中文摘要：

颗粒体系是远离平衡态的复杂耗散体系，在外力驱动下，能表现出类似于固体、液体或气体的特性．在稀疏的颗粒气体中，由于耗散，体系中常常会形成颗粒在局部的凝聚．这种凝聚行为具有自发对称性破缺的特征，使得颗粒气体中自动形成高密度区域和低密度区域共存的非均匀状态，类似于气体中液滴的形成．本文利用零重力条件下的三维颗粒气体理论模型进行计算和分析，揭示了颗粒气体这一相变行为的不稳定性根源及类气液相变本质，给出了颗粒气体中这种相分离发生的临界条件，并通过分子动力学模拟进行检验，结果从定性上和定量上都能够很好地吻合，为进一步的空间实验提供了理论依据和相关实验参数．

英文摘要：

Granular systems are intrinsically far from equilibrium for their many-body and dissipative nature. Driven by external forces, they can behave like that of solids, liquids, or gases. In dilute granular gases, clusters usually form due to dissipation, which leads to inhomogeneous density distributions. Such a local condensation shows spontaneous symmetry-breaking instability and neg- ative compressibility instability, just as droplets form in vapor. In this paper, a three-dimensional model for granular gases under zero gravity is investigated, and calculations predict such a gas-liquid like phase transition. Molecular dynamics simulations show good agreements with the theoretical results. The numerical results shall provide detailed experimental parameters for the further related space experiments.