Battat Sarah, Weitz David A, Whitesides George M
John A. Paulson School of Engineering and Applied Sciences, Harvard University, Cambridge, Massachusetts 02138, USA.
Department of Physics, Harvard University, Cambridge, Massachusetts 02138, USA.
Soft Matter. 2023 May 3;19(17):3190-3198. doi: 10.1039/d2sm01635d.
The question of melting has been addressed theoretically and experimentally for two-dimensional crystals in thermal equilibrium. However, as it pertains to out-of-equilibrium systems, the question is unresolved. Here, we present a platform to study the melting of a two-dimensional, binary Coulombic crystal composed of equal numbers of nylon and polytetrafluoroethylene (PTFE) beads that measure a couple of millimeters in diameter. The beads are tribocharged-nylon positively and PTFE negatively-and they experience long-range electrostatic interactions. They form a square crystal in which nylon and PTFE beads sit at alternating sites on a checkerboard lattice. We melt the crystal by agitating the dish in which it resides using an orbital shaker. We compare the melting behavior of the crystal without impurities to that of the crystal with impurities, where we use gold-coated nylon beads as impurities because they tribocharge negligibly. Our results reveal that impurities do not influence the melting of the crystal. Instead, the crystal undergoes shear-induced melting, beginning from its edges, due to its collisions with the dish. As a result of repetitive collisions, the beads acquire kinetic energy, undergo rearrangements, and become disordered. Unlike most examples of shear-induced melting, portions of the crystal remain locally ordered given the persistence of electrostatic interactions and the occurrence of some collisions that are favorable to ordering clusters of beads. Our work clarifies the melting behavior of sheared crystals whose constituents have persistent long-range interactions. It may prove valuable in determining the conditions under which such materials are immune to disorder.
对于处于热平衡状态的二维晶体，熔化问题已在理论和实验上得到解决。然而，对于非平衡系统，这个问题尚未解决。在此，我们展示了一个平台，用于研究由数量相等的尼龙珠和聚四氟乙烯（PTFE）珠组成的二维二元库仑晶体的熔化，这些珠子直径为几毫米。珠子通过摩擦带电——尼龙带正电，PTFE带负电——并且它们经历长程静电相互作用。它们形成一种方形晶体，其中尼龙珠和PTFE珠位于棋盘格晶格的交替位置上。我们通过使用轨道振荡器搅拌放置晶体的盘子来使其熔化。我们将无杂质晶体的熔化行为与有杂质晶体的熔化行为进行比较，其中我们使用镀金尼龙珠作为杂质，因为它们的摩擦带电可忽略不计。我们的结果表明，杂质不会影响晶体的熔化。相反，晶体由于与盘子碰撞而从其边缘开始经历剪切诱导熔化。由于反复碰撞，珠子获得动能，发生重排并变得无序。与大多数剪切诱导熔化的例子不同，由于静电相互作用的持续存在以及一些有利于珠子有序聚集的碰撞的发生，晶体的部分区域仍保持局部有序。我们的工作阐明了其成分具有持久长程相互作用的剪切晶体的熔化行为。这可能在确定此类材料不受无序影响的条件方面具有价值。
