Bulletin of the American Physical Society
APS March Meeting 2017
Volume 62, Number 4
Monday–Friday, March 13–17, 2017; New Orleans, Louisiana
Session H17: Out of Equilibrium Colloids and Gels |
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Sponsoring Units: GSOFT Chair: Jennifer Schwarz, Syracuse University Room: 276 |
Tuesday, March 14, 2017 2:30PM - 2:42PM |
H17.00001: Unstable colloidal rollers: a new kind of fingering instability Michelle Driscoll, Blaise Delmotte, Mena Youssef, Stefano Sacanna, Aleksandar Donev, Paul Chaikin When colloidal particles are rotated adjacent to nearby floor, strong advective flows are generated around them, even quite far away. When a group of these microrollers is driven, the strong hydrodynamic coupling between particles leads to formation of new structures: an initially uniform front of microrollers evolves first into a shock-like structure, which then quickly becomes unstable, emitting fingers of a well-defined wavelength. Our experiments and simulations confirm that this instability is quite different than typical fingering instabilities, where size scale selection is a consequence of competing stresses. Here, this instability arises only due to hydrodynamic interactions, and it is controlled by a single geometric parameter, the particle-floor height. Our measurements of the growth rate in both experiments and simulations agree with results from our continuum model. This instability is a direct consequence of the inward flows created by the interactions between the particles and the nearby solid surface. [Preview Abstract] |
Tuesday, March 14, 2017 2:42PM - 2:54PM |
H17.00002: The surprising strength of hydrodynamic attraction Blaise Delmotte, Michelle Driscoll, Aleksandar Donev, Paul Chaikin Self-assembly in colloidal systems often requires finely tuning the interactions between particles. When colloids are active, or moving due to an external drive, the assembly is even harder to achieve. In this talk, we will focus on an experimental system made of colloids rotating parallel to a floor. We will show how hydrodynamic interactions alone can generate compact motile structures. These structures are persistent, robust to thermal diffusion and are an attractor of the system. Combining experiments, large scale simulations and theory, we will explain the origin of such hydrodynamic attraction and how it could be used for practical applications. [Preview Abstract] |
Tuesday, March 14, 2017 2:54PM - 3:06PM |
H17.00003: Effect of nanoparticle surfactants on the Plateau-Rayleigh instability. Anju Toor, Thomas Russell, Brett Helms Nanoparticle (NP) surfactant systems consists of nanoparticles dispersed in one liquid, and polymers with a complementary functionality dispersed in another immiscible fluid. Particles and polymers interact at a liquid-liquid interface to form NP surfactants. These surfactants could enable the generation of structured fluids using microfluidic methods based on Plateau and Rayleigh instabilities. We have investigated the interfacial tension of an aqueous dispersion of carboxylated Silica nanoparticles suspended in a toluene solution of amine-terminated Polydimethylsiloxane. The interfacial tension undergoes significant reduction due to the formation of NP surfactants. The effect of NP surfactants on the breakup of a laminar water jet in oil phase is investigated. As the aqueous suspension of nanoparticles is drawn from an orifice into a solution of amine-terminated polymer, NP surfactants will form at the oil-water interface. Experiments with water-oil systems with and without NP surfactants have been performed, varying fluid flow rate, jet diameter, and NP surfactant concentration. NP surfactants were found to significantly affect the breakup of laminar liquid jets resulting in longer jet breakup lengths and dripping to jetting phase transitions. [Preview Abstract] |
Tuesday, March 14, 2017 3:06PM - 3:18PM |
H17.00004: Structural Transitions and Hysteresis in Clump and Stripe-forming Colloids Under Dynamic Compression Danielle McDermott, Cynthia J. Olson Reichhardt, Charles Reichhardt Using numerical simulations, we study the dynamical evolution of particles interacting via competing long-range repulsion and short-range attraction in 2D. The particles are compressed using a time-dependent quasi-one dimensional trough potential that controls the local density, causing a series of structural phase transitions from a low density clump lattice to stripes, voids, and a high density uniform state. The compression proceeds via slow elastic motion that is interrupted with avalanche-like bursts of activity as the system collapses via plastic rearrangements. The plastic events vary in magnitude from small rearrangements of particles, including the formation of quadrupole-like defects, to large-scale vorticity and structural phase transitions. In the dense uniform phase, the system compresses through row reduction transitions mediated by a disorder-order process. We characterize the rearrangement events by measuring changes in the energy, the fraction of sixfold coordinated particles, the local density, and the velocity distribution. At high confinements, we find power law scaling of the velocity distribution during row reduction transitions. We observe hysteresis under a reversal of the compression when relatively few plastic rearrangements occur. [Preview Abstract] |
Tuesday, March 14, 2017 3:18PM - 3:30PM |
H17.00005: Probing active turbulence by self-assembled spinners Gasper Kokot, Igor Aronson, Alex Snezhko Active magnetic colloidal systems are driven locally by a global energizing field. They exhibit not only a wealth of directed collective behavior, but also regimes where the collective motion is on average non-directional, which gives rise to an active (self-driven) diffusion. We exploit a system of Ni microparticles suspended at the water-air interface and subject to a uniaxial oscillating magnetic field applied along the interface. The self-assembled spinners, emerging as a result of spontaneous symmetry breaking of self-assembled chainsâ€™ rotations, generate a local flow vorticity leading to strong quasi-two dimensional flows in the container. Experiments reveal the dependence of the diffusion constant on the frequency of the driving magnetic field, active particle density and tracer size, the last being, intriguingly, non-monotonic. Erratic motion of spinners in the container results in highly disordered and non-periodic flow velocity field which we show to be of a turbulent-like nature despite the low Reynolds number ($Re \sim 30$) associated with the spinners. [Preview Abstract] |
Tuesday, March 14, 2017 3:30PM - 3:42PM |
H17.00006: Bubbles are responsive materials interesting for nonequilibrium physics Daria Andreeva, Steve Granick Understanding of nature and conditions of non-equilibrium transformations of bubbles, droplets, polysomes and vesicles in a gradient filed is a breath-taking question that dissipative systems raise. We ask: how to establish a dynamic control of useful characteristics, for example dynamic control of morphology and composition modulation in soft matter. A possible answer is to develop a new generation of dynamic impactors that can trigger spatiotemporal oscillations of structures and functions. We aim to apply acoustic filed for development of temperature and pressure oscillations at a microscale area. We demonstrate amazing dynamic behavior of gas-filled bubbles in pressure gradient field using a unique technique combining optical imaging, high intensity ultrasound and high speed camera. We find that pressure oscillations trigger continuous phase transformations that are considered to be impossible in physical systems. [Preview Abstract] |
Tuesday, March 14, 2017 3:42PM - 3:54PM |
H17.00007: Homogenizing a viscous suspension without fine tuning Jikai Wang, Jennifer Schwarz, Joseph Paulsen Particle suspensions, present in many natural and industrial settings, typically contain aggregates and other microstructures. The breaking up of such inhomogeneities can ease processing demands. Recent work has shown that applying uniform periodic shear near a critical transition can lead to an extremely homogeneous spatial distribution of particles. However, this strategy requires fine-tuning of the strain amplitude. Here we explore a model of sedimenting particles under periodic shear. Previous work [1] found that for particles that sink slower than a finite threshold speed, the system is automatically driven towards a critical state. Our simulations and scaling arguments reveal a different phase boundary this behavior. Within the slow-sedimentation regime, we show that the critical state is robust at homogenizing the suspension. [1] Corte et al., Phys. Rev. Lett. 103, 248301 (2009). [Preview Abstract] |
Tuesday, March 14, 2017 3:54PM - 4:06PM |
H17.00008: Lattice-Boltzmann-based simulations of diffusiophoresis of colloids and cells Jennifer Kreft Pearce, Joshua Castigliego Increasing environmental degradation due to plastic pollutants requires innovative solutions that facilitate the extraction of pollutants without harming local biota. We present results from a lattice-Boltzmann-base Brownian Dynamics simulation on diffusiophoresis and the separation of particles within the system. A gradient in viscosity that simulates a concentration gradient in a dissolved polymer allows us to separate various types of particles based on their deformability. As seen in previous experiments, simulated particles that have a higher deformability react differently to the polymer matrix than those with a lower deformability. Therefore, the particles can be separated from each other. The system described above was simulated with various concentration gradients as well as various Soret coefficients in order to optimize the separation of the particles. This simulation, in particular, was intended to model an oceanic system where the particles of interest were motile and nonmotile plankton and microplastics. The separation of plankton from the microplastics was achieved. [Preview Abstract] |
Tuesday, March 14, 2017 4:06PM - 4:18PM |
H17.00009: Mapping the viscoelastic response of hydrogels at the nanometer scale. David Haviland, Per-Anders Thoren, Riccardo Borgani, Daniel Forchheimer, Daniel Platz, Illia Dobryden, Per Claesson We demonstrate a powerful new method for high-resolution mapping of the viscoelastic response of soft material surfaces at the nanometer scale. Dynamic Atomic Force Microscopy is performed with a special multi-frequency lockin amplifier that captures very high order intermodulation distortion of the cantilever motion, resulting from the nonlinear tip-surface interaction. Frequency domain analysis of this distortion reveals the conservative and dissipative forces between the tip and the surface, giving detailed information about the nonlinear interaction. We describe a new type of interaction model that treats the motion of the tip and surface as a dynamic two-body problem [1]. The model works extremely well with a wide variety of soft materials. Comparing simulations of this model to experimental data, we extract the viscous and elastic force coefficients on the surface of hydrogels, revealing heterogeneity at the nanometer scale. [1] D. B. Haviland et al. Soft Matter, DOI: 10.1039/c5sm02154e [Preview Abstract] |
Tuesday, March 14, 2017 4:18PM - 4:30PM |
H17.00010: Shear-Induced Heterogeneity in Associating Polymer Gels Ahmad Omar, Zhen-Gang Wang We study associating polymer gels under steady shear using Brownian dynamics simulation to explore the interplay between the network structure, dynamics and rheology. For a wide range of flow rates, we observe the formation of shear bands with a pronounced difference in shear rate, concentration and structure. This shear-concentration coupling is shown to result from a striking increase in the polymer pressure in the gradient direction along with the inherently large compressibility of the gels. We find that shear has only a modest influence on the degree of association, but induces marked spatial heterogeneity in the network connectivity. We attribute the increase in the polymer pressure (and polymer mobility) to this structural reorganization. [Preview Abstract] |
Tuesday, March 14, 2017 4:30PM - 4:42PM |
H17.00011: Shear-induced rejuvenation and overaging of laponite clay suspensions Michael Rogers, Kui Chen, Robert Leheny, James Harden Disordered soft solids have elaborate shear-induced mechanical properties that are connected to their complex hierarchical structure from nano- to micrometer scales. The nonequilibrium nature of many disordered soft solids makes identifying and understanding these connections difficult: their properties depend not only on thermodynamic conditions, but also on their history. For example, laponite suspensions are well-known to exhibit characteristic mechanical aging. Above a critical concentration, they evolve into gel phases with relaxation times that grow with age. We have studied aging in laponite suspensions using X-ray Photon Correlation Spectroscopy (XPCS). This technique involves auto-correlating characteristic speckle patterns from coherent x-ray scattering to uncover collective particle dynamics. Laponite suspensions were aged between parallel plates of a shear cell that allowed for in-situ XPCS measurements. This enabled occasional disruption of the aging process by the application of controlled shear. Our results show that aging can be transiently reversed (the gel is "rejuvenated") by applying oscillatory or step shear. Moreover, we have also identified a small regime of low amplitude oscillatory shear that causes "overaging", where shear enhances the aging process. [Preview Abstract] |
Tuesday, March 14, 2017 4:42PM - 4:54PM |
H17.00012: Simulating large, anisotropic density fluctuations in colloidal gels under shear James Swan, Zsigmond Varga The steady shear of weak colloidal gels results in vorticity aligned density fluctuations. These have been measured in neutron scattering and flow dichroism experiments and observed with microscopy coupled to rheometer tools of varying geometry. The origins of this instability remain a mystery, and discrete element simulations of colloidal gels have to date, failed to reproduce the phenomena. We use new Brownian Dynamics simulations to show that this instability is fluid mechanical in origin, and results from long-ranged hydrodynamic interactions among particles in the gel. Squeeze flows between vorticity aligned flocs prevent mutual collisions and realignment, thus promoting stability of large-scale anisotropic density fluctuations. The nonlinear rheology in sheared colloidal gels and measures of their structural anisotropy determined from simulations agree well with a wide variety of experiments. Finally, we demonstrate collapse of this data across different shear rates, strengths of interaction, and volume fractions using a single force scale, the most probable rupture force for the inter-colloid bonds. [Preview Abstract] |
Tuesday, March 14, 2017 4:54PM - 5:06PM |
H17.00013: Dissipation, diffusion and phase separation of driven disks Clara del Junco, Suriyanarayanan Vaikuntanathan Active matter systems have been used to elucidate many aspects of non-equilibrium self-assembly. However, the tradeoffs between energy consumption and organization in these systems are still not well-understood. I will present simulations of a minimal but non-trivial model of a driven non-equilibrium liquid that enables us to explore this question. Phase separation has been broadly observed in active and driven systems. Likewise, this system exhibits rich phase behavior - specifically, a transition from a mixed steady state to a time-periodic, separated state and back to a mixed state as the activity is increased. Surprisingly, the phase behavior of this system is explained by a quantitative connection between the work done on the system by the non-equilibrium forces, and the renormalized diffusion constant of the liquid. I will show how we can use a set of analytical models to argue that the relation between energy dissipation and diffusion can be generalized to other settings, and demonstrate that it holds for a second non-equilibrium liquid with qualitatively different activity. These results lay the groundwork for exploring tradeoffs between dissipation and organization in many-body non-equilibrium systems. [Preview Abstract] |
Tuesday, March 14, 2017 5:06PM - 5:18PM |
H17.00014: Kepler orbits in the Stokesian sedimentation of discs Rahul Chajwa, Narayanan Menon, Sriram Ramaswamy We study experimentally the settling dynamics of a pair of falling discs in a viscous fluid (Re $\sim 10^{-4}$), in a quasi-two-dimensional geometry with the vector normal to the discs, and the trajectory of the centres of the discs, lying in a plane. For initial conditions that are symmetric about the settling direction, we find periodic or scattering orbits of the settling pair [S. Jung et al., PRE {\bf 74}, 035302 (2006)], and account for these in a purely far-field analysis [S. Kim, Int J Multiphase Flow {\bf 11}, 699 (1985)]. In particular, we show that the problem of a symmetrically settling pair of spheroids can be mapped to the Kepler two-body problem. The solution to this problem gives a sharp transition between bound and scattering trajectories which is consistent with experimental observations. For initial conditions where the motions of the particles are not symmetric about the settling direction, we obtain yet another separatrix between full rotations and periodic oscillations which we study within an effective Hamiltonian description of this inertialess and entirely dissipative dynamical system.\\ Present addresses -- RC: ICTS-TIFR, Hessarghatta, Bengaluru 560 089; NM: Physics Department, UMass Amherst MA 01003; SR: Dept of Physics, IISc, Bengaluru 560 012 [Preview Abstract] |
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