The Role of Hydrodynamics in the Behavior of Active Matter

许多生物体的一个显著特征是它们能够移动、自我推进和活跃。“活性物质”系统的组成部分能够通过将燃料转化为机械运动来独立地自我推进。活性物质的例子包括微观实体，如我们细胞内的微生物和运动蛋白，以及大型实体，如鱼类和鸟类。无生命、无生命的物体也可以通过不同于生物体的机制实现自我推进。这些无生命的活性系统的集体行为的结果不一定与有生命的活性系统不同。事实上，所有规模的活性物质系统都有联系在一起并集体移动的趋势，从菌落、虫群、鸟群、鱼群到牛群。本研究解决的问题是微观力学，流体动力学，生物（和非生物）有机体展示集体和连贯运动的起源，以及如何使用简单的物理原理来解释和建模。这种洞察力将使预测、设计和控制活跃的软物质系统及其在自然界和工业中的开发成为可能。内在活动性赋予活性物质新的行为，使其区别于平衡凝聚态系统。活性物质系统产生它们自己的内应力，这使它们远离平衡状态，从而使它们从传统的热力学约束中解放出来，通过这样做可以控制和指导它们自己的行为和周围环境的行为。活性物质至少总是一个由两部分组成的系统——活性物质和由活性物质自我推进的嵌入介质。本研究首次纳入了自走体之间流体介导的水动力相互作用。流体力学显著地影响主动粒子对边界或其他物体施加的力，并且可以通过改变机械“游动压力”深刻地影响主动系统中的相分离。游动压力为活性物质提供了一种压力-浓度关系，可以定量预测活性体系中的冷凝和相分离，并为确定从活性体系的随机运动中可以获得的功的量提供了一条途径。我们还表明，一般来说，游动应力具有非对角线或偏差贡献，特别是当一个活动系统受到剪切运动时。游动应力预测，在非常一般的条件下，活性颗粒可以将悬浮液的有效粘度降低到零，从而使活性物质自发流动。机械游泳应力的观点使人们能够理解、分析和利用广泛的活性软物质系统，从游泳细菌到催化纳米机器人，再到激活细胞骨架的分子马达。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

A distinguishing feature of many living organisms is their ability to move, to self-propel, to be active. Constituents of "active matter" systems are capable of independent self-propulsion by converting fuel into mechanical motion. Examples of active matter include both microscopic entities like microorganisms and motor proteins within our cells and large bodies like fishes and birds. Inanimate, nonliving bodies can also achieve self-propulsion using mechanisms that are different than living organisms. The outcome of the collective behavior of these nonliving active systems is not necessarily different from living active systems. Indeed, active matter systems of all scales have the tendency to associate together and move collectively, from colonies of bacteria, swarms of insects, flocks of birds, schools of fish, to herds of cattle. The question addressed in this research is the micromechanical, hydrodynamic, origin for living (and nonliving) organisms to exhibit collective and coherent motion and how it can be explained and modeled using simple physical principles. Such insight will enable the prediction, design, and control of active soft matter systems and their exploitation in nature and in industry.The intrinsic activity imparts new behaviors to active matter that distinguish it from equilibrium condensed matter systems. Active matter systems generate their own internal stress, which drives them far from equilibrium and thus frees them from conventional thermodynamic constraints, and by so doing can control and direct their own behavior and that of their surrounding environment. Active matter is always at least a two-component system - the active body and the embedding medium off of which the active body self-propels. In this research fluid-mediated hydrodynamic interactions among self-propelled bodies are incorporated for the first time. Hydrodynamics significantly affect the forces active particles exert on boundaries or other objects and can profoundly affect the phase separation in active systems by modifying the mechanical "swim pressure." The swim pressure provides a pressure-concentration relation for active matter that can quantitatively predict condensation and phase separation in active systems and provides a route for determining the amount of work that can be harvested from the often random motion of active systems. We also show that, in general, the swim stress has off-diagonal or deviatoric contributions, especially when an active system is subject to shearing motion. The swim stress predicts that, under very general conditions, active particles can reduce the suspension effective viscosity to zero, enabling spontaneous flow of active matter. The mechanical swim stress perspective allows one to understand, analyze and exploit a wide class of active soft matter systems, from swimming bacteria to catalytic nanobots to molecular motors that activate the cellular cytoskeleton.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.