Penn State Physicists Measure Critical Adsorption of Nitrogen in a Simple Geometry

When a fluid is brought close to the liquid-vapor critical point, the density of the fluid near the wall of the container is expected to increase dramatically. This increase, called critical adsorption, is expected in the framework of renormalization group theory of critical points. Earlier measurements of critical adsorption were carried out in porous media with complicated geometry making comparison with theory difficult. At Penn State University, Sarah Scheidemantel, Klaus Knorr, Rafael Garcia, and Moses Chan have finished an experiment measuring the critical adsorption of nitrogen in a simple, tractable parallel plate geometry with a gap of three micrometers. The Penn State group was able to extend the measurements to within a reduced temperature of 10-6 (over two orders-of-magnitude closer in temperature to the critical point than previous experiments) and they also measured the phenomenon over a wide range of densities near the critical point. Data closest to the critical point were strongly influenced by gravity. However, when the data were corrected for gravitational effects, the results appeared consistent with the renormalization group predictions.