Wetting film dynamics during evaporation under weightlessness in a near-critical fluid.

By performing near-critical fluid experiments in the weightlessness of an orbiting space vehicle, we have suppressed buoyancy-driven flows and gravitational constraints on the liquid-gas interface of a large gas bubble. At equilibrium, the liquid completely wets the walls of a cylindrical cell, and the bubble is pushed to the sidewall. In these experiments the system's temperature T is increased at a constant rate past the critical temperature T(C), pushing it slightly out of equilibrium. The wetting film shows a large mechanical response to this heating, including contact lines that recede on a solid surface and a spreading bubble. Near T(C), the receding contact lines make the entire bubble appear to spread along the copper sidewall. The spreading bubble is a manifestation of the boiling crisis near the critical point. We present quantitative data of the receding contact lines that are observed prior to the near-critical boiling crisis. We analyze the receding contact lines in detail, and find that they are driven by vapor recoil from evaporation, as is the spreading bubble of the boiling crisis.