Critical boiling phenomena observed in microgravity

We report experimental observations of the critical boiling when co-existing gas and liquid phases of pure fluid are heated under weightlessness through the critical point. We find that when the system's temperature T is being increased to the critical temperature T-c so that it's slightly out of equilibrium, the apparent contact angle becomes very large (up to 110 degrees). The gas appears to <<wet>> the solid surface, In addition we detect large temperature gradients between the (hot) gas phase and the (cold) liquid phase in the interferometric cell. These unexpected results are robust: they are observed either under continuous heating (ramping) or stepping by positive temperature quenches, for various morphologies of the gas bubble and in different fluids (SF6 and CO2). The difference in isentropic thermal responses of gas and liquid during heating due to the adiabatic heating by the << Piston Effect >>, is responsible for the temperature non-homogeneities. The vapour recoil force due to liquid evaporation, which is involved in the boiling crisis in heat exchangers, is presumably at the origin of the interface deformation.