhang, T. Seydel, M. Hennig, F. Roosen-Runge, M. Skoda, R. Jacobs and others. [Preview Abstract] |
Tuesday, March 19, 2013 9:12AM - 9:24AM |
F34.00003: ABSTRACT WITHDRAWN |
Tuesday, March 19, 2013 9:24AM - 9:36AM |
F34.00004: Protein clusters in biomembranes Nicolas Destainville We propose that proteins embedded in lipidic bio-membranes can spontaneously self-organize into stable membrane nano-domains (or clusters), due to the competition between short-range attractive and longer-range repulsive forces between proteins, specific to these systems, and propagated by the lipidic membrane. We compare different long-range potentials (including notably three-body terms) and we demonstrate that the existence of cluster phases in this context should be quite generic. Furthermore, a real membrane contains hundreds of different protein species that are far from being randomly distributed in these nano-domains, which is crucial in terms of biological functions. We take this protein diversity into account by modulating protein-protein interactions both at short and longer range. Both theoretical and numerical investigations explain why protein clusters recruit only a few protein species, thus leading to cluster biological specialization. In this respect, we highlight that cluster phases can turn out to be an advantage at the biological level, for example by enhancing the cell response to external stimuli. [Preview Abstract] |
Tuesday, March 19, 2013 9:36AM - 9:48AM |
F34.00005: Assembly of Spherical Colloids by Short-range Out-of-plane Attraction and Long-range In-plane Repulsion Fuduo Ma, David T. Wu, Ning Wu The electric-field assembly of spherical colloids with isotropic surface properties has been studied in both two- and three- dimensions. Structures, such as FCC, HCP, and BCT crystals were observed. Recently, we have found surprisingly new types of structures within a previously unexplored experimental regime: low frequency regime (100 Hz to 10 kHz) and low salt concentrations (below 10$^{-4}$ M). At low particle concentrations, a family of well-defined clusters, ranging from 3 to 10 was observed. Statistical analysis of the population distribution reveled non-trivial peaks for trimers, tetramers, hexamers, and nonamers. We attribute these new types of non-planar structures to a short-range out-of-plane (the plane refers to the substrate) attraction and a long-range in-plane repulsion. For example, the double layer and in-plane dipolar repulsion could make bottom particles in the clusters separate from each other. While the out-of-plane dipolar attraction and particle-substrate attraction could be responsible for the formation of the clusters, i.e., the top central sphere is associated with the bottom spheres. Phase diagrams from experiments and simulation will be compared. These clusters could be used as building blocks for making photonic crystal, filtration, and plasmonic structures. [Preview Abstract] |
Tuesday, March 19, 2013 9:48AM - 10:00AM |
F34.00006: Colloidal stability in concentrated electrolyte solutions using large counterions Guillermo Guerrero Garcia, Pedro Gonzalez Mozuelos, Monica Olvera de la Cruz The stability of charged colloids in solution has been widely studied because it has ubiquitous applications in science and engineering. According to the classical DLVO theory, the electrostatic repulsion among charged colloids is significantly screened at high electrolyte concentrations. As a result, highly charged particles are expected to aggregate due to short-range van der Waals attractive interactions. Nevertheless, the classical DLVO theory relies in the linear Poisson-Boltzmann equation, which is usually restricted to low electrolyte concentrations and weakly charged colloids. In this work, we propose a novel mechanism beyond the classical DLVO picture that uses large counterions to prevent highly charged nanoparticles from aggregating in salt solutions with concentrations up to 1 M, in agreement with experimental observations. [Preview Abstract] |
Tuesday, March 19, 2013 10:00AM - 10:12AM |
F34.00007: Size and interaction-strength effects on the phase behavior of colloidal particle assemblies Ray Sehgal, David Ford, Dimitrios Maroudas We report the findings of a systematic computational study of the inherently complex phase behavior of thermodynamically small assemblies (clusters) of colloidal particles interacting via a potential that includes electrostatic repulsion and depletion-based short-ranged attraction. Using Monte Carlo umbrella sampling with coarse graining in two order parameters and a biasing scheme based on a genetic algorithm, we generate free-energy landscapes (FELs) that can indicate coexistence between fluid-like and crystalline phases. We have used the data mining technique of diffusion mapping to determine the dimensionality of the order-parameter space and assess the suitability of chosen order parameters that represent metrics of cluster density and crystallinity. Evaluation of phase behavior metrics from analysis of the FELs leads to predictions of conditions for formation of stable phases of such small colloidal clusters. A stable crystalline phase emerges as the number of particles in the assembly increases beyond a critical value. We find that the critical cluster size for the onset of crystallization decreases with increasing strength of the interparticle attraction. This FEL analysis also enables a mean-field description of the phase transitions undergone by these assemblies. [Preview Abstract] |
Tuesday, March 19, 2013 10:12AM - 10:24AM |
F34.00008: Distinguishing cluster phases as a unique scenario of intermediate range order in colloidal suspensions and protein solutions Paul Douglas Godfrin, Ramon Casta\~neda-Priego, Yun Liu, Norman J. Wagner A state of stable clusters is characterized by the reversible aggregation of colloidal particles to a finite, energetically favored size. Clusters can arise from a competition between short range attraction, driving aggregation, and long range repulsion, stabilizing clusters. Recent interest in systems with these interactions has brought attention to the formation of a low-q peak in the structure factor and the proposition that this peak directly indicates cluster formation. To understand the structures that produce a low-q peak, Metropolis Monte Carlo simulations are performed to calculate the partial structure factors by decomposing the system into cluster-cluster, monomer-monomer, and cross-correlations. We find that a low-q peak appears in fluids with strong cluster-cluster correlations but also in systems dominated by monomer-monomer correlations and percolated states. Thus, this low-q peak is more appropriately termed the intermediate range order (IRO) peak. Consequently, an IRO peak does not necessarily signal the existence of a cluster state in solution. Rather, it reflects the presence of a preferred length scale related to the two competing potential features. Determining cluster formation is most accurately accomplished by combining experiment with simulation. [Preview Abstract] |
Tuesday, March 19, 2013 10:24AM - 10:36AM |
F34.00009: Clustering and state diagram of charged colloids with short-range attraction in shear flows Alessio Zaccone, Massimo Morbidelli Under static conditions, the superposition of short-range (e.g. van der Waals) attraction and electrostatic repulsion gives rise to interesting phases such as equilibrium clusters in globular protein suspensions. What is much less understood is their behavior under external flow, which is important for the physiological aggregation of proteins and for industrial systems as well. I will present theoretical and experimental results showing that clustering of these systems in shear flow is characterized by the crossover from a reaction-limited clustering kinetics at low shear into a convection-dominated aggregation regime at high Peclet numbers. The kinetics may rise by up to many orders of magnitude in the crossover regime. This behavior is due to the singularly-perturbed character of the governing diffusion equation where the shear drift term induces a singularity and a boundary-layer at large interparticle distances. This understanding, together with a theoretical description of cluster breakup, is used to rationalize the peculiar nonequilibrium state diagram (including gelation) of these colloidal suspensions in shear flow with applications ranging from microfluidic self-assembly to proteins. [Preview Abstract] |
Tuesday, March 19, 2013 10:36AM - 10:48AM |
F34.00010: Percolation and local density fluctuations for a Colloidal System with competing interactions Nestor Valadez-Perez, Yun Liu, Ramon Castaneda-Priego The gelation is believed to result from the particle aggregation in a complex structure. The aggregate span in the entire volume gives it a capability for supporting stresses. Gelled systems possess a high degree of inhomogeneity, while locally the particles and their near neighbors present a defined array as can be seen in their coordination number and bonding angles. Using Monte Carlo simulations, we investigate the structure of a system of hard spheres interacting through a combined potential: a short-ranged Square Well (SW) and a long-ranged repulsive Yukawa potential (RY). We made an exhaustive study for several conditions of temperature (T*) and concentration ($\varphi$) corresponding to different repulsion strengths ($A$). Our results show that the percolation threshold is shifted to lower concentrations when the repulsion is increased, but this shift gradually disappears at low temperature. Besides we also computed the local density through the system; we particularly identified a length scale at which the density fluctuations are attenuated. This length coincides with the intermediated range order recently identified in protein systems. [Preview Abstract] |
Tuesday, March 19, 2013 10:48AM - 11:00AM |
F34.00011: Drag coefficient of an electrophoretic colloidal particle Kathryn Reddy, Ming-Tzo Wei, Joel A. Cohen, H. Daniel Ou-Yang Electrophoretic mobility is a measure to determine the electric charges on a colloidal particle. Zeta potential, a concept originated by Smoluchowski, has been a standard for quantifying the surface charge density for the electric double-layers that are thin compared to the particle radius. Various models have been suggested to improve Smoluchowski's theory for systems with Debye length not thin compared to the particle radius. Central to the issue is that the fluid flow due to the external field-induced counter-ion motion is unknown. Using optical tweezers to trap a colloidal particle in a low-frequency electric field, we found the drag coefficient of the particle in the field to be non-Stokes. We discuss how the non-Stokes' drag coefficient as a function of salt concentration and particle size may be useful for interpreting different models of Zeta potential. [Preview Abstract] |
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