Acoustic Studies of Liquid-Vapor Critical Behavior of Neon and Helium in Aerogels

We have used an acoustic resonator to study the liquid-vapor coexistence curve and critical region of simple fluids (neon and helium) confined in high porosity silica aerogels. A diverging isothermal compressibility at a critical point appears as a sharp minimum in the sound velocity. The low frequencies of the resonator (a few kHz) allow a more direct probe of the fluid compressibility than was possible in previous ultrasonic measurements. We have also made direct density-pressure measurements in aerogels, using a capacitive technique to avoid complications from any bulk fluid present in the experiment. The presence of the aerogel lowers the fluidsʹʹ critical temperatures and narrows their coexistence curves. We compare our results to those for bulk neon and to previous measurements in aerogels.