Critical boiling phenomena observed in microgravity

We report experimental observations of the critical boiling when co-existin g gas and liquid phases of pure fluid are heated under weightlessness throu gh the critical point. We find that when the system's temperature T is bein g increased to the critical temperature T-c so that it's slightly out of eq uilibrium, the apparent contact angle becomes very large (up to 110 degrees ). The gas appears to <<wet>> the solid surface, In addition we detect larg e temperature gradients between the (hot) gas phase and the (cold) liquid p hase in the interferometric cell. These unexpected results are robust: they are observed either under continuous heating (ramping) or stepping by posi tive temperature quenches, for various morphologies of the gas bubble and i n different fluids (SF6 and CO2). The difference in isentropic thermal resp onses 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 inte rface deformation.