Department of Materials Science and Engineering, University of California, Berkeley, California 94720, USA.
NERSC, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA.
J Chem Phys. 2023 Feb 21;158(7):074904. doi: 10.1063/5.0138256.
In striking contrast to equilibrium systems, inertia can profoundly alter the structure of active systems. Here, we demonstrate that driven systems can exhibit effective equilibrium-like states with increasing particle inertia, despite rigorously violating the fluctuation-dissipation theorem. Increasing inertia progressively eliminates motility-induced phase separation and restores equilibrium crystallization for active Brownian spheres. This effect appears to be general for a wide class of active systems, including those driven by deterministic time-dependent external fields, whose nonequilibrium patterns ultimately disappear with increasing inertia. The path to this effective equilibrium limit can be complex, with finite inertia sometimes acting to accentuate nonequilibrium transitions. The restoration of near equilibrium statistics can be understood through the conversion of active momentum sources to passive-like stresses. Unlike truly equilibrium systems, the effective temperature is now density dependent, the only remnant of the nonequilibrium dynamics. This density-dependent temperature can in principle introduce departures from equilibrium expectations, particularly in response to strong gradients. Our results provide additional insight into the effective temperature ansatz while revealing a mechanism to tune nonequilibrium phase transitions.
与平衡系统形成鲜明对比的是，惯性可以深刻地改变主动系统的结构。在这里，我们证明了驱动系统可以表现出类似于有效平衡的状态，随着粒子惯性的增加，尽管严格违反了涨落耗散定理。增加惯性可以逐渐消除由运动引起的相分离，并恢复活性布朗球的平衡结晶。这种效应似乎对广泛的一类主动系统都是普遍存在的，包括那些由确定性时变外部场驱动的系统，这些系统的非平衡模式最终随着惯性的增加而消失。通往这个有效平衡极限的路径可能是复杂的，有时有限的惯性会加剧非平衡转变。通过将主动动量源转换为类似于被动的应力，可以理解接近平衡统计的恢复。与真正的平衡系统不同，有效温度现在是密度依赖的，这是唯一剩余的非平衡动力学。原则上，这个密度依赖的温度可能会导致偏离平衡的预期，特别是在强梯度下。我们的结果为有效温度假设提供了更多的见解，同时揭示了一种调节非平衡相变的机制。
