Gas “Wets” a Solid Wall in Orbit

When coexisting gas and liquid phases of a pure fluid are heated through their critical point, large-scale density fluctuations make the fluid extremely compressible and expandable and slow the diffusive transport. These properties lead to perfect wetting by the liquid phase (zero contact angle) near the critical temperature Tc . However, when the system’s temperature T is increased to Tc, so that it is slightly out of equilibrium, the apparent contact angle is very large (up to 110°), and the gas appears to ‘‘wet’’ the solid surface. These experiments were performed and repeated on several missions on the Mir space station using the Alice-II instrument, to suppress buoyancy-driven flows and gravitational constraints on the liquid–gas iInterface. These unexpected results are robust, i.e., they are observed under either continuous heating (ramping) or stepping by positive temperature quenches, for various morphologies of the gas bubble and in different fluids (SF6 and CO2 ). Possible causes of this phenomenon include both a surface-tension gradient, due to a temperature gradient along the iInterface, and the vapor recoil force, due to evaporation. It appears that the vapor recoil force has a more dominant divergence and explains qualitatively the large apparent contact angle far below Tc .