Caprini Lorenzo, Marini Bettolo Marconi Umberto
School of Sciences and Technology, University of Camerino, Via Madonna delle Carceri, I-62032, Camerino, Italy.
Soft Matter. 2021 Apr 21;17(15):4109-4121. doi: 10.1039/d0sm02273j.
Recently, it has been discovered that systems of active Brownian particles (APB) at high density organise their velocities into coherent domains showing large spatial structures in the velocity field. This collective behavior occurs spontaneously, i.e. is not caused by any specific interparticle force favoring the alignment of the velocities. This phenomenon was investigated in the absence of thermal noise and in the overdamped regime where inertial forces could be neglected. In this work, we demonstrate through numerical simulations and theoretical analysis that velocity alignment is a robust property of ABP and persists even in the presence of inertial forces and thermal fluctuations. We also show that a single dimensionless parameter, such as the Péclet number customarily employed in the description of self-propelled particles, is not sufficient to fully characterize this phenomenon either in the regimes of large viscosity or small mass. Indeed, the size of the velocity domains, measured through the correlation length of the spatial velocity correlation, remains constant when the swim velocity increases and decreases as the rotational diffusion becomes larger. We find that, contrary to the common belief, the spatial velocity correlation not only depends on inertia but is also non-symmetrically affected by mass and inverse viscosity variations. We conclude that in self-propelled systems, at variance with passive systems, variations in the inertial time (mass over solvent viscosity) and mass act as independent control parameters. Finally, we highlight the non-thermal nature of the spatial velocity correlations that are fairly insensitive both to solvent and active temperatures.
最近，人们发现高密度的活性布朗粒子（ABP）系统会将其速度组织成相干域，在速度场中呈现出大尺度的空间结构。这种集体行为是自发产生的，即不是由任何有利于速度对齐的特定粒子间力引起的。该现象是在没有热噪声且处于过阻尼状态（惯性力可忽略不计）的情况下进行研究的。在这项工作中，我们通过数值模拟和理论分析证明，速度对齐是ABP的一种稳健特性，即使在存在惯性力和热涨落的情况下也依然存在。我们还表明，在大粘度或小质量的情况下，像描述自推进粒子时通常使用的佩克莱数这样的单一无量纲参数，不足以完全表征这一现象。实际上，通过空间速度相关性的关联长度来衡量的速度域大小，在游动速度增加时保持不变，而随着旋转扩散变大而减小。我们发现，与普遍看法相反，空间速度相关性不仅取决于惯性，还受到质量和反粘度变化的非对称影响。我们得出结论，在自推进系统中，与被动系统不同，惯性时间（质量除以溶剂粘度）和质量的变化起着独立控制参数的作用。最后，我们强调了空间速度相关性的非热性质，其对溶剂温度和活性温度都相当不敏感。
