Bulletin of the American Physical Society
APS March Meeting 2014
Volume 59, Number 1
Monday–Friday, March 3–7, 2014; Denver, Colorado
Session W18: Colloidal Particles at Interfaces |
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Chair: Matthew Lohr, University of Pennsylvania Room: 403 |
Thursday, March 6, 2014 2:30PM - 2:42PM |
W18.00001: Solids under Stress: Lessons from Simple Problems of Elastic String Depinning Stefanos Papanikolaou When stress is applied on solid structures, deformation ultimately becomes permanent/plastic. Plasticity at the mesoscale proceeds through abrupt events, naturally resembling the jumps that a rubber band makes when driven through a landscape of pins. Elastic string depinning has been a very useful analogy for elucidating the statistical character of either crystalline or amorphous plasticity. Recent experiments and simulations, however, point out that the analogy is not complete, as naturally expected: In crystals, plasticity-mediating dislocation defects may ``jump'' through multiple slow relaxation channels beyond the fast gliding one. In amorphous solids, the ``pins'' for each particle have dynamics that may not be neglected since they correspond to neighboring particles. For each case, I will describe a generalized depinning model that aims to minimally include the additional physical mechanism and then compare to recent experiments and simulations. These simple depinning models suggest that plasticity of solids may cross-over to a state of stochastic relaxation oscillations through a mechanism that resembles a singular Hopf bifurcation. [Preview Abstract] |
Thursday, March 6, 2014 2:42PM - 2:54PM |
W18.00002: Hysteresis of the Contact Angle around Spheres Adsorbed at Curved Fluid Interfaces Nesrin Senbil, Wei He, Benjamin Davidovitch, Anthony Dinsmore When a particle adsorbs to a fluid interface, the geometry of the contact between the interface and the particle determines the force acting on the particle. We find a significant hysteresis in the contact angle, and --surprisingly-- a strong dependence of the hysteresis on the shape of the interface. Hysteresis in the wetting of a fluid on a flat substrate is well known, whereby two contact angles are typically defined, corresponding to the advancing and receding cases. We find that the receding angle around the sphere changes with the shape of the interface. We use millimeter-sized glass spheres coated with PDMS and adsorbed at an air-water interface. High-resolution images are analyzed to obtain the contact geometry as the spheres are raised or lowered across the interface. We find advancing contact angles of approximately 107$^{\mathrm{o}}$ and receding angles that range between 90$^{\mathrm{o}}$ and 97$^{\mathrm{o}}$ depending on the interface shape. Our results are important for understanding interactions between particles at interfaces and may shed new light on the origin of contact-angle hysteresis. This work is funded by the NSF through CBET-0967620 and by the Gulf of Mexico Research Initiative through the C-MEDS consortium. [Preview Abstract] |
Thursday, March 6, 2014 2:54PM - 3:06PM |
W18.00003: Squares of spheres: capillarity-induced ordering of spherical colloids on an interface with anisotropic curvature Jasper Van der Gucht, Dmitry Ershov Objects floating at a liquid interface, such as breakfast cereals floating in a bowl of milk or bubbles at the surface of a soft drink, clump together in space-saving hexagons to minimize the disruption of the liquid interface. Micrometer-sized colloidal particles embedded in a liquid interface normally do not disrupt the interface, so that such clustering does not occur. Here, we show that this is different when the interface has a curvature that is anisotropic. We find that in this case the condition of constant contact angle along the three-phase contact line can only be satisfied when the interface is deformed. We present experiments and numerical calculations that demonstrate how this leads to quadrupolar capillary interactions between the particles, giving rise to organization into regular square lattices. We demonstrate that the strength of the governing anisotropic interactions can be rescaled with the deviatoric curvature alone, irrespective of the exact shape of the liquid interface. Our results suggest that anisotropic interactions can easily be induced between isotropic colloids through tailoring of the interfacial curvature. [Preview Abstract] |
Thursday, March 6, 2014 3:06PM - 3:18PM |
W18.00004: Dynamics of 2D Colloidal Crystals Under Microscopic Shear Colm Kelleher, Paul Chaikin Since the early 1980's, 2D colloidal crystals have been used as model systems for studying a variety of basic problems in condensed matter physics - for instance, the KTHNY theory of melting, and the ``Thomson problem'' of finding the ground states of crystals in curved space. However, many non-equilibrium phenomena, such as the response of these crystals to external forces, remain poorly understood. We study systems of 2D colloidal crystals which are formed when charged PMMA microspheres bind to a flat oil-water interface. Using optical tweezers, we apply forces to individual particles, or selected groups of particles, in the crystal lattice. These forces can be precisely controlled in time, space and intensity. We then use video microscopy to study defect formation and dynamics in the crystal. We are particularly interested in the issue of reversibility of dislocation dynamics - as shown recently [Irvine et al. \textit{PNAS} \textbf{2013} 110 (39)], simple dislocation-dislocation interactions tend to be reversible, while more complex, many-dislocation interactions tend to be irreversible. This talk will discuss the above topics in the context of an experiment where the crystal was sheared periodically between two parallel rows of optically trapped colloids. [Preview Abstract] |
Thursday, March 6, 2014 3:18PM - 3:30PM |
W18.00005: Crowding and Ordering in the Assembly of Nanoparticles at Liquid Interfaces Konrad Schwenke, Lucio Isa, Emanuela Del Gado Experiments with self-assembly of nanoparticles at liquid interfaces suggest that cooperative and slow dynamical processes due to particle crowding at the interface govern the adsorption and properties of the final assembly [1]. We report a numerical approach to study non-equilibrium adsorption, which elucidates these experimental observations. The analysis of particle rearrangements shows that local ordering processes are directly related to adsorption events at high interface coverage. Interestingly, this feature and the mechanism coupling local ordering to adsorption do not seem to change qualitatively upon increasing particles size polydispersity, although the latter changes the interface microstructure and its final properties. Our results indicate how adsorption kinetics can be used for the fabrication of two-dimensional nano-composites with controlled microstructure. \newline \newline [1] L. Isa, E. Amstad, K. Schwenke, E. Del Gado, P. Ilg, M. Kroger and E. Reimhult, Soft Matter, 2011, 7, 7663-7675.\newline [2] K. Schwenke, L. Isa and E. Del Gado, Assembly of nanoparticles at liquid interfaces: Crowding and ordering, submitted [Preview Abstract] |
Thursday, March 6, 2014 3:30PM - 3:42PM |
W18.00006: Ordering in Conformal Crystals Vishal Soni, Leopoldo Gomez, William Irvine Condensed matter systems commonly undergo ordering processes that are frustrated by geometric constraints. 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 flat colloidal Wigner crystals immersed in an axially symmetric potential. By relating the resulting inhomogenous structure to a lattice with Gaussian curvature, we investigate the role of topological defects in organizing the conformal crystal-like ground state. [Preview Abstract] |
Thursday, March 6, 2014 3:42PM - 3:54PM |
W18.00007: Dynamic Regimes for Driven Colloidal Particles on a Periodic Substrate at Commensurate and Incommensurate Fillings Danielle McDermott, Jeffery Amelang, Cynthia Reichhardt, Charles Reichhardt We numerically examine colloidal particles driven over a muffin tin substrate. Previous studies of this model identified a variety of commensurate and incommensurate static phases in which topological defects can form domain walls, ordered stripes, superlattices, or disordered patchy regimes as a function of the filling fraction. Here we show that the addition of an external drive to these static phases can produce distinct dynamical responses. At incommensurate fillings the flow occurs in the form of localized pulses or solitons correlated with topological defect structures. Transitions between different modes of motion can occur as a function of increasing drive. We measure the average particle velocity for specific ranges of external drive and show that changes in the velocity response correlate with changes in the topological defect arrangements. We also demonstrate that in the different dynamic phases, the particles have distinct trajectories and velocity distributions. Dynamic transitions between ordered and disordered flows exhibit hysteresis, while in strongly disordered regimes there is no hysteresis and the velocity-force curves are smooth. When stripe patterns are present, transport can occur at an angle to the driving direction. [Preview Abstract] |
Thursday, March 6, 2014 3:54PM - 4:06PM |
W18.00008: Orientation Waves as the Order Parameters for Crystals Rolfe Petschek Even rather simple particle-particle interactions can result in very complicated crystal structures. Simple Landau or density theories that use particle density as the primary order parameter generically suggest that crystal structures should generically be simple -- e.g. bcc, particularly if the interactions are short ranged and uncomplicated. Convincing evidence is presented that the order parameters for the complicated crystal that forms in the crystallization of hard tetrahedra is an orientation wave: dependent on the periodic ordering of a third and sixth rank traceless symmetric tensor rather than a density is presented. A simple Landau / density-like theory presentation of such orientation waves is presented. Such orientation waves are known to be order parameters for the blue phases which have crystalline symmetry, in which a second rank tensor varies periodically in space. It is argued that the complicated nature of the Landau theory of high rank tensor order parameters makes it plausible that they result in complicated crystals and quasicrsystals. This, in turn, suggests that complicated crystals and quasicrystals have high rank tensor order parameters. [Preview Abstract] |
Thursday, March 6, 2014 4:06PM - 4:18PM |
W18.00009: Colloidal diffusion over a periodic energy landscape Xiao-guang Ma, Pik-Yin Lai, Penger Tong A two-layer colloidal system is developed for the study of colloidal diffusion over a two-dimensional periodic energy landscape. The energy landscape is made from the bottom layer of colloidal spheres forming a honey-comb crystalline pattern above a glass substrate. The corrugated surface of the bottom colloidal crystal provides a gravitational potential field for the diffusing particles in the top layer. The obtained population probability histogram $P(x,y)$ of the diffusing particles is used to fully characterize the energy landscape $U(x,y)$ via the Boltzmann distribution. The dynamical properties of the diffusing particle, such as its escape time $t_R$ and diffusion coefficient $D$ are simultaneously measured from the particle's trajectories. The long-time diffusion coefficients $D$ is found to be in good agreement with the theory for all colloidal samples studied. The experiment demonstrates the applications of this newly constructed colloidal energy landscape. *Work supported in part by the Research Grants Council of Hong Kong SAR. [Preview Abstract] |
Thursday, March 6, 2014 4:18PM - 4:30PM |
W18.00010: Hydrodynamic damping of collective motion in a quasi-two-dimensional dense colloidal particle suspension Michael Ryan, Tim Still, Arjun Yodh, Kevin Aptowicz Dense colloidal suspensions confined to a monolayer are often used to explore physical phenomena such as the glass transition, crystallization, and frustration. Although hydrodynamic damping is known to play a significant role in the dynamics of these systems, it is difficult to quantify due to the collective nature of the particle motions. In this work, we employ digital video-microscopy to explore the phonon dynamics of an entropic 2D colloidal crystal. Friction coefficients along high symmetry directions in q-space are extracted and provide insight about the hydrodynamic forces at play. Preliminary results suggest the friction coefficient decreases with increasing phonon wavelength, but it does not appear to vanish.~ [Preview Abstract] |
Thursday, March 6, 2014 4:30PM - 4:42PM |
W18.00011: 2D Melting in General: Solid/hexatic/liquid Phase Transitions in Soft Spheres using Event-Chain Monte Carlo Sebastian C. Kapfer, Manon Michel, Werner Krauth The melting of two-dimensional solids has been the subject of continued research for more than fifty years, with the prevalent scenarios being the KTHNY theory of defect unbindung and a conventional first-order liquid/solid transition. For hard disks, a rather unexpected hybrid transition has recently been found with both a first-order transition and an intermediate hexatic phase [1], while magnetic colloid experiments support the KTHNY scenario [2]. To resolve this discrepancy, we here address the melting problem for soft interaction potentials, in particular the nature of the liquid/hexatic and hexatic/solid transitions, and the defects driving melting. Simulations were performed using a new rejection-free irreversible Monte Carlo algorithm generalizing event-chain Monte Carlo to arbitrary pair potentials. In addition to fast equilibration, this algorithm permits to deduce the pressure in the NVT ensemble without any additional computations [3]. References: [1] E. P. Bernard and W. Krauth, Phys. Rev. Lett. 107, 155704 (2011). [2] P. Keim et al. Phys. Rev. Lett. 92, 215504 (2004). [3] M. Michel, S. C. Kapfer and W. Krauth, preprint at arXiv:1309.7748. [Preview Abstract] |
Thursday, March 6, 2014 4:42PM - 4:54PM |
W18.00012: ABSTRACT WITHDRAWN |
Thursday, March 6, 2014 4:54PM - 5:06PM |
W18.00013: A field-directed colloidal phase transition Eric Furst, James Swan, Jonathan Bauer Suspensions of polarizable colloids are expected to form crystalline equilibrium phases when exposed to a steady, uniform field. However, when colloids become localized this field-induced phase transition arrests and the suspension persists indefinitely as a kinetically trapped, percolated structure. We show that by toggling the applied field on and off gels formed in MR fluids can be annealed. There is a stark boundary as a function of magnetic field strength and toggle frequency that distinguishes arrested states from phase separation. A key advantage of self-assembly in toggled fields is the relatively large range of field-strengths (effective temperatures) that lead to phase separation. Finally, we demonstrate that such directed self-assembly can be used to create colloidal crystals of uniform size. These results demonstrate how kinetic barriers to a colloidal phase transition are subverted through measured, periodic variation of driving forces. [Preview Abstract] |
Thursday, March 6, 2014 5:06PM - 5:18PM |
W18.00014: The role of hydrodynamic forces in the confinement and assembly of magnetic dipoles M. Prikockis, A. Chen, R. Sooryakumar The confinement of interacting magnetic dipoles provides a means to probe the assembly of and many-body coupling within a mesoscopic system. Using a previously developed confinement method (Scientific Reports 3, 3124 (2013)), we investigate the role of hydrodynamic forces in one such mesoscopic system that supports a fluid borne suspension of microscopic beads that contain embedded superparmagnetic particles. Our confinement platform consists of a thin permalloy disk patterned on a silicon surface and a precessing magnetic field. By adjusting the orientation of the field, inter-particle dipolar and trap confinement forces are tuned - thereby enabling the plane-confined beads to repel or attract one another. At a specific field orientation, the dipolar interaction is weakened to provide a regime where the hydrodynamic forces, stemming from rotational motion of the beads, play a role in bead assembly. We investigate the dependence of dipole ordering on the hydrodynamic forces by varying the frequency of the field rotation in this special field configuration. This represents a unique system where the hydrodynamic forces of fluid borne magnets are tuned independently of the magnetic forces in a magnetic dipolar confinement scheme. [Preview Abstract] |
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