EAGER: Emergent order of hydrodynamically coupled microrotors

1544196（Vlahovska）所提出的研究的目的是从理论和实验上研究一类新的活性流体，即自旋转颗粒的悬浮液。活性流体是以独特的方式表现的流体，因为存在可以自组装的活性颗粒，或者可以以不同的方式移动和包装，从而赋予流体不同的宏观性质。某些复杂流体和生物流体属于这一类。即使是一群鸟或一群鱼，其中每一个移动的动物都自己移动，但所有的运动都遵循一个比个体大得多的模式，都是活动流体的例子。最近已经发现，在最初随机分布的向上或向下自旋的单层转子中，相同自旋的转子自发地分离并集体地在交通车道上移动或在大涡流中循环。当转子密度接近最大填充时，转子会堵塞在不断熔化、重新组装和移动的晶体中。建议用计算和实验相结合的方法来研究这些现象，以了解这种集体行为是如何从转子之间的流体动力相互作用中出现的。数值模拟是基于浸入边界法。该实验系统依赖于昆克效应，这是一个电介质球在施加均匀电场中的自发旋转。本研究的目的是（1）在数值模拟中考虑静电相互作用，（2）研究一对和单层昆克转子的动力学。除了推进基础知识，研究还将发现新的动态结构，可用于设计对外部环境敏感的“智能”材料。PI将把这项研究的结果纳入研究生课程，并将利用布朗大学成功的推广计划，向公众宣传这项工作的相关性和重要性。

1544196(Vlahovska)The objective of the proposed research is to investigate theoretically and experimentally a new class of active fluids, that of suspensions of self-rotating particles. Active fluids are fluids that behave in unique ways, because of the presence of active particles that can self-assemble, or can move and pack in different ways, giving different macroscopic properties to the fluid. Certain complex fluids and biofluids fall in this category. Even a flock of birds, or a school of fish, where each moving animal moves on its own, but the motion of all follows a pattern at a much larger scale than the individual, are examples of active fluids.It is proposed to examine dense suspensions of rotating spheres (rotors). It has very recently been found that in a monolayer of rotors with initially randomly distributed up or down spins, same-spin rotors spontaneously segregate and collectively move in traffic lanes or circulate in large vortices. When the rotor density gets close to maximum packing, the rotors jam into crystals that continuously melt, reassemble, and move. It is proposed to study these phenomena with a combined computational and experimental approach to understand how this collective behavior emerges from the hydrodynamic interactions between the rotors. The numerical simulations are based on the immersed boundary method. The experimental system relies on the Quincke effect, which is the spontaneous spinning of a dielectric sphere in an applied uniform electric field. The proposed research aims to (1) include the electrostatic interactions in the numerical simulations, and (2) investigate the dynamics of a pair and monolayer of Quincke rotors. In addition to advancing basic knowledge, the research will uncover novel dynamic structures that could be exploited for design of `smart' materials responsive to the external environment. The PIs will incorporate the results from this research in graduate courses and will also leverage successful outreach programs at Brown University to communicate the relevance and significance of the work to the general public.