Molecular dynamic simulation of adhesional release of particles from surfaces

The generalized form of the Lennard-Jones soft-sphere pair potential is used to examine the adhesional attachment and release behavior of particles on surfaces for materials of varying "ductility". Using a two-dimensional simulation, with the repulsive term held constant at m = 12, the attractive term is varied from n = 2 to n = 10 at a constant binding energy to provide a controlled way of changing the effective range of interaction between atoms. Molecular dynamics simulates the placement of particles on a free surface and the subsequent removal of these particles by controlling the displacement of the center of mass. Simulations indicate that the longer-ranged removal forces literally tear out a chunk of the surface by pulling out a tether-like strand connecting the ball and plate. As the forces become shorter range, the size of the region of disturbed material after separation decreases until, at the shortest range, only three atoms are transferred and there is relatively little damage to the system on separation as indicated by the lack of slip steps in the plate. Results are discussed both mechanistically and from the point of view of more traditional approaches to surface force behavior, such as the JKR and DMT models.