Bulletin of the American Physical Society
APS March Meeting 2016
Volume 61, Number 2
Monday–Friday, March 14–18, 2016; Baltimore, Maryland
Session H37: Clustering and Gelation with Competing Interactions IIFocus
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Sponsoring Units: GSOFT Chair: Yun Liu, NIST Room: 340 |
Tuesday, March 15, 2016 2:30PM - 2:42PM |
H37.00001: Implicit depletion of anisotropic particles Jens Glaser, Andrew Karas, Sharon Glotzer Entropy mediates depletion interactions between particles with excluded volume. We implement a novel algorithm [1] to simulate this emergent attraction between hard anisotropic particles in the presence of penetrable hard spheres. Our algorithm is efficient because it integrates out the degrees of freedom of the ideal depletant gas in parallel, which makes it well suited for high-performance computing. The algorithm can achieve several orders of magnitude speed-up over explicit algorithms. As an application we study the interplay between phase separation and kinetic arrest in the anisotropic clustering of colloidal discoids [2]. We also discuss applications of the algorithm for the assembly of hemispheres [1] and of cuboctahedra [3]. [1] J. Glaser, A. S. Karas, and S. C. Glotzer, arXiv:1508.07077. [2] L. C. Hsiao, B. A. Schultz, J. Glaser, M. Engel, M. E. Szakasits, S. C. Glotzer, and M. J. Solomon, Nat. Commun. 6, 8507 (2015). [3] A. S. Karas, J. Glaser, and S. C. Glotzer, arXiv:1510.04236. [Preview Abstract] |
Tuesday, March 15, 2016 2:42PM - 2:54PM |
H37.00002: Interplay of directional and isotropic interactions in self-assembly Debra Audus, Francis Starr, Jack Douglas Patchy particle models, composed of hard spheres with decorated with attractive patches, have been introduced as models of micron-sized particles with anisotropic interactions, as well as solutions of globular proteins. Here, we extend the canonical model of the patchy particles to include a short-ranged isotropic interaction in order to probe of the coupling of the directional and isotropic interactions on the self-assembly process. In particular, we evaluate basic properties characterizing self-assembly including average cluster mass and the fraction of particles in the clustered state using both Monte Carlo simulation and analytic Wertheim theory. This combination allows for validation of the theory and for insight into analyzing experimental data. We also find that Flory-Stockmayer theory describes the cluster size distribution data found in our simulations remarkably well, despite its erroneous mass-scaling exponent. This result, coupled with Wertheim theory, predicts both a master curve for the average cluster mass and a method to parameterize patchy particle models using experimental data. [Preview Abstract] |
Tuesday, March 15, 2016 2:54PM - 3:06PM |
H37.00003: Effective potentials in concentrated colloid-polymer mixtures with competing interactions Marco Laurati, Nestor Valadez Perez, Ronja Capellmann, Stefan Egelhaaf, Ramon Casta{\~n}eda-Priego We determine the effective potentials describing the interactions between colloidal particles in concentrated colloid-polymer mixtures in which depletion attraction competes with electrostatic repulsion. To obtain the potentials, the method of Monte-Carlo inversion is applied to experimental pair distribution functions obtained by confocal microscopy. Both fluid and gel states are investigated. We compare the results of the inversion method with those obtained by describing the interactions using a combination of a square well potential for the attractive component and a Yukawa potential for the repulsive component. This allows us to test the validity range of the one-component pair-potential. [Preview Abstract] |
Tuesday, March 15, 2016 3:06PM - 3:18PM |
H37.00004: Short-time dynamics in dispersions with competing short-range attraction and long-range repulsion Riest Jonas, Gerhard Naegele The dynamic clustering of globular particles in suspensions exhibiting competing short-range attraction and long-range repulsion such as in protein solutions has gained a lot of interest over the past years. We investigate theoretically the influence of clustering on the dynamics of globular particle dispersions [1]. To this end, we systematically explore various pair potential models by a combination of state-of-the-art analytic methods in conjunction with computer simulations where the solvent-mediated hydrodynamic interactions are likewise included. Our theoretical results show that the cluster peak (intermediate-range-order peak) is present also in the hydrodynamic function characterizing the short-time dynamics, in accord with new experimental data [2]. Enhanced short-range attraction leads to a smaller self-diffusion coefficient and a larger dispersion viscosity. The behavior of the (generalized) sedimentation coefficient is more intricate, e.g. showing a non-monotonic interaction strength dependence. \begin{itemize} \item [{[1]}] J. Riest \& G. N\"agele, \textit{Soft Matter} (2015). doi:10.1039/C5SM02099A \item [{[2]}] Collaboration with D. Godfrin (NIST \& MIT), Y. Liu (NIST) and N. Wagner (UDEL), work in progress \end{itemize} [Preview Abstract] |
Tuesday, March 15, 2016 3:18PM - 3:30PM |
H37.00005: Rheology of clustering protein solutions Steven Hudson, Vishnu Dharmaraj, P. Douglas Godfrin, Yun Liu Here we explore the rheology of low-salt lysozyme solutions, with special interest in the extremes of high concentration and low temperature. Under these conditions, reversible clustering of protein governed by their competing short-range attraction and long-range repulsion markedly enhances viscosity. Even in these conditions, the solutions exhibit Newtonian behavior over a wide range of shear rates. To test for departures from Newtonian behavior, we examined still higher shear rates. At shear rates in excess of 10,000 /s, we find reversible shear thinning at 40 {\%} mass fraction. These results reveal dynamics of the protein clusters and are compared with other measurements of solution dynamics by neutron spin echo scattering and dynamic light scattering. [Preview Abstract] |
Tuesday, March 15, 2016 3:30PM - 3:42PM |
H37.00006: Monte Carlo simulation studies of diffusion in crowded environments Prithviraj Nandigrami, Brandy Grove, Andrew Konya, Robin Selinger Anomalous diffusion has been observed in protein solutions and other multi-component systems due to macromolecular crowding. Using Monte Carlo simulations, we investigate mechanisms that govern anomalous diffusive transport and pattern formation in a crowded mixture. We consider a multi-component lattice gas model with ``tracer'' molecules diffusing across a density gradient in a solution containing sticky ``crowder'' molecules that cluster to form dynamically evolving obstacles. The dependence of tracer flux on crowder density shows an intriguing re-entrant behavior as a function of temperature with three distinct temperature regimes. At high temperature, crowders segregate near the tracer sink but, for low enough overall crowder density, remain sufficiently disordered to allow continuous tracer flux. At intermediate temperature, crowders segregate and block tracer flux entirely, giving rise to complex pattern formation. At low temperature, crowders aggregate to form small, slowly diffusing obstacles. The resulting tracer flux shows scaling behavior near the percolation threshold, analogous to the scenario when the obstacles are fixed and randomly distributed. Our simulations predict distinct quantitative dependence of tracer flux on crowder density in these temperature limits. [Preview Abstract] |
Tuesday, March 15, 2016 3:42PM - 4:18PM |
H37.00007: On shape and charges in colloidal dispersions Invited Speaker: Emmanuel Trizac Coulomb interactions are paramount in determining structural and dynamical properties for a wealth of anisotropic soft matter systems (clays, mineral crystallites, exfoliated nanosheets, patchy colloids, cement etc). The interplay between screening effects and anisotropy leads to distinctive yet often overlooked features, that will be discussed. In turn, the competition between the resulting effective potential and hard core constraints will be addressed. This competition can lead to non trivial structures, or impose strong dynamical slowing down. A distinction will be operated between weak and strong Coulomb coupling regimes. [Preview Abstract] |
Tuesday, March 15, 2016 4:18PM - 4:30PM |
H37.00008: Wetting-induced clustering and phoretic motions of colloidal particles Theyencheri Narayanan, Enrico Semeraro, Rajiv Dattani In recent years, self-propelled colloidal systems have received considerable attention as models for active matter. Most commonly used synthetic self-propelled systems involve Janus particles with asymmetric chemical composition in a catalytic medium. An analogous behavior can be obtained when particles are suspended in a phase separating binary liquid mixture due to preferential adsorption of one of the liquid species on the colloidal particles. Above an aggregation temperature ($T_{A}$), particles become attractive and aggregate to form compact colloidal clusters. In the two phase region of the binary mixture, particles partition into the phase rich in adsorbed component. We have used silica colloids suspended in a binary mixture of 3-methyl pyridine and heavy water to probe this adsorption-induced phoretic motion of particles. Using ultra small-angle X-ray scattering and photon correlation spectroscopy, we investigated the static and dynamic behavior of this system. In the one phase region below $T_{A}$, particles display a repulsive structure factor with diffusive dynamics. In the two-phase region of the host liquid, the static structure is similar but the dynamics is strongly enhanced with the onset of phase separation reminiscent of self-propelled motion. [Preview Abstract] |
Tuesday, March 15, 2016 4:30PM - 4:42PM |
H37.00009: Multiscale simulations of nanoribbon structures from chromophore amphiphile self-assemblies Dongxu Huang, Zhenwei Yao, Monica Olvera, Samuel Stupp Finite-width self-assembled one-dimensional nanostructures have many potential applications as electronically or biologically active materials. Understanding the driving forces for supramolecular self-assembly is essential for the molecular design of new highly functional structures. Here we use multi-scale molecular dynamics simulations to study the self-assembly of chromophore amphiphiles into a nanoribbon previously shown to be useful in photocatalysis [1]. We demonstrate that the nanoribbon structure is a result of the competition between electrostatics and the hydrophobic effect. We incorporate a scaling analysis that correlates the electrostatic strength with the finite width of the ribbon. These results with additional numerical calculations show that anisotropy of the short-range intermolecular interactions and long-range electrostatics can be used to control the dimensionality of these systems. [1] Adam S. Weingarten, Roman V. Kazantsev, Liam C. Palmer, et al. {\&} Samuel I. Stupp, Nature Chemistry, 2014 [Preview Abstract] |
Tuesday, March 15, 2016 4:42PM - 4:54PM |
H37.00010: Self-assembly of Ionic Chromonic Liquid Crystals Hythem Sidky, Jonathan K. Whitmer Chromonic liquid crystals exhibit a unique self--assembly process which is of both theoretical and practical interest. A characteristic feature of chromonics is the occurrence of molecular association through stacking at extremely low concentrations. Experimental evidence has suggested that this process is approximately isodesmic across a broad concentration range. To date, only a handful of computational studies have managed to reproduce crucial aspects of chromonic phases, using expensive atomistic simulations. Here, we present a minimal model capable of capturing key features of the lyotropic chromonic phase. Molecular simulations of coarse-grained mesogens are used to map out the phase behavior and explore how structural and energetic anisotropies influence their ordering and response. [Preview Abstract] |
Tuesday, March 15, 2016 4:54PM - 5:06PM |
H37.00011: Lipid domains in zwitterionic-anionic lipid mixtures induced by combined effect of monovalent and divalent ions Hongcheng Xu, Sai Ganesan, Silvina Matysiak Lipid domain formation is an important process for many cellular processes. In experiment, the effects of Ba$^{2+}$, Sr$^{2+}$, Ca$^{2+}$ and Mg$^{2+}$ in inducing lateral phase separation in the binary phosphatidylcholine-phosphatidylserine (PC-PS) bilayer are quite different, of which the molecular mechanism remains to be understood. We have explored the effect of monovalent (MI) and divalent (MII) cationic radii on lipid domain formation in mixed zwitterionic-anionic lipid bilayers. We propose a mechanism for the formation of divalent-cation-induced lipid domains based on MD simulations with our Water-Explicit Polarizable MEMbrane (WEPMEM) coarse-grained model, which uses PC as the model for zwitterionic and PS for anionic lipids. Lipid aggregation only occurs with limited range of monovalent and divalent ion sizes in agreement with experimental observations. More ordering and closer packing of the lipids are noted within the domains, which correlate with bilayer thickness, curvature and lipid asymmetry. The results of the simulations reveal that the lipid domain consists of MII-mediated anionic lipid dimer/trimer complexes bridged by monovalent ions MI and provide a stereochemical insight in understanding the experimentally observed calcium-induced phase separation. [Preview Abstract] |
Tuesday, March 15, 2016 5:06PM - 5:18PM |
H37.00012: The morphology of small sized clusters in a system with the competing interactions Yun Liu, NÉSTOR VALADEZ-PÉREZ, Ramon CASTAÑEDA-PRIEGO We have systematically investigated the morphological changes of clusters in a system with both a short-ranged attraction and long-ranged repulsion, which is ubiquitous for protein solutions. Interestingly, even though the delicate balance between the attraction and repulsion controls the fractal dimension of the large clusters, the overall sizes of small clusters seem to be sensitive only to the short-ranged attraction. This microscopic structure change is thus consistent with and provides a microscopic physical picture of the recently proposed general phase diagram where the attractive interaction controls the formation of clustered fluid in the one phase region. [Preview Abstract] |
Tuesday, March 15, 2016 5:18PM - 5:30PM |
H37.00013: Viscoelastic properties of DNA coated colloid suspensions Christine Middleton, David Pine DNA coated colloids are a unique gel forming system because of their tunable short range attraction. The interparticle potential can be varied through the chosen DNA sticky end, the particle coating density, and the temperature. We present studies of how these parameters affect the viscoelastic properties of suspensions of DNA coated colloids around the gel transition. [Preview Abstract] |
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