Supercritical transition mechanism of immiscible ethanol/hexadecane droplets

Abstract

Although the properties of supercritical fluids have been significantly focused on, few studies have focused on the transition process of multicomponent fluids from the subcritical to supercritical state. Herein, the trans-critical behavior of an immiscible binary ethanol/hexadecane (EtOH/C16) droplet under supercritical nitrogen environments is analyzed for the first time using molecular dynamics. The ambient pressure (p) and ambient temperature (T) of the droplets exceed the critical conditions for both EtOH and C16. Moreover, trans-critical EtOH/C16 droplets undergo a two-stage bulging-to-shrinking process. Liquid fraction λ is introduced to quantify the structural characteristics of trans-critical fluids. Fluids inside the droplet are shown to transform from the vapor to liquid phase, signified by the increase in λ with p. Three droplet evolution types are classified based on p: micro-explosion, puffing, and mixing types. The occurrence of each trans-critical droplet evolution type is determined by the competition between the subcritical gasification of EtOH and the supercritical pseudo-boiling of C16. The supercritical transition of C16 can be detected under each condition, while that of EtOH only occurs for the mixing-type droplet evolution. Furthermore, a p–T diagram is provided to analyze the combined effect of T and p on the trans-critical droplet evolution types.