Das Soumen, Ghosh Shankar, Sadhu Tridib, Klamser Juliane U
Department of Condensed Matter Physics and Material Sciences, Tata Institute of Fundamental Research, Mumbai 400005, India.
Department of Theoretical Physics, Tata Institute of Fundamental Research, Mumbai 400005, India.
PNAS Nexus. 2025 Aug 18;4(9):pgaf266. doi: 10.1093/pnasnexus/pgaf266. eCollection 2025 Sep.
Active-matter systems are inherently out-of-equilibrium and perform mechanical work by utilizing their internal energy sources. Breakdown of time-reversal symmetry (BTRS) is a hallmark of such dissipative nonequilibrium dynamics. We introduce a robust, experimentally accessible, noninvasive, quantitative measure of BTRS in terms of the Kullback-Leibler divergence in collision events, demonstrated in our novel artificial active matter, comprised of battery-powered spherical rolling robots whose energetics in different modes of motion can be measured with high precision. Our model independent, dimensionless measure characterizes how dissipation and internal energetics are influenced by kinematic constraints from interactions with the environment. We propose this measure of BTRS as an empirical estimate of the distance from equilibrium. An energetic insight into this departure of active matter from equilibrium comes from our demonstration of a nontrivial fluctuation symmetry, which reveals a potentially universal thermodynamic characteristic of active energetics. As a many-body consequence of BTRS in our experimental active system, we demonstrate the emergence of herding, which has no equilibrium analog.
活性物质系统本质上是远离平衡态的，它们通过利用自身的内部能量来源来执行机械功。时间反演对称性破缺（BTRS）是这种耗散非平衡动力学的一个标志。我们引入了一种稳健的、实验上可实现的、非侵入性的、基于碰撞事件中库尔贝克-莱布勒散度的BTRS定量测量方法，这在我们新型的人工活性物质中得到了证明，该活性物质由电池供电的球形滚动机器人组成，其在不同运动模式下的能量学可以高精度测量。我们的与模型无关的无量纲测量方法表征了耗散和内部能量学如何受到与环境相互作用的运动学约束的影响。我们提出这种BTRS测量方法作为对远离平衡态距离的一种经验估计。对活性物质偏离平衡态的一种能量学洞察来自于我们对一种非平凡涨落对称性的证明，这揭示了活性能量学潜在的普遍热力学特征。作为我们实验活性系统中BTRS的多体结果，我们证明了群聚现象的出现，这在平衡态中没有类似物。
