Institute for Theoretical Physics, Georg-August-Universität Göttingen, 37 077 Göttingen, Germany;
Institute for Theoretical Physics, Georg-August-Universität Göttingen, 37 077 Göttingen, Germany.
Proc Natl Acad Sci U S A. 2021 Sep 28;118(39). doi: 10.1073/pnas.2101964118.
Dense assemblies of self-propelled particles that can form solid-like states also known as active or living glasses are abundant around us, covering a broad range of length scales and timescales: from the cytoplasm to tissues, from bacterial biofilms to vehicular traffic jams, and from Janus colloids to animal herds. Being structurally disordered as well as strongly out of equilibrium, these systems show fascinating dynamical and mechanical properties. Using extensive molecular dynamics simulation and a number of distinct dynamical and mechanical order parameters, we differentiate three dynamical steady states in a sheared model active glassy system: 1) a disordered state, 2) a propulsion-induced ordered state, and 3) a shear-induced ordered state. We supplement these observations with an analytical theory based on an effective single-particle Fokker-Planck description to rationalize the existence of the shear-induced orientational ordering behavior in an active glassy system without explicit aligning interactions of, for example, Vicsek type. This ordering phenomenon occurs in the large persistence time limit and is made possible only by the applied steady shear. Using a Fokker-Planck description with parameters that can be measured independently, we make testable predictions for the joint distribution of single-particle position and orientation. These predictions match well with the joint distribution measured from direct numerical simulation. Our results are of relevance for experiments exploring the rheological response of dense active colloids and jammed active granular matter systems.
自推进粒子的密集组装体可以形成固态，也被称为活性或活玻璃，在我们周围比比皆是，涵盖了广泛的长度尺度和时间尺度：从细胞质到组织，从细菌生物膜到车辆交通堵塞，从两面胶体到动物群。由于结构无序且强烈偏离平衡，这些系统表现出迷人的动力学和力学特性。使用广泛的分子动力学模拟和许多不同的动力学和力学有序参数，我们在受剪切的模型活性玻璃状系统中区分出三种动力学稳态：1）无序状态，2）推进诱导有序状态，3）剪切诱导有序状态。我们用基于有效单粒子福克-普朗克描述的分析理论来补充这些观察结果，以解释在没有明确对齐相互作用（例如 Vicsek 型）的情况下，活性玻璃状系统中剪切诱导取向有序行为的存在。这种有序现象发生在大持续时间极限中，并且仅通过施加的稳态剪切才成为可能。使用可以独立测量的参数的福克-普朗克描述，我们对单粒子位置和取向的联合分布做出了可测试的预测。这些预测与从直接数值模拟测量的联合分布吻合良好。我们的结果与探索密集活性胶体和堵塞活性颗粒物质系统的流变响应的实验有关。
