Physics of the Liquid and Supercritical States of Matter.

PhD Theses.The liquid and supercritical states of matter are respectively the least understood and most misunderstood states of ordinary matter, from a theoretical view. The work exposited in this thesis aims to elucidate the nature of the supercritical state and its relationship to the liquid and gas states which ank it on the phase diagram. Contrary to the belief of the supercritical state as lacking transitions, these works present several transitions to be found in this state. This is done with molecular dynamics simulations data. We begin with structural crossovers discovered in the two most important supercritical uids from an industrial point of view: water and carbon dioxide. These crossovers coincide with calculations of the dynamical crossover called the Frenkel line, which marks termination of oscillatory molecular motion, giving way to purely di usive motion. These structural crossovers across the Frenkel line demonstrate the universal applicability of the Frenkel picture of uids. I then perform simulations of argon to calculate the dynamical instability of its supercritical state. Using tools from chaos theory, I show that the dynamical instability undergoes a crossover at the Frenkel line, which demonstrates that the supercritical state sports a transition in the fundamental geometry of phase space. The \c"-transition is presented next. This is a universal interrelation between the dynamics and thermodynamics across the supercritical state and a transition which provides an unambiguous separation of liquidlike and gaslike states in several di erent supercritical uids. This discovery was completely unanticipated and is like nothing else ever seen in the supercritical state. The \c"-transition is suggestive of a phase transition, and the thesis concludes with the beginnings of a search for it. The heat capacity is calculated from molecular dynamics simulations of argon with unprecedented precision, and other methods used to nd a phase transition are discussed