ADSORPTION AND DESORPTION OF A WETTING FLUID IN VYCOR STUDIED BY ACOUSTIC AND OPTICAL TECHNIQUES

We study the adsorption and desorption of hexane in porous Vycor, as t he ambient vapor pressure is varied, through sorption isotherm, ultras onic velocity and attenuation, and light scattering measurements. On a dsorption, we show that the fluid fills the pore space uniformly until capillary condensation occurs; however, small, randomly distributed, vapor bubbles remain, as detected by a large increase in the attenuati on of the ultrasound. On desorption, the mismatch in the index of refr action between the empty pores and the surrounding filled pores leads to intense scattering of light that reveals the presence of long-range correlations in the pore space. These correlations have a fractal dim ension of 2.6, which is very near the value predicted for invasion per colation. Finally, we also investigate the time dependence of the chan ges in the adsorbed fluid mass and use these measurements to identify three distinct regimes with vastly differing mechanisms for mass trans port. The results presented here provide information on the difference s in pore-space correlations on filling and drainage, and highlight th e critical role of the connectivity of the pores to the surface in det ermining the desorption behavior.