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.
