Soni Harsh, Kumar Nitin, Nambisan Jyothishraj, Gupta Rahul Kumar, Sood A K, Ramaswamy Sriram
Department of Physics, Indian Institute of Science, Bangalore 560 012, India.
TIFR Centre for Interdisciplinary Sciences, Tata Institute of Fundamental Research, Hyderabad 500 107, India.
Soft Matter. 2020 Aug 12;16(31):7210-7221. doi: 10.1039/c9sm02552a.
We present a large-scale numerical study, supplemented by experimental observations, on a quasi-two-dimensional active system of polar rods and spherical beads confined between two horizontal plates and energised by vertical vibration. For a low rod concentration Φr, our observations are consistent with a direct phase transition, as bead concentration Φb is increased, from the isotropic phase to a homogeneous flock. For Φr above a threshold value, an ordered band dense in both rods and beads occurs between the disordered phase and the homogeneous flock, in both experiments and simulations. Within the size ranges accessible, we observe only a single band, whose width increases with Φr. Deep in the ordered state, we observe broken-symmetry "sound" modes and giant number fluctuations. The direction-dependent sound speeds and the scaling of fluctuations are consistent with the predictions of field theories of flocking; sound damping rates show departures from such theories, but the range of wavenumbers explored is modest. At very high densities, we see phase separation into rod-rich and bead-rich regions, both of which move coherently.
我们展示了一项大规模数值研究，并辅以实验观测，该研究针对的是一个准二维活性系统，该系统由夹在两个水平板之间并通过垂直振动供能的极性棒和球形珠组成。对于较低的棒浓度Фr，随着珠浓度Фb的增加，我们的观测结果与从各向同性相到均匀聚集体的直接相变一致。对于高于阈值的Фr，在实验和模拟中，无序相和均匀聚集体之间都会出现一个棒和珠都密集的有序带。在可及的尺寸范围内，我们只观察到一个带，其宽度随Фr增加。在深度有序状态下，我们观察到破缺对称性的“声”模式和巨大的数量涨落。与方向有关的声速和涨落的标度与群聚场论的预测一致；声阻尼率与这些理论有所偏离，但所探索的波数范围较小。在非常高的密度下，我们看到相分离成富含棒的区域和富含珠的区域，这两个区域都相干移动。
