Bulletin of the American Physical Society
APS March Meeting 2019
Volume 64, Number 2
Monday–Friday, March 4–8, 2019; Boston, Massachusetts
Session L53: Controlling Local Structure With Time-Dependent External FieldsInvited
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Sponsoring Units: GSOFT Chair: Craig Maloney, Northeastern University Room: BCEC 253C |
Wednesday, March 6, 2019 11:15AM - 11:51AM |
L53.00001: Symmetric and Asymmetric Time-Varying Magnetic Fields to Control and Drive Paramagnetic Colloidal Assemblies Invited Speaker: Sibani Biswal Micron-sized superparamagnetic colloids placed in time-varying magnetic fields can provide us with rich dynamic behavior depending on the magnetic field applied and the particle concentration. In symmetric magnetic fields, a long-range attractive potential is induced resulting in the formation of colloidal clusters, which assemble into circular shapes that exhibit fluid-like behavior at relatively low fields, while becoming crystallized and highly ordered at higher field strengths. These systems exhibit interesting coarsening and spinodal decomposition structural transitions for larger particle systems. |
Wednesday, March 6, 2019 11:51AM - 12:27PM |
L53.00002: Driving and assembling magnetic particles with incoherent fields Invited Speaker: Randy Erb Magnetic particles in suspension can chain under static applied magnetic fields or be transported to target sites with static magnetic field gradients. This is clear and has led to innovations from magnetorheological fluids to magnetic targeting of tumors to magnetic delivery of drugs. But how do magnetic particles behave in more complicated fields and field gradients? Can we use these complications to our advantage? Can we understand the physics? There has been significant research toward these questions for coherent (simple rotating) fields by many research groups including ours. Dynamically tuning the direction (but not magnitude) of the field will disrupt the simple chaining of particles. For two isolated particles, this disruption is found to be an elegant systematic dance between particle pairs. For large populations of particles, fascinating group dynamics occur. We have looked at these dynamics for chains as well as for multipolar ring formations. In addition, we have been investigating the behavior of magnetic particles under incoherent fields, in which there is a mismatch in the frequency domain for x, y, and z field functions. Surprisingly, we find that incoherent fields can generate coherent assemblies! We apply our findings to the realms of magnetic assembly, magnetic drug delivery and magnetic rheology in an attempt to enhance current practices through complicating the applied fields. |
Wednesday, March 6, 2019 12:27PM - 1:03PM |
L53.00003: A touch of non-linearity in fluid fields: where spheres “think” collectively and swim together Invited Speaker: Daphne Klotsa From crawling cells to orca whales, swimming in nature occurs at different scales. The study of swimming across length scales can shed light onto the biological functions of natural swimmers or inspire the design of artificial swimmers with applications ranging from targeted drug delivery to deep-water explorations. In this talk, I will present experiments and simulations of how oscillating spheres, universally simple geometric objects, can utilize non-linearities to demonstrate complex pattern formation in a granular system, or different swimming behaviors in a spherobot (robot made out of spheres) when placed in a fluid at intermediate Reynolds numbers, 1<Re. |
Wednesday, March 6, 2019 1:03PM - 1:39PM |
L53.00004: Deck the walls with tunable energy fields for colloidal particles in nematic liquid crystals Invited Speaker: Kathleen Stebe Confined nematic liquid crystals provide new opportunities to direct colloidal motion and assembly. Nematic liquid crystal director fields molded by confinement can present domains of bend, splay or twist with associated elastic energies. Colloidal particles also distort the nematic director field, and are repelled or attracted to these domains to minimize the elastic energy cost in these systems. We have been studying colloids adjacent to a wavy wall in a nematic liquid crystal with a smoothly varying, non-singular director field that features bend and splay distortions. Colloids within this field can have multi-stable states. Wall-colloid equilibrium distances and ranges of interaction can be tuned by varying wavy wall geometry. Colloids move toward and dock at attractive sites and away from repulsive loci; this repulsion can propel objects in the domain. Extensions to anisotropic colloids are discussed. Manipulation by application of external fields to complement the nematic director fields are discussed in the context of reconfigurable systems and microrobotics. |
Wednesday, March 6, 2019 1:39PM - 2:15PM |
L53.00005: Emergent dynamics and self-assembly of colloids in time-dependent magnetic fields Invited Speaker: Alexey Snezhko Strongly interacting colloids subject to an external periodic forcing often exhibit nontrivial collective dynamics and self-organization. Driven magnetic colloids proved to be an excellent model system to explore emergent collective behavior and out-of-equilibrium self-assembly. New self-assembled structures emerging in time-dependent magnetic fields are often not accessible under equilibrium conditions. In this presentation I will demonstrate that dispersions of magnetic particles suspended at a liquid-air, liquid-liquid interfaces or in the bulk and driven out-of-equilibrium by an alternating magnetic field develop nontrivial dynamic self-assembled phases and structures. Experiments reveal new types of nontrivially ordered phases and collective dynamics emerging in such systems in a certain range of excitation parameters. These remarkable non-equilibrium structures emerge as a result of the competition between magnetic and hydrodynamic forces. The dynamic phases are reversible and fine-tuned by the parameters of the driving magnetic field. Above certain frequency threshold or on external perturbation some of the dynamic structures spontaneously break the symmetry of self-induced flows and turn into swimmers, spinners or can be used as robotic manipulators at microscale. Furthermore, collective motion of self-assembled dynamic structures at interfaces often generates chaotic fluid flow reminiscent of two-dimensional turbulence - active turbulence. In case of a rotational magnetic field two-dimensional active spinner materials can be realized. |
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