Digregorio Pasquale, Levis Demian, Cugliandolo Leticia F, Gonnella Giuseppe, Pagonabarraga Ignacio
Centre Européen de Calcul Atomique et Moléculaire (CECAM), Ecole Polytechnique Fédérale de Lausanne (EPFL), Batochimie, Avenue Forel 2, 1015 Lausanne, Switzerland.
Departament de Fisica de la Materia Condensada, Universitat de Barcelona, Marti i Franques 1, 08028 Barcelona, Spain.
Soft Matter. 2022 Jan 19;18(3):566-591. doi: 10.1039/d1sm01411k.
We provide a comprehensive quantitative analysis of localized and extended topological defects in the steady state of 2D passive and active repulsive Brownian disk systems. We show that, both in and out-of-equilibrium, the passage from the solid to the hexatic is driven by the unbinding of dislocations, in quantitative agreement with the KTHNY singularity. Instead, extended clusters of defects largely dominate below the solid-hexatic critical line. The latter percolate in the liquid phase very close to the hexatic-liquid transition, both for continuous and discontinuous transitions, in the homogeneous liquid regime. At critical percolation the clusters of defects are fractal with statistical and geometric properties that are independent of the activity and compatible with the universality class of uncorrelated critical percolation. We also characterize the spatial organization of point-like defects and we show that the disclinations are not free, but rather always very near more complex defect structures. At high activity, the bulk of the dense phase generated by Motility-Induced Phase Separation is characterized by a density of point-like defects, and statistics and morphology of defect clusters, set by the amount of activity and not the packing fraction. Hexatic domains within the dense phase are separated by grain-boundaries along which a finite network of topological defects resides, interrupted by gas bubbles in cavitation. This structure is dynamic in the sense that the defect network allows for an unzipping mechanism that leaves free space for gas bubbles to appear, close, and even be released into the dilute phase.
我们对二维被动和主动排斥布朗圆盘系统稳态下的局域和扩展拓扑缺陷进行了全面的定量分析。我们表明，无论是在平衡态还是非平衡态，从固态到六方相的转变都是由位错的解束缚驱动的，这与KTHNY奇点在定量上是一致的。相反，在固态 - 六方相临界线以下，扩展的缺陷簇占主导地位。在均匀液相区域，对于连续和不连续转变，后者在非常接近六方相 - 液相转变的液相中发生渗流。在临界渗流时，缺陷簇是分形的，其统计和几何性质与活性无关，并且与不相关临界渗流的普适类兼容。我们还表征了点状缺陷的空间组织，并表明位错不是自由的，而是总是非常接近更复杂的缺陷结构。在高活性下，由运动诱导相分离产生的致密相的主体特征是点状缺陷的密度，以及由活性量而非堆积分数决定的缺陷簇的统计和形态。致密相内的六方域由晶界分隔，沿晶界存在有限的拓扑缺陷网络，在空化过程中被气泡打断。这种结构是动态的，因为缺陷网络允许一种解拉链机制，为气泡的出现、闭合甚至释放到稀相留出自由空间。
