Non-Equilibrium Phase Transitions

人们对非平衡相变的兴趣与日俱增，但我们对它们的理解还远未完成。这项拟议的研究将探索几个特定系统中远离平衡的转变：流体入侵、磁畴壁运动、流体扩散、摩擦以及固体和流体的剪切。除了允许洞察非平衡转变的基本特征外，这项工作还将讨论具有技术重要性的系统。前三个过程是界面在无序介质中运动的典型例子。无序阻碍了界面的运动，需要一个有限的力来启动流动。在运动开始时已经确定了两种类型的临界现象：脱钉和渗流。这项研究的目标之一将是对特定模型的这些转变进行完整的表征。研究了脱钉力、临界指数和界面形态随空间尺寸、无序性质和无序强度的变化。不同普适性类之间的相变也将被研究。对摩擦和剪切系统的研究将探索内部自由度在产生不连续相变中的作用。其中一个焦点将是胶体悬浮液中剪切诱导的熔融转变。这项工作还将提供有关高剪应力下固体流动的信息。第二个焦点将是粘滑摩擦--两个表面之间交替的粘合和滑动。粘滑运动会导致铰链发出吱吱声，并导致运动部件磨损。最近的研究表明粘滑摩擦是由周期性的非平衡相变引起的，这一研究将是对最近的研究的扩展。

There has been growing interest in non-equilibrium phase transitions, yet our understanding of them is far from complete. The proposed research will explore transitions far from equilibrium in several specific systems: fluid invasion, magnetic domain wall motion, fluid spreading, friction, and shearing of solids and fluids. In addition to allowing insight into basic features of non-equilibrium transitions, the work will address systems of technological importance. The first three processes are prototypical examples of interface motion through disordered media. Disorder impedes interface motion and a finite force is needed to initiate flow. Two types of critical phenomena have been identified at the onset of motion: depinning and percolation. One goal of the research will be a complete characterization of these transitions for specific models. The variation of the depinning force, the critical exponents, and the interface morphology with the spatial dimension and nature and strength of the disorder will be explored. Phase transitions between different universality classes will also be studied. Studies of friction and sheared systems will explore the role of internal degrees of freedom in producing discontinuous phase transitions. One focus will be the shear-induced melting transition in colloidal suspensions. This work will also provide information about flow of solids under high shear stresses. A second focus will be stick-slip friction - alternate sticking and sliding between two surfaces. Stick-slip motion leads to squeaks in hinges and to wear of moving parts. The proposed research will extend recent studies which indicate that stick-slip friction is caused by periodic non-equilibrium phase transitions.