Fig. 1

Schematic of the calculation of individual free paths in molecular dynamics (MD) simulations: the parameters l and v are the individual free path and velocity of molecules, respectively; r_ij is the distance between two molecules, σ_col is the collision diameter which is used to determine the collision event; Δt is the time step in MD, and n is the number of time steps between two successive collisions of the ith molecule. It should be noted that x and y are orthogonal coordinate directions (i.e. the walls are oriented along the xy-plane) and the distance between the walls is along z direction