New critical phenomena observed under weightlessness

Abstract

A review is given of several new phenomena observed when the effects of gravity are suppressed in fluids and liquid mixtures near a critical point. A new mechanism of heat transport, the “Piston effect”, has been discovered. This mechanism is due to the expansion of a conductive thermal boundary layer and leads to a “critical speeding up” instead of the usual “critical slowing-down”. In the presence of an external force (e.g. gravity) the boundary layer is affected by convection and the effect is reduced. Under certain circumstances, a jet may be formed. New results have been obtained concerning the kinetics of phase ordering. After a temperature quench, which leads to the formation of two phases (e.g. liquid and gas), the morphology and growth of the droplet pattern can be connected and leads to only two universal growth laws: growth linear with time at high volume fraction (Φ>0.35) where the droplets are interconnected, and growth as (time)^1/3 for smaller volume fraction (Φ<0.35). This universality seems to be ruled only by the volume fraction and should apply to any fluid phase ordering process. The effect of an electric field leads to interesting observations, where electrostriction is able to reproduce some features of the gravity phenomena. These observations were made by using density-matched liquid mixtures and fluids onboard Texus rockets, furthermore with experiments on the space shuttle missions D-2, IML-1 and IML-2 and the MIR orbital station during the Antares and Altair missions.