Study on mechanism of abnormal variation of n-dodecane film thickness under supercritical condition by molecular dynamic simulations

Abstract

Though extensive studies have been carried out to study the supercritical transition behaviors, some key issues in the supercritical fluids have not been well understood. Recently, some researchers proposed that there is an abnormal increase of droplet diameter during the supercritical transition. However, it is unclear whether the occurrence of this abnormal increase would be affected by different definitions of vapor–liquid interface, and the underlying physics has not been unraveled. Besides, the relationship between the occurrence of supercritical transition and abnormal variation has not been clearly demonstated. In this study, the evolutions of n-dodecane film thickness are predicted by three vapor–liquid interface definitions. Although the values of film thickness are different among definitions, the evolution trends are similar, confirming the negligible effect of interface definition on the occurrence of abnormal variation. Based on the microscopic structure and thermodynamic analyses, the abnormal thickness variation is observed to occur after the supercritical transition rather than the transition instant. This is attributed to the sparse molecular distribution due to the abrupt variations of film temperature and supercritical diffusion rate.