Bulletin of the American Physical Society
APS March Meeting 2016
Volume 61, Number 2
Monday–Friday, March 14–18, 2016; Baltimore, Maryland
Session E36: Colloids: Assembly, Mechanics and Techniques |
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Sponsoring Units: GSOFT Chair: Daniel Blair, Georgetown University Room: 339 |
Tuesday, March 15, 2016 8:00AM - 8:12AM |
E36.00001: Fluid to solid transition of hard regular polygons Joshua Anderson, Michael Engel, James Antonaglia, Andres Millan, Sharon Glotzer We perform simulations of hard regular polygons and determine the nature of the fluid to solid transition. Hard disks have a first order phase transition from fluid to hexatic and a continuous transition from hexatic to solid [1,2]. Hard polygons have shape and an additional degree of freedom. Directional entropic forces cause the polygons to attract edge to edge, which alters the phase transition. Polygons with enough edges have phase behavior similar to hard disks, with the density of the transition shifted lower. Polygons with few edges show a range of different behaviors. We develop and use HPMC [3] to run these simulations on the Titan supercomputer at the OLCF. HPMC is a scalable GPU-accelerated hard particle Monte Carlo simulation engine built on top of HOOMD-blue. [1] E. P. Bernard, W. Krauth, PRL 107, 155704 (2011). [2] M. Engel, J. A. Anderson, S. C. Glotzer, M. Isobe, E. P. Bernard, W. Krauth, PRE 87, 042134 (2013). [3] J. A. Anderson, M. E. Irrgang, S. C. Glotzer, arxiv:1509.04692 [Preview Abstract] |
Tuesday, March 15, 2016 8:12AM - 8:24AM |
E36.00002: Shape Sensitive Assembly and Colloidal Superball Phase Transitions Vishal Soni, Laura Rossi, Douglas J. Ashton, David J. Pine, Albert P. Philipse, Paul M. Chaikin, Marjolein Dijkstra, Stefano Sacanna, William T. M. Irvine Guiding the self-assembly of materials by controlling the shape of the individual particle constituents is a powerful approach to material design. In particular the assembly of colloidal particles driven by depletants is a versatile playground for investigating this potential. We find that colloidal “superballs” may assemble into distinct phases that depend on both their shape and the size of the depletants. By using a mixture of depletants, one of which is size-tunable, we can explore reversible transitions between these phases. [Preview Abstract] |
Tuesday, March 15, 2016 8:24AM - 8:36AM |
E36.00003: Heaps of Shapes: Flow-Stabilized Solids with Non-Spherical Colloids Scott Lindauer, C. Wyatt Shields IV, Gabriel P. Lopez, Karen E. Daniels, Robert Riehn Flow-stabilized solids are a class of fragile matter that are formed when a dense suspension of hard colloids is accumulated against a semipermeable barrier. We build a microfluidic device to confine Brownian particles in a quasi-2D channel; a controlled flow rate above a critical value forms flow-stabilized solids against the barrier. We extend prior work on submicron spherical particles, to particles of size 2-5 microns, and of various shapes: circular, rectangular, hexagonal, and triangular prisms. We perform experiments on these flow-stabilized solids to observe the angle of repose, packing fraction, and orientational order as a function of flow rate. We vary the flow rate quasi-statically in order to conduct the experiment at steady state. We find a critical flow rate below which no pile forms. In general, particles with less-circular shape form more stable heaps. [Preview Abstract] |
Tuesday, March 15, 2016 8:36AM - 8:48AM |
E36.00004: Digital Alchemy for Materials Design: Colloids and Beyond Greg van Anders, Daphne Klotsa, Andrew Karas, Paul Dodd, Sharon Glotzer Starting with the early alchemists, a holy grail of science has been to make desired materials by manipulating basic building blocks. Building blocks that show promise for assembling new complex materials can be synthesized at the nanoscale with attributes that would astonish the ancient alchemists in their versatility. However, this versatility means that connecting building-block attributes to bulk structure is both necessary for rationally engineering materials and difficult because building block attributes can be altered in many ways. We show how to exploit the malleability of colloidal nanoparticle “elements” to quantitatively link building-block attributes to bulk structure through a statistical thermodynamic framework we term “digital alchemy”. We use this framework to optimize building blocks for a given target structure and to determine which building-block attributes are most important to control for self-assembly, through a set of novel thermodynamic response functions. We thereby establish direct links between the attributes of colloidal building blocks and the bulk structures they form. Moreover, our results give concrete solutions to the more general conceptual challenge of optimizing emergent behaviors in nature and can be applied to other types of matter. [Preview Abstract] |
Tuesday, March 15, 2016 8:48AM - 9:00AM |
E36.00005: Shaping Crystal-Crystal Phase Transitions Xiyu Du, Greg van Anders, Julia Dshemuchadse, Sharon Glotzer Previous computational and experimental studies have shown self-assembled structure depends strongly on building block shape. New synthesis techniques have led to building blocks with reconfigurable shape and it has been demonstrated that building block reconfiguration can induce bulk structural reconfiguration. However, we do not understand systematically how this transition happens as a function of building block shape. Using a recently developed “digital alchemy”[1] framework, we study the thermodynamics of shape-driven crystal-crystal transitions. We find examples of shape-driven bulk reconfiguration that are accompanied by first-order phase transitions, and bulk reconfiguration that occurs without any thermodynamic phase transition. Our results suggest that for well-chosen shapes and structures, there exist facile means of bulk reconfiguration, and that shape-driven bulk reconfiguration provides a viable mechanism for developing functional materials. [1] G. van Anders, D. Klotsa, A. S Karas, P. M Dodd, and S. C Glotzer, ACS Nano, 9, 9542 (2015). [Preview Abstract] |
Tuesday, March 15, 2016 9:00AM - 9:12AM |
E36.00006: Discontinuous Shear Thickening using Boundary Stress Microscopy Vikram Rathee, Daniel Blair, Jeffery Urbach The microscopic picture of particle chain formation in discontinuous shear thickening suspensions remains unclear. In order to identify the role of localized stresses arising from particle chains we have applied the technique of Boundary Stress Microscopy to shear thickening suspension. By imaging deformations of an elastic boundary of the sheared suspension, we observe the appearance of localized forces on elastic substrate above critical stress value. These forces possibly arise from particles forming local network under shear. At the onset of thickening, we observe a change in first normal stress difference from negative to positive, inferring frictional contacts. However, the localized forces are only evident when the viscosity increases by an order of magnitude. [Preview Abstract] |
Tuesday, March 15, 2016 9:12AM - 9:24AM |
E36.00007: Viscosity of Sheared Helical filament Suspensions Matthew Sartucci, Jeff Urbach, Dan Blair, Walter Schwenger The viscosity of suspensions can be dramatically affected by high aspect ratio particles. Understanding these systems provides insight into key biological functions and can be manipulated for many technological applications. In this talk, the viscosity as a function of shear rate of suspensions of helical filaments is compared to that of suspensions of straight rod-like filaments. Our goal is to determine the impact of filament geometry on low volume fraction colloidal suspensions in order to identify strategies for altering viscosity with minimal volume fraction. In this research, the detached flagella of the bacteria \textit{Salmonella Typhimurium} are used as a model system of helical filaments and compared to mutated straight flagella of the \textit{Salmonella}. We compare rheological measurements of the suspension viscosity in response to shear flow and use a combination of the rheology and fluorescence microscopy to identify the microstructural changes responsible for the observed rheological response. [Preview Abstract] |
Tuesday, March 15, 2016 9:24AM - 9:36AM |
E36.00008: Holographic Characterization of Imperfect Spheres Mark Hannel, Christine Middleton, David Grier Holographic snapshots of colloidal spheres can be fit to Lorenz-Mie theory, yielding the radius, refractive index and position of individual colloids in situ. This procedure assumes that the scatterer is a uniformly dense ideal sphere. Via experimentation and simulation, we demonstrate that small deviations from ideal sphericity produce palatable errors (approximately 1{\%}) in our estimation of the particle's physical properties. [Preview Abstract] |
Tuesday, March 15, 2016 9:36AM - 9:48AM |
E36.00009: Thermo-responsive cross-linked liquid crystal bowl-shaped colloids Wei-Shao Wei, Yu Xia, Shu Yang, A. G. Yodh In this work we create and investigate cross-linked bowl-shaped nematic liquid crystal (NLC) colloidal particles. Janus colloids are first formed via solvent-induced phase separation in emulsions consisting of NLC monomers and isotropic polymers. This scheme enables us to realize different particle morphologies such as bowl-shape by fine-tuning the confinement of NLCs within the droplets, e.g. by varying the size of droplets, the volume ratio between NLC and polymer, and the type/concentration of surfactants in aqueous background phase. The NLC compartment is composed of RM82 (1,4-Bis-[4-(6-acryloyloxyhexyloxy)benzoyloxy]-2-methylbenzene) monomers, which are then photocrosslinked by dithiol groups to form nematic liquid crystal elastomer. Finally, we remove the polymer parts of Janus colloids to obtain the target structures, which are temperature sensitive due to change of elasticity and molecular alignment of NLC near the isotropic to nematic phase transition temperature. We will explore novel mechanical and optical properties from the thermo-responsive structures as well as their applications, such as biomimic swimming behaviors and adjustable lensing effects. [Preview Abstract] |
Tuesday, March 15, 2016 9:48AM - 10:00AM |
E36.00010: Simulating Electrophoresis with Discrete Charge and Drag Aaron J. Mowitz, Thomas A. Witten A charged asymmetric rigid cluster of colloidal particles in saline solution can respond in exotic ways to an electric field: it may spin or move transversely. These distinctive motions arise from the drag force of the neutralizing countercharge surrounding the cluster. Because of this drag, calculating the motion of arbitrary asymmetric objects with nonuniform charge is impractical by conventional methods. Here we present a new method of simulating electrophoresis, in which we replace the continuous object and the surrounding countercharge with discrete point-draggers, called Stokeslets. The balance of forces imposes a linear, self-consistent relation among the drag and Coulomb forces on the Stokeslets, which allows us to easily determine the object's motion via matrix inversion. By explicitly enforcing charge+countercharge neutrality, the simulation recovers the distinctive features of electrophoretic motion to few-percent accuracy using as few as 1000 Stokeslets. In particular, for uniformly charged objects, we observe the characteristic Smoluchowski independence of mobility on object size and shape. We then discuss electrophoretic motion of asymmetric objects, where our simulation method is particularly advantageous. [Preview Abstract] |
Tuesday, March 15, 2016 10:00AM - 10:12AM |
E36.00011: Dielectrophoresis force of colloidal nanoparticles Hao Huang, Daniel Ou-Yang Dielectrophoresis (DEP) is the motion of a polarizable colloidal particle in a nonuniform electric field. The magnitude of the DEP force is known to be proportional to the gradient of $E^{2}$. The DEP force also depends on the relative polarizability of the particle to that of the surrounding medium. Due to its ease of use, DEP has been proposed for a variety of applications to manipulate colloidal particles in a microfluidic setting. However, accurate measurements of the DEP force on colloidal nanoparticles are lacking. A new method is proposed to measure accurately the DEP potential force of colloidal nanoparticles by using confocal fluorescence imaging to determine the density distributions of dilute colloidal nanoparticle in a DEP potential force field. The DEP potential field can be calculated from the particle density distributions since the spatial distribution of the particle number density follows the Boltzmann distribution of the DEP potential energy. The validity of the measured DEP force is tested by examining the force as a function of the E field strength and particle size. The classic MaxwellWagnerO’Konski is found to be inadequate to fully describe the frequency dependence of the DEP force. [Preview Abstract] |
Tuesday, March 15, 2016 10:12AM - 10:24AM |
E36.00012: Extremely Small and Incredibly Fast Microscopy: 1 nm and 10 us dynamics in concentrated colloidal suspensions Brian Leahy, Matthew Bierbaum, Alexander Alemi, Itai Cohen, James Sethna Recently we developed PERI, a technique for locating colloidal sphere's positions and radii to within 1 nm from ordinary light microscopy images. PERI provides unprecedented access to the physics of colloidal suspensions at small length scales. We use this for high precision measurements of the pair-correlation function g(r) and colloidal interactions at 1-nm distances. Finally, we couple PERI with high-speed brightfield light microscopy to examine fast dynamics of concentrated suspensions. [Preview Abstract] |
Tuesday, March 15, 2016 10:24AM - 10:36AM |
E36.00013: Confocal light microscopy at 1 nm: Locating colloids at maximum resolution Matthew Bierbaum, Brian D. Leahy, Alexander A. Alemi, Itai Cohen, James P. Sethna We present PERI, a method to locate colloidal spheres at the information theoretic limit using a generative model for confocal microscope images. Without modification to the microscope we resolve positions and radii to 1 nm, which we verify with experimental data. Employing Monte Carlo techniques, we recover the probability distributions for all particle positions and radii, microscope point spread function, laser intensity fluctuation, scan parameters, and signal to noise ratio in a single fit. Using this technique we explore precision measurements of dense colloidal suspensions including standard quantities such as mean squared displacement and the pair correlation function. [Preview Abstract] |
Tuesday, March 15, 2016 10:36AM - 10:48AM |
E36.00014: Dissecting diffusive and advective motion in colloidal sedimentation by multi-speckle Ultra-Small-Angle XPCS Johannes Möller, Theyencheri Narayanan In colloidal suspensions internal or external fields can induce directed motions of particles in addition to Brownian diffusion. Here, gradients in temperature or chemical potential, shear flow as well as gravity can act as an external field. Examples for internal motions can be found in synthetic self-propelling particles and microorganisms, generally coined as active matter. We present multi-speckle X-ray photon correlation spectroscopy measurements in the Ultra-Small-Angle scattering range which probes an expanded length scale comparable to DLS and optical microscopy. To demonstrate the advanced capabilities, we show measurements probing the motions within a settling suspension of sub-micron sized silica particles. A global fitting procedure has been applied to separate the diffusive and advective contributions to the particle dynamics. With this, macroscopic parameters such as the sedimentation velocity can be probed on a microscopic level in highly opaque and concentrated systems, which are in general difficult to access for optical investigations. This procedure may prove its value for investigating various kinds of non-equilibrium systems. [Preview Abstract] |
Tuesday, March 15, 2016 10:48AM - 11:00AM |
E36.00015: Enthalpy versus Entropy: the Thermodynamic Origin of Hard Particle Ordering Mitchell Anthamatten, Shaw Chen, Jane Ou, Jeffrey Weinfeld The topic of hard particle ordering transitions is important in the development of molecular to mesoscale materials with potential applications in biomedicine, catalysis, optoelectronics, and renewable energy. The first step toward deterministic materials design rests on understanding the thermodynamic nature of ordering transitions involving two phases in equilibrium. We apply classical thermodynamics to show that (i) first-order, hard particle ordering transitions are forbidden at constant volume; and that (ii) hard-particle ordering is driven by a loss in enthalpy through volume reduction that outweighs a concomitant entropy loss upon ordering. The traditional approach considers minimization of Helmholtz energy, at constant volume, whereas the current study exclusively focuses on equilibrium phase transitions and, therefore, targets minimization of Gibbs energy at constant pressure. The Gibbs energy platform offers physically intuitive interpretations consistent with existing computation and experiments. The prevalent idea of entropy-driven ordering at constant V is restricted to transitions from non-equilibrium initial states that have yet to be properly defined for quantification. [Preview Abstract] |
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