Bulletin of the American Physical Society
APS March Meeting 2013
Volume 58, Number 1
Monday–Friday, March 18–22, 2013; Baltimore, Maryland
Session M30: Self-Assembly: Janus and other Colloids |
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Sponsoring Units: DCMP Chair: Stefano Sacana, New York University Room: 338 |
Wednesday, March 20, 2013 8:00AM - 8:12AM |
M30.00001: Directed Self-Assembly of Colloidal Janus Matchsticks Kundan Chaudhary, Qian Chen, Jaime Juarez, Steve Granick, Jennifer Lewis The ability to assemble anisotropic colloidal building blocks into ordered configurations is scientifically and technologically important for developing new classes of soft materials. We are studying the fabrication and electric field driven assembly of end- and side-coated Janus rods. Specifically, we fabricate silica rods (L/D $=$ 2-4) functionalized with hydrophobic gold (Au) patches using a multistep process involving electric field alignment and crystallization, microcontact printing, and selective metallization. In the absence of an applied electric filed, the Janus matchsticks (end-coated rods) self-assemble into multi pods (e.g., bi-, tri- and tetrapods) of varying coordination number and patch angle in aqueous solution. By contrast, both Janus matchsticks and side-coated Janus rods form complex chains in applied AC electric fields of varying magnitude and frequency, whose configurations vary significantly from those formed by pure silica rods. [Preview Abstract] |
Wednesday, March 20, 2013 8:12AM - 8:24AM |
M30.00002: Theory of crystallization and orientational ordering of spherical Janus colloids Homin Shin, Kenneth Schweizer Amphiphilic Janus particles have two chemically distinct surfaces, one hydrophobic (attractive) and the other hydrophilic (repulsive), resulting in orientationally anisotropic interparticle interactions. In contrast to homogeneous spherical particles, broken rotational symmetry can result in more exotic crystals that possess distinct orientational patterns, and also plastic crystals. We study the rich phase behavior of Janus colloids using a self-consistent phonon theory that includes coupled translational and rotational entropic and enthalpic contributions to the free energy. Ground states are identified based on the compatibility between the patch geometry of particles (e.g., patch coverage, number, shape) and the lattice symmetry. The coupled self-consistent equations for translational and rotational localization parameters are then solved for a given crystal symmetry, thermodynamic state, and patch orientational order, and their stability determined. For two-dimensional diblock AB Janus crystals, we predict the phase sequence of stripes, modulated stripes (zig-zag), and plastic crystals (rotator phases), which depends sensitively on particle chemical composition and pressure. We also study triblock Janus colloids, including the possibility a Kagome lattice. [Preview Abstract] |
Wednesday, March 20, 2013 8:24AM - 8:36AM |
M30.00003: Modeling of tunable structural re-configuration of Janus colloidal particles Daniel Beltran, Ronald Larson Colloidal particles can assemble into a myriad of structures by virtue of the many interaction forces available to them. Variable range attraction and repulsion and the recently explored non-isotropic character, exemplified by Janus particles, are examples of the versatility of colloidal particles as building blocks. A systematic approach to understand the assembly of Janus colloids, as a function of Janus balance and particle concentration is not yet available. In this work we study the phase behavior of Janus particles as a function of the strength of interaction, Janus balance and volume fraction of spherical particles. A secondary goal of this work is the assessment of re-configurability of the structures found. Our results show the range of stability of several structures, including a fluid phase of small clusters, bilayers and worm-like aggregates. We find the bilayer structures are very stable over a range of phase space and provide a good pre-cursor to hexagonally close-packed structures. These findings enable the understanding of the assembly process of Janus building blocks and provide a framework with which to study the kinetics of structure change. [Preview Abstract] |
Wednesday, March 20, 2013 8:36AM - 8:48AM |
M30.00004: Janus Magnetic Rods, Ribbons, and Rings Jing Yan, Kundan Chaudhary, Sung Chul Bae, Jennifer Lewis, Steve Granick Dipolar particles are fundamental building blocks in nature and technology but the roles of anisotropy are seldom explored in their assembly. Here, we fabricate colloidal silica rods coated on one hemicylinder with a thin magnetic layer to satisfy multiple criteria: nearly monodisperse, easily imaged, and magnetic interaction dominant over gravity. We confirm long-predicted features of dipolar assembly and stress the microstructural variety brought about by shape and chemical anisotropy, especially by borrowing knowledge learned from molecules. We describe analogies to liquid crystalline deformations with bend, splay and twist; an analogy to cis/trans isomerism in organic molecules, which in this system can be controllably and reversibly switched; and a field-switching methodology to direct single ribbons into not only single but also multiple rings that can subsequently undergo hierarchical self-assembly. Going beyond earlier investigations of phase behavior, we show that dynamic reconfigurability presents subtle materials issues and possibilities. [Preview Abstract] |
Wednesday, March 20, 2013 8:48AM - 9:00AM |
M30.00005: Study of Aggregation of Janus Ellipsoids Donovan Ruth, Wei Li, Shreeya Khadka, Jeffrey Rickman, James Gunton We perform numerical simulations of a quasi-square well potential model of one-patch colloidal particles to investigate the collective structure of a system of Janus ellipsoids. We show that for Janus ellipsoids such that one half is an attractive patch, while the entire ellipsoid has a hardcore repulsion, the system organizes into a distribution of orientationally ordered micelles and vesicles. We analyze the cluster distribution at several temperatures and low densities and show that below certain temperatures the system is populated by stable clusters and depending on temperature and density the system is populated by either vesicles or micelle structures. [Preview Abstract] |
Wednesday, March 20, 2013 9:00AM - 9:12AM |
M30.00006: Amphiphilic Janus cylinders at fluid-fluid interfaces Daeyeon Lee, Bum Jun Park, Chang-Hyung Choi, Chang-Soo Lee We study the configuration and assembly of amphiphilic Janus cylinders at fluid-fluid interfaces at the single- and two-particle levels using experimental and theoretical approaches. We observe that high aspect ratio Janus cylinders have two configurations -- upright and tilted orientation, whereas Janus cylinders with small aspect ratios adopt only the upright orientation. These configurations are confirmed by numerically calculating and minimizing the attachment energy of each Janus cylinder as a function of the orientation angle as well as the vertical displacement with respect to the interface. Unlike homogenous cylinders which show deterministic assembly behaviours at fluid-fluid interfaces, Janus cylinders exhibit a variety of assembly behaviours. We show the origin of such a diversity stems from the attractive capillary interactions between tilted Janus cylinders, which could be explained by the quasi-quadrupolar interface deformation that is caused by the wetting of the fluids on the particle surface. We will also describe our recent results involving the configuration and interactions of asymmetrically hydrophilic cylinders at an air-water interface. [Preview Abstract] |
Wednesday, March 20, 2013 9:12AM - 9:24AM |
M30.00007: Thermodynamically Stable Pickering Emulsions Stabilized by Janus Dumbbells Fuquan Tu, Bum Jun Park, Daeyeon Lee Janus particles have two sides with different, often opposite, surface properties. Janus dumbbell is one type of Janus particles that consists of two partially fused spherical lobes. It is possible to independently control the geometry and surface wettability of Janus dumbbells. Janus dumbbells can also be produced in a large quantity, making them useful for practical applications such as emulsion stabilization. In this work, we calculate the free energy of emulsion formation using amphiphilic Janus dumbbells as solid surfactants. In contrast to kinetically stable emulsions stabilized by homogeneous particles, emulsion stabilized by Janus dumbbells can be thermodynamically stable. There also exists an optimal radius of droplets that can be stabilized by infinite or limited number of amphiphilic dumbbells in the continuous phase. We demonstrate that the optimal radius of dumbbell-stabilized droplets can be predicted based on the volume of the dispersed phase and the volume fraction of dumbbells in the continuous phase. We believe our calculation will provide guidelines for using Janus dumbbells as colloid surfactants to generate stable emulsions. [Preview Abstract] |
Wednesday, March 20, 2013 9:24AM - 9:36AM |
M30.00008: Analytic Solutions and Numerical Simulation of Self-Assemble Magnetic Colloidal Structures David Piet, Igor Aronson, Alex Snezhko, Athur Straube Self-assembled magnetic colloidal structures that lie at a fluid-air interface can have a wide range of behavior, from localized axisymmetric star-like objects to linear, snake-like ones. Modeling these structures requires both the extensive use of the Navier-Stokes Equations from an analytic standpoint as well as the ability to numerically solve and simulate them alongside Newton's Equations. Analytically, these equations are approximated by an asymptotic expansion with a small viscosity. Using those expressions, simulations are run on GPU's to utilize their parallel capability. The result is a remarkable, qualitative recapturing of the experimentally observed behavior, namely, the formation of both snakes and stars from a randomized initial condition. [Preview Abstract] |
Wednesday, March 20, 2013 9:36AM - 9:48AM |
M30.00009: Dynamic phases in non-equilibrium magnetic colloids at liquid interfaces under in-plane magnetic field driving Alexey Snezhko, Gasper Kokot, David Piet, Igor Aranson Ensembles of interacting colloidal particles subject to an external periodic forcing often develop nontrivial collective behavior. We study emergent phenomena in magnetic colloidal ensembles suspended at a liquid-air interface and driven out of equilibrium by alternating magnetic fields. We use ferromagnetic colloidal micro-particles (so the magnetic moment is fixed in each particle and interactions between colloids is highly anisotropic and directional) suspended over a water-air interface and energized by alternating magnetic fields applied in-pane of the interface. Experiments reveal new types of dynamic self-assembled phases (in particular, ``wires,'' ``rotators'') emerging in such systems in a certain range of excitation parameters. Transition between different self-assembled phases with parameters of external driving magnetic field is observed. Molecular dynamic simulations captures some of the non-equilibrium self-assembled phases in our system. [Preview Abstract] |
Wednesday, March 20, 2013 9:48AM - 10:00AM |
M30.00010: Strictly Polyhedral Colloids Challenged by Electric Field Nobuhiro Yanai, Melinda Sindoro, Jing Yan, Steve Granick We have succeeded in fabricating monodisperse polyhedral metal-organic framework (MOF) crystals. Here, the micron-sized rhombic dodecahedra are suspended in liquid as candidates for directed self-assembly. The application of AC electric field is found to produce assembly at various facets truncations, probably owing to induced dipole attraction, with linear chaining that we observe and analyze based on direct in-situ imaging. The facet-to-facet preference during assembly produces striking selectivity for these1D chains. [Preview Abstract] |
Wednesday, March 20, 2013 10:00AM - 10:12AM |
M30.00011: Crystalline aggregates of magnetic colloidal particles Joshua E.S. Socolar, Catherine C. Marcoux, Lin Fu, Patrick Charbonneau, Ye Yang, Benjamin B. Yellen A colloidal system of magnetic and non-magnetic spheres confined to a 2D monolayer has been found to form a variety of structures, including Kagome, honeycomb, and square lattices, as well as various chain and ring configurations [1]. In these experiments, the layer of beads is immersed in a ferrofluid and placed in an external magnetic field and the different structures are obtained for different values of the relative concentrations of the bead types, the susceptibility of the ferrofluid, and the angle of the field with respect to the assembly plane. We study an approximate model for the potential energy of the system based on self-consistent solutions for the magnetic moments of point dipoles. We find that the model accounts well for the stability of the observed phases and we identify additional possible phases via a genetic algorithm that searches for crystal structures with up to ten atoms per unit cell. Further calculations suggest the possibility of creating materials with strong elastic responses to applied magnetic fields.\\[4pt] [1] K. S. Khalil, A. Sagastegui, Y. Li, M. A. Tahir, J. E. S. Socolar, B. J. Wiley, and B. B. Yellen, {\it Nat. Comm.}, {\bf 3}, Article number: 794 (2012). [Preview Abstract] |
Wednesday, March 20, 2013 10:12AM - 10:24AM |
M30.00012: Using chaotic Faraday waves to create a two-dimensional pseudo-thermal bath for floating particles with tunable interaction potentials Kyle Welch, Isaac Hastings-Hauss, Raghuveer Parthasarathy, Eric Corwin Whether chaos in actively driven systems can be described by an effective temperature is an unresolved question in the study of nonlinear physics. We use chaotic Faraday waves to create a two-dimensional pseudo-thermal bath to investigate tunable interactions between floating particles. By vertically oscillating a liquid with an acceleration greater than $g$ we excite the Faraday instability and create surface waves. Increasing this acceleration above some critical value causes this instability to become chaotic with fluctuations over a broad range of length scales. Particles placed on the surface are buffeted by random excitations in analogy to Brownian motion. We can change the ``temperature'' of the pseudo-thermal bath by manipulating the driving frequency and amplitude, a feature of the system we verify using real-time tracking to follow the diffusive movement of a single particle. With an eye toward creating complex self-assembling systems we use this system to measure the tunable interaction potential in two-, three-, and many-particle systems and to probe the effects of particle size, shape, symmetry, and wetting properties. [Preview Abstract] |
Wednesday, March 20, 2013 10:24AM - 10:36AM |
M30.00013: Twisted results on interior packing and surface energy for filament bundles Isaac Bruss, Gregory Grason Twisted filament bundles are a common structural motif found in both natural and synthetic systems. Examples range from protein assemblies such as collagen and fibrin, to artificial structures such as carbon nanotube ropes and microfabricated materials. They are oftentimes found to self-assemble from fibers via various adhesive interactions, be they depletion, capillary, or other such forces. In these assemblies, twist frustrates the perfect crystalline order of the fibers, requiring the presence of defects in the packing. Through numerical simulations, we discover defect organizations ranging from dislocations and grain boundaries for low twist, to multi-five-fold disclination clusters for high twist. And furthermore, by developing and employing an analytical continuum model, we find that for sufficiently long fibers, twist reduces the surface energy of the assembly. Together, this suggests that the equilibrium lowest energy state of a filament bundle may be twisted regardless of any intrinsic chirality present in the system. [Preview Abstract] |
Wednesday, March 20, 2013 10:36AM - 10:48AM |
M30.00014: Kinetics of Phase Separation in Binary Mixtures Shaista Ahmad, Subir K. Das, Sanjay Puri We present numerical simulation results of the phase separation kinetics in three-dimensional symmetric binary fluid mixtures and binary solid mixtures. In the former system, our extensive molecular dynamics simulation is able to probe an extended period where the domain size grows linearly with time, leading to an unambiguous confirmation of the viscous hydrodynamic regime. On the other hand, for the binary solid mixture, we use Monte Carlo simulation with spin-exchange dynamics to verify the Lifshitz-Slyzov growth law. In spite of the differences in the growth mechanisms, the pair correlation functions and structure factors of the two systems overlap, indicating similarity in the morphologies during phase separation. [Preview Abstract] |
Wednesday, March 20, 2013 10:48AM - 11:00AM |
M30.00015: Cooperative Symmetry Breaking from One to Three Dimensions in Multi-Component Double Emulsions Laura Adams, Jacy Bird, Jaiwei Yang, Thomas Franke, Vinothan Manoharan, David Weitz We follow the evolution of aqueous inner drops confined in a thin sheath of oil in the dimensional crossover from one to three dimensions using a fast camera and microfluidics. Surprisingly, inner drops interact cooperatively to pair with their next nearest neighbor to transform their linear configuration into a three dimensional composite sphere. The measured time scales of transforming these multi-component double emulsions are investigated as a function of number, size, and composition of inner drops. We model the dynamics to understand and predict how both folding and buckling occur in these complex microfluidic systems. [Preview Abstract] |
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