Detection of liquid-vapour interface in molecular dynamics simulation

The liquid-vapour interface plays an important role in many physical and chemical processes. In molecular-dynamics simulation of the liquid-vapour coexistence system, it is important to detect the liquid-vapour interface. In the present work, a detection method based on coordination number is proposed to identify the liquid molecules at the liquid-vapour interface. The coordination of a fluid molecule is defined as the number of its neighboring molecules that are within a certain distance. These neighboring molecules are actually confined in a sphere, whose center is the fluid molecule and whose radius is referred to as the reference radius. A small reference radius was employed to distinguish liquid molecules from vapour molecules. By using a larger reference radius, the present method is not only able to identify the liquid molecules but also detect the liquid molecules at liquid-vapour interface. The coordination number of the liquid molecules at the liquid-vapour interface is found between two critical values. The lower critical value is determined based on the density distribution perpendicular to the liquid-vapour interface while the upper critical value is determined by a trial and error method. The proposed detection method is applied to analyze the dynamic evolution of the coalescence-induced droplet jumping phenomenon.