The phase transition of gases and liquids

The behavior of gases and liquids during a phase transition is severely affected by gravity effects which unavoidably drive the denser phase downwards and the lighter phase upwards. When such gravity effects are suppressed a number of new and surprising behavior are observed. The studies are performed mostly in the vicinity of a critical point where the behavior can be expressed in scaled units of a natural length scale (the correlation length of the order parameter fluctuations) and a natural timescale (the characteristic relaxation time of the order parameter fluctuations). We will review the main results obtained with binary liquids and simple fluids. Firstly, a phase change is initiated by quenching down the fluid from above the coexistence curve, where the fluid is homogeneous, to inside the coexistence curve, where its phase separates into two phases. The main result is the recognition that the hydrodynamics of coalescence eventually induces the pattern morphology and the phase transition kinetics. The volume fraction, is the key parameter which decides whether drops fuse because of random, Brownian collisions [pattern of drops, growth law in (time)1/3], or coalesce in a continuous process where the flow due to a coalescence event induces another coalescence (growth proportional to time, interconnected drop pattern). The presence of a wall modifies by its geometry and wetting properties the phase development. Coalescence is constrained and leads to new growth laws in the immediate vicinity of the wall. We emphasize that these phase ordering processes are quite general; they can be applied with success to quite different ordering situations, such as the sorting of embryonic tissues, an important process in morphogenesis. Secondly, when a gas–liquid, two-phase fluid, is heated from below the critical point to above, the process appears to be driven by the wetting and thermal properties of the boundary layer near the heating wall. Under terrestrial gravity, the dynamics is driven by a Rayleigh–Taylor instability. Under weightlessness, the liquid/vapor contact angle on the wall is modified in such a way that the gas seemingly “wets” the wall. We propose the vapor recoil force to be at the origin of this non-equilibrium “wetting” transition.