Maitra Ananyo, Srivastava Pragya, Marchetti M Cristina, Ramaswamy Sriram, Lenz Martin
Sorbonne Université and CNRS, Laboratoire Jean Perrin, F-75005, Paris, France.
LPTMS, CNRS, Univ. Paris-Sud, Université Paris-Saclay, 91405 Orsay, France.
Phys Rev Lett. 2020 Jan 17;124(2):028002. doi: 10.1103/PhysRevLett.124.028002.
We present a comprehensive theory of the dynamics and fluctuations of a two-dimensional suspension of polar active particles in an incompressible fluid confined to a substrate. We show that, depending on the sign of a single parameter, a state with polar orientational order is anomalously stable (or anomalously unstable), with a nonzero relaxation (or growth) rate for angular fluctuations, not parallel to the ordering direction, at zero wave number. This screening of the broken-symmetry mode in the stable state does lead to conventional rather than giant number fluctuations as argued by Bricard et al., Nature 503, 95 (2013), but their bend instability in a splay-stable flock does not exist and the polar phase has long-range order in two dimensions. Our theory also describes confined three-dimensional thin-film suspensions of active polar particles as well as dense compressible active polar rods, and predicts a flocking transition without a banding instability.
我们提出了一种关于二维极性活性粒子悬浮液在限制于基底的不可压缩流体中的动力学和涨落的综合理论。我们表明，取决于单个参数的符号，具有极性取向序的状态异常稳定（或异常不稳定），在零波数时，对于不平行于有序方向的角涨落具有非零的弛豫（或增长）速率。如Bricard等人在《自然》503, 95 (2013)中所论证的，稳定状态下这种破缺对称模式的筛选确实导致了常规而非巨大的数涨落，但他们所提出的在展曲稳定的群集中的弯曲不稳定性并不存在，并且极性相在二维中具有长程有序。我们的理论还描述了限制于三维的活性极性粒子薄膜悬浮液以及密集可压缩的活性极性棒，并预测了一种没有条带不稳定性的聚集转变。
