Non-Ising-Like Effect on the Sound Propagation in a Critical Fluid

Abstract

Sound attenuation and dispersion near the critical point reveals important aspects of order parameter dynamics hidden in direct observations of order parameter. Until recently it has been believed that the mode· coupling theory of Kawasaki l ) fairly well describes the overall features of sound pro· pagation in single· component classical fluids. However, it has become awared that discrep· ancies between the theory and experimental observations are quite serious. 2 ) Since the original theory is confined to the two heatmode intermediate states, attention has been called to the possible importance of mode coupling processes involving three, four, ... heat·modes in intermediate states in the sound propagation problem. In fact, it is shown ) that taking four heat·mode contribu· tion to sound attenuation and dispersion into account resolves the existing discrepancies. In so doing three heat·mode intermediate states were neglected by simply assuming a built· in symmetry of the Ising model (the lattice·gas model) in which fluctuations in the number density and those in the energy density are statistically independent, whereas this is not so in the real liquid· vapor system. 4 )