Capillary rise, wetting layers, and critical phenomena in confined geometry

Abstract

We have used an interferometric technique to measure the capillary rise of sulfur hexafluoride (SF6) between closely spaced, nearly parallel plates. The data span the range of plate separations 0.8–3.5 μm and span temperatures from 2 mK below the critical temperature (Tc=319 K) to 540 mK below Tc. The data demonstrate that a thin wetting layer of liquid SF6 intrudes between each interferometer plate and the SF6 vapor. Close to Tc, where the capillary rise is quite small, the wetting layer becomes as thick as 0.7 μm. These layers are a factor of 3 thicker than our theoretical estimates which are based on very simple models which assume that the thickness is governed by a competition between the gravitational force which tends to thin the layers and long-ranged van der Waals forces which tend to thicken the layers. Although our data are close to Tc, the correlation length in the SF6 is always much less than the separation between the interferometer plates; thus, we did not expect to observe strong departure from three-dimensional critical behavior towards two-dimensional critical behavior. In agreement, we find that the sample of SF6 between the interferometer plates has the same critical temperature as that of bulk SF6 (±1 mK). Furthermore the capillary rise data are consistent with the hypotheses that the surface tension and the difference between the liquid density and the vapor density of this confined sample of SF6 are the same as those of bulk SF6 at the same temperature. These results concerning critical phenomena contrast with results obtained in certain earlier studies of binary liquid mixtures near their consolute temperatures. The earlier experiments were interpreted without consideration of wetting layers to indicate that large critical temperature shifts and a crossover to two-dimensional behavior did occur in a confined geometry and temperature range similar to the one we use. We briefly discuss the effects that wetting layers have on other experiments near the critical point of SF6.