Liquid Water—Acoustic Properties: Absorption and Relaxation

Abstract

In general, liquids differ from solids primarily in their ability to flow. While in the process of flowing, however, a liquid is not in thermodynamic equilibrium. On a miscoscopic basis, for flow to occur, the molecules must rearrange relative to each other. In thermodynamic equilibrium, a liquid is completely characterized by a small number of state variables (e.g., the internal energy, volume, number of moles, total magnetic and electric moments, etc.). When a system initially in equilibrium is perturbed into a nonequilibrium state, the method by which equilibrium is restored is known as a relaxation process. Any such nonequilibrium state requires that parameters in addition to the state variables be specified for a complete characterization. In the present chapter, we will be concerned with structural relaxation. This is the process by which the molecules of a system “flow” from a nonequilibrium configuration to a new equilibrium configuration.