Maire R, Plati A, Smallenburg F, Foffi G
Université Paris-Saclay, CNRS, Laboratoire de Physique des Solides, 91405 Orsay, France.
J Chem Phys. 2025 Mar 28;162(12). doi: 10.1063/5.0250643.
Non-equilibrium phase coexistence is commonly observed in both biological and artificial systems, yet understanding it remains a significant challenge. Unlike equilibrium systems, where free energy provides a unifying framework, the absence of such a quantity in non-equilibrium settings complicates their theoretical understanding. Granular materials, driven out of equilibrium by energy dissipation during collisions, serve as an ideal platform to investigate these systems, offering insights into the parallels and distinctions between equilibrium and non-equilibrium phase behavior. For example, the coexisting dense phase is typically colder than the dilute phase, a result usually attributed to greater dissipation in denser regions. In this article, we demonstrate that this is not always the case. Using a simple numerical granular model, we show that a hot solid and a cold liquid can coexist in granular systems. This counterintuitive phenomenon arises because the collision frequency can be lower in the solid phase than in the liquid phase, consistent with equilibrium results for hard-disk systems. We further demonstrate that kinetic theory can be extended to accurately predict phase temperatures even at very high packing fractions, including within the solid phase. Our results highlight the importance of collisional dynamics and energy exchange in determining phase behavior in granular materials, offering new insights into non-equilibrium phase coexistence and the complex physics underlying granular systems.
非平衡相共存现象在生物系统和人工系统中都普遍存在，但对其进行理解仍然是一项重大挑战。与平衡系统不同，在平衡系统中自由能提供了一个统一的框架，而在非平衡环境中缺乏这样一个量使得对它们的理论理解变得复杂。颗粒材料在碰撞过程中由于能量耗散而偏离平衡状态，成为研究这些系统的理想平台，为洞察平衡相行为和非平衡相行为之间的异同提供了契机。例如，共存的致密相通常比稀薄相温度更低，这一结果通常归因于较致密区域有更大的耗散。在本文中，我们证明情况并非总是如此。通过一个简单的颗粒数值模型，我们表明在颗粒系统中热的固体和冷的液体可以共存。这种违反直觉的现象之所以出现，是因为碰撞频率在固相可能低于液相，这与硬磁盘系统的平衡结果一致。我们进一步证明，即使在非常高的填充率下，包括在固相中，动力学理论也可以扩展以准确预测相温度。我们的结果突出了碰撞动力学和能量交换在决定颗粒材料相行为方面的重要性，为非平衡相共存以及颗粒系统背后的复杂物理提供了新的见解。
