What is Weightlessness

This chapter addresses the very particular thermal behavior that supercritical fluids exhibit when nearing their critical point. In this region, supercritical fluids exhibit strong anomalies in their thermodynamic and transport properties. Pressure change associated to a temperature variation leads to a nearly isentropic thermalization of the fluid, the “piston effect,” which leads to a paradoxical “critical speeding-up.” Bulk fluid temperature is uniform, and temperature gradients are confined in thermal boundary layers, making the bulk fluid a thermal short-circuit. It follows very particular behavior, as dynamic heat pipes or heat going seemingly backward, in apparent contradiction with the second principle of thermodynamics. Under an acceleration field, thermal convection occurs only in the boundary layers, which paradoxically can enhance the fluid stability or even cool the fluid after a heat pulse. These effects can deeply modify the supercritical fluids thermal behavior in space and energy activities, giving to these effects socio-economic relevance.