Bulletin of the American Physical Society
APS March Meeting 2020
Volume 65, Number 1
Monday–Friday, March 2–6, 2020; Denver, Colorado
Session A31: Microflows Meet Soft Matter I: Crystals, Colloids, & ParticlesFocus
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Sponsoring Units: DSOFT DFD GSNP Chair: Ivan Christov, Purdue Univ Room: 503 |
Monday, March 2, 2020 8:00AM - 8:36AM |
A31.00001: Wetting and dewetting of nanoscale films of nematic liquid crystal Invited Speaker: Linda Cummings The evolution of ultra-thin films (tens of nm) of nematic liquid crystals (NLCs) is considered. Such free-surface films can undergo complex dewetting behavior, as observed in experiments. We present a simplified thin-film model for the free surface evolution that includes strong spatially-varying planar anchoring at the substrate, and weak antagonistic anchoring at the free surface. A number of large-scale simulations are presented, showing good qualitative agreement with experiments. Ongoing work including the effect of spatially-varying electric fields on film evolution is briefly highlighted. |
Monday, March 2, 2020 8:36AM - 8:48AM |
A31.00002: A phase-field model for a modulated-disordered interface with varying density Eduardo Vitral, Perry H. Leo, Jorge Vinals Soft modulated phases have been shown to undergo complex morphological transitions at high temperatures, in which the orientation of their layers and the Gaussian curvature play a major role. This is the case of smectic films under thermal treatment, where focal conics can be reshaped into conical pyramids and concentric ring structures. While evaporation-condensation mechanisms have been theoretically and numerically studied for a smectic-isotropic interface, hydrodynamic stresses at the interface and their resulting flows are yet to be analyzed. This is particularly challenging in the case of a smectic-air boundary, due to the large density ratio between the phases. We derive a phase-field model that accounts for a varying density field and represents the smectic layering by an order parameter. The resulting equations govern the evolution of an interface between a modulated and a disordered phase with distinct densities, being able to account for compressibility effects at the interface and accommodate topological transitions. By integrating the equations in time, we investigate the interfacial flow on a disturbed smectic, and verify the implementation based on the derived dispersion relation. |
Monday, March 2, 2020 8:48AM - 9:00AM |
A31.00003: Non-reciprocal motion in superparamagnetic magnetoelastic membrane patches Chase Brisbois, Mykola Tasinkevych, Monica Olvera De La Cruz Magnetic fields allow for the remote manipulation of bio-orthogonal microscale robots that have applications in drug delivery or microsurgery. We develop a theoretical approach for autonomous locomotion of superparamagnetic membranes through viscous media. Oscillations in triaxial magnetic fields induce non-reciprocal, wave-like motions on circular membrane patches. We show how the strength, angle and frequency of rotating magnetic fields affects circumferential and radial membrane wave propagation. Using the lattice Boltzmann method, we implement hydrodynamic interactions that reveal the flow field of the surrounding fluid and show how truncated circular patches swim at low Reynolds numbers. |
Monday, March 2, 2020 9:00AM - 9:12AM |
A31.00004: Particle motion nearby rough surfaces Christina Kurzthaler, Amir Pahlavan, Lailai Zhu, Howard A Stone Interactions between particles and boundaries are ubiquitous in nature and play a pivotal role in microfluidic applications. Here, we study the hydrodynamic couplings between particles and solid, rough boundaries that are characterized by periodic and random surface shapes. Using the Lorentz reciprocal theorem, we derive analytical expressions for the mobility tensor of a spherical particle and investigate its gravity driven sedimentation near a random rough wall. Our theory and experiments show that the particle exhibits translation perpendicular to the gravitation force, in striking contrast to its motion near a planar wall, and follows the surface shape in close proximity to the boundary. Overall, our results should lay the foundation to study microswimmer motion close to random, heterogeneous boundaries. |
Monday, March 2, 2020 9:12AM - 9:24AM |
A31.00005: Growth of clogs in parallel microchannels Emilie Dressaire, Emmanuel Villermaux, Alban Sauret During the transport of colloidal suspensions in microchannels, the deposition of particles can lead to the formation of clogs. Once a clog is formed in a microchannel, advected particles form an aggregate upstream from the site of the blockage. This aggregate called filter cake grows over time, which leads to a dramatic reduction of the flow rate. We present an analytical description that captures the time evolution of the volume of the aggregates. The results are compared with experiments performed using a pressure-driven suspension flow in an array of parallel microchannels. The coupled dynamics of the aggregates is key to bridge clogging at the pore scale with macroscopic observations of the flow rate evolution at the filter scale. |
Monday, March 2, 2020 9:24AM - 9:36AM |
A31.00006: Flow and Fouling in Elastic Membrane Filters with Complex Pore Morphology Pejman Sanaei, Shi Yue Liu, Zhengyi Chen Filtration technology has been increasingly used for industrial purposes and the study of membrane science is beneficial for filtration efficacy prediction and performance analysis. Real membranes have complex geometry, with pores inside the membrane branch and interconnected with each other. Membrane fouling, as an indispensable consequence for removing particles, occurs in course of filtration process and deteriorates the membrane permeability. In this work, we consider standard blocking as a fouling mechanism, which decreases membrane porosity. However, for membranes with elastic materials, the pressure within the membrane results in membrane pore radius expansion and further influences the filtration performance. We present a mathematical model with multi-layer bifurcating interior morphology, where two pervasive filtration forcing mechanisms are considered: (i) constant pressure drop; and (2) constant flux through membrane. |
Monday, March 2, 2020 9:36AM - 9:48AM |
A31.00007: Hydrodynamic shock and instability in sedimenting colloidal suspensions along a surface Shake Karapetyan, Sam Wilken, Michio Tanaka, Brennan Sprinkle, Aleksandar Donev, Paul M Chaikin We combine experiments, large-scale simulations, and a continuum model to study the emergence of a coherent density profile in a suspension of passive particles sedimenting near an inclined plane. Sedimenting colloids form a shock when there are sharp density gradients in the suspension and agree well with a solution to a modified Burger's equation. We also observe the formation of an instability at the front of the shock that is different from the case of driven microrollers1 and other fluid-like instabilities in that the amplitude does not grow exponentially. The instability is characterized by a wavelength controlled by the gravitational height, the typical height of the particles above the inclined plane. |
Monday, March 2, 2020 9:48AM - 10:00AM |
A31.00008: Unsteady Sedimentation of a Colloidal Sphere in a Horizontal Channel Lauren Altman, David G Grier The elementary system of a sphere sedimenting through a viscous fluid under gravity becomes remarkably difficult to treat analytically when the host fluid is bounded by parallel horizontal walls. The simplest treatment, due to Oseen, involves linear superposition of Faxén’s classic single-wall correction to the mobility. We investigate the limits of the Oseen superposition approximation in this canonical system by measuring the trajectories of colloidal spheres sedimenting through water in slit pores. Measurements are performed by lifting solid or liquid droplets to the top glass-water interface with holographic optical tweezers and tracking their descent with nanometer precision by interpreting holographic video microscopy data with the Lorenz-Mie theory of light scattering. This analysis also yields precise measurements of the particles’ refractive indexes that can be interpreted with Maxwell Garnett effective medium theory to estimate the particles’ buoyant masses. Agreement between the hydrodynamic theory and these measurements establishes the limits of validity of the measurement technique and the hydrodynamic model. |
Monday, March 2, 2020 10:00AM - 10:12AM |
A31.00009: Stresslet of colloidal suspensions in a spherical cavity Emma del Carmen Gonzalez Gonzalez, Roseanna Zia Early studies of force and torque hydrodynamic functions for a colloid in a spherical cavity1-3 paved the way to more sophisticated computational methods, such as the Confined Stokesian dynamics algorithm4,5. However, these studies are restricted to equilibrium situations owing to the lack of stresslet coupling. Currently, the study of confined colloids is gaining traction given its direct application to intracellular transport. In cells, the dynamics are driven out-of-equilibrium by active transport, concentration gradients, and metabolic responses. Furthermore, a predictive model for intracellular transport needs to characterize rheological parameters (η, OP, …), this is only possible by accurately incorporating the stresslet coupling. Here we present the exact solution for stresslet hydrodynamic functions of a colloid in a spherical cavity, and its application to more concentrated suspensions via the Confined Stokesian dynamics algorithm. With this algorithm, we predict high-frequency dynamic viscosities that show non-monotonic behavior throughout the confined domain. |
Monday, March 2, 2020 10:12AM - 10:24AM |
A31.00010: Shape induced segregation and anomalous diffusion of particles under confinement Jiyuan Li, Abhinendra Singh, Xikai Jiang, Juan P. Hernandez-Ortiz, Juan De Pablo, Heinrich M. Jaeger Diffusive behaviors in confined environment is a fundamental problem that finds applications in various areas of science and engineering, including cells, supercooled liquids, and mesoporous materials, etc. These behaviors should intuitively be affected by particle shapes and concentrations. However, these effects remain poorly understood due to the computational difficulties in solving hydrodynamic interactions (HIs) between arbitrarily shaped particles in confined space. Here, an immersed boundary–General geometry Ewald-like method (IB-GgEm) is adopted to simulate the dynamics of a mixture of particles of different shapes and relative concentrations with the consideration of both short- and long-range fluctuating HIs. We find that increasing the fraction of cylinders induces particle segregation, where the spherical particles are pushed towards the wall, while the cylinders prefer to be near the center of the cavity. In addition, increasing the fraction of cylinders also affects the diffusive-to-anomalous transition and the degree of anomaly. We believe that our results offer a pathway to understanding fundamental questions in biology, e.g., anomalous macromolecular diffusion in cells, and serve as a route to design and optimize drug-delivering platforms. |
Monday, March 2, 2020 10:24AM - 10:36AM |
A31.00011: Equilibrium diffusion, thermodynamics, and rheology of confined Brownian suspensions Alp Sunol, Roseanna Zia Computational modeling of spherically confined, hydrodynamically interacting colloids has led to a new framework for modeling biological cells. While modeling of cellular behavior is robust in atomistic-scale structural biology, with little time evolution, and kinetics-based systems-biology, which abstracts away space, many cellular processes operate over colloidal length scales, where interparticle interactions and particle motion play central and nontrivial roles in whole-cell behavior. Here, we present the results of our dynamic simulation studies using both Confined Stokesian Dynamics and Confined Brownian Dynamics algorithms. We compare the role of thermodynamic structure induced by confinement on the short- and long-time transport properties with and without hydrodynamic interactions. Additionally, we find relations between the variables of particle size and volume fraction within the confinement to rheological properties, such as osmotic pressure and viscosity, and highlight how these findings can play an important role in understanding how prokaryotic cells regulate their function. |
Monday, March 2, 2020 10:36AM - 10:48AM |
A31.00012: Auto-phoretic nanorods driven up the wall by gravity Quentin Brosseau, Florencio Balboa Usabiaga, Enkeleida Lushi, Yang Wu, Leif Ristroph, Michael Ward, Michael John Shelley, Jun Zhang Gravitaxis is the directed upward motion of micro-organisms against gravity, and is observed for a few ciliated organisms like Chlamydomonas, Euglenas or Paramecium. Lacking a dedicated sensor, their gravitactic response relies on bottom-heaviness or shape anisotropy to induce a bias in their swimming direction. |
Monday, March 2, 2020 10:48AM - 11:00AM |
A31.00013: When microrollers meet anisotropy Ernest van der Wee, Ramakrishna Kotni, Alfons van Blaaderen, Michelle R Driscoll Driven colloidal particles can display an array of collective effects. Here we study a system in which these collective interactions are largely driven by hydrodynamics: microrollers. Microrollers can be experimentally realized by driving (weakly) magnetic colloidal particles suspended in a liquid above a wall with a rotating magnetic field. Using fluorescence microscopy and particle tracking we can study their response to the field, as well as their collective interactions. Here, we study how anisotropy in the shape of the rollers alters their dynamics. We explore the collective response of rod-shaped hematite-silica microrollers and how this response is modified by adjusting their concentration and shape. Additionally, we study the role of the alignment of the magnetic moment of the rollers with respect to the anisotropic axis of the rollers themselves. Finally, we will show how these microrollers interact with passive obstacles. |
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