Phase change of a confined subcooled simple liquid in a nanoscale cavity

The phase transition of a simple liquid bounded between two parallel walls a few nanometers apart is investigated with molecular dynamics simulations. Vapor nucleation in a liquid confined in a microchannel of only a few nanometers in size cannot be achieved by increasing the temperature at the wall. Already small changes in temperature cause a large rise in pressure, in terms of orders of magnitude. On the other hand, using the fact that some fluids thermally contract on cooling, e.g., the argon liquid investigated here, reducing the temperature places the fluid in the liquid-vapor coexistence regime. If the bottom wall temperature is further reduced, the fluid will crystallize starting from the bottom surface, creating a "frozen" bubble in the crystallized state. It was found that the confining walls and not the quenching rate primarily affect the crystallization process. However, the fastest cooling rate investigated herein led to a decrease of the boiling initiation temperature. At a lower cooling rate, the vapor nucleation temperature was the same as the equilibrium boiling temperature for the confined liquid. Small temperatures in the confined system result in dominating attraction forces at the fluid-wall interface exposing the fluid to tensile stress. The increased influence of the walls results in a significant decrease of the boiling as well as freezing temperatures.