EAGER: Emergent order of hydrodynamically coupled microrotors

1544196(Vlahovska)拟议研究的目的是从理论和实验上研究一类新的活性流体--自旋转颗粒悬浮液。活性流体是以独特方式表现的流体，因为活性颗粒的存在可以自组装，或者可以以不同的方式移动和填充，从而赋予流体不同的宏观性质。某些复杂的流体和生物流体就属于这一类。即使是一群鸟或一群鱼，其中每个移动的动物都自己移动，但所有动物的运动遵循的模式比个人的规模大得多，都是主动流体的例子。有人建议研究旋转球体(转子)的稠密悬浮。最近发现，在最初具有随机向上或向下自旋分布的单层转子中，相同自旋的转子自发地分离并在车道上集体运动或在大漩涡中循环。当转子密度接近最大堆积密度时，转子会卡在晶体中，不断熔化、重新组装和移动。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。数值模拟采用浸没边界法。该实验系统依赖于Quincke效应，即介质球在外加均匀电场中的自发自旋。这项研究的目的是(1)将静电相互作用纳入数值模拟，(2)研究Quincke转子对和单层的动力学。除了提高基础知识外，这项研究还将发现新的动态结构，可以用来设计对外部环境做出反应的“智能”材料。PIS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外展计划，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.