Critical adsorption in a well-defined geometry.

A fluid's density profile near a wall is predicted to assume a universal shape near the liquid-vapor critical point, a phenomenon termed critical adsorption. This universal shape is predicted to depend on the boundary conditions of the fluid at the walls and is predicted to be a function of the ratio z/xi, where z is the distance from the wall and xi is the bulk correlation length. A body of evidence confirms the analogous phenomenon of critical adsorption in binary fluids near the critical demixing point, but in the simple liquid-vapor system the experimental situation is not as clear. For example, critical adsorption of SF6 was observed in porous glass for reduced temperature t=T/T(c)-1>10(-3). However, for t<10(-3) a desorption behavior is seen. This desorption has so far resisted rigorous theoretical explanation. We report measurements of the critical adsorption of nitrogen inside a capacitor gap with a simple parallel plate geometry and open gap of 3 microm. Unlike the previous experiments with SF6, the data show a monotonic increase in the adsorption between t=5 x 10(-4) and t=10(-6), consistent with theoretical prediction and without any indication of desorption.