Editorial: Transport phenomena in microgravity

It is in fluids that the effects of gravity—and its absence, microgravity—mainly matter. Fluids are essential in the functioning of spacecrafts and satellites and for sustaining life in crewed missions. It happens that the phenomena of transport in fluids and mixtures of fluids are specially affected by the absence of Earth gravity. On Earth, the variations of mass density under the action of temperature or concentration indeed lead to stratification and/or convection phenomena due to buoyancy effects. Transport phenomena related to other causes, e.g., thermal or mass diffusion, can be thus misleadingly minimized or even ignored. The question that arises when the buoyancy-induced convection and sedimentation effects are canceled is therefore how altered are the transport phenomena. The latter is concerned with different processes and can be classified into three main classes: transport of heat, mass, and momentum. In the absence of forced convection, the transport of heat under microgravity conditions becomes only due to a diffusion process, natural convection under the action of buoyancy forces being canceled. This effect becomes particularly important near the critical point of fluids. Here, due to the critical anomalies in compressibility and thermal expansion, heat transport becomes very unstable on Earth even for minutes temperature gradients. Microgravity allows a very close approach to the critical point and the detailed study of thermal diffusion processes without spurious gravity-induced convection flows (Oprisan et al. contribution). Other phenomena, however, become important. It is the case of the isentropic heating or cooling of a fluid by the expansion or contraction of a thermal boundary layer (the “Piston Effect”). This phenomenon, which provokes a slight motion of the fluid near the edge of the boundary layer, is all the more pronounced that the fluid is close to its critical point (Beysens et al. contribution). It is present on Earth but is masked by the natural convection flows; it was detected thanks to microgravity. Mass transport in a phase separation process under weightlessness is no more dominated by the denser constituent going down and the lighter going up. The effects of diffusion and surface tension lead to only two specific patterns and growth evolution. Boiling, which is a liquid-vapor transition, has been the object of many studies OPEN ACCESS