Friction of two-dimensional colloids on the disordered substrate

The Langevin molecular dynamics simulations are applied to investigating the friction of two-dimensional (2D) colloids on the disordered substrate. It is found that for lower initial velocities, the frictional force increases linearly with the initial velocity, in accordance with Stokes's law. For higher initial velocities, the frictional force increases nonlinearly with the initial velocity and then tends to saturate, consistent with recent experiments [Phys. Rev. Lett. 131 , 266201 (2023)] and exhibiting a decrease in the size of large islands and the appearance of individual particles in the moving configuration. In addition, it is observed that when the normal load is below a certain threshold, the frictional force is independent of the normal load, also consistent with recent experiments [Phys. Rev. Lett. 131 , 266201 (2023)], while when the normal load is above the threshold, the frictional force is proportional to the normal load, in accordance with the macroscopic friction law. The frictional force remains constant as the normal load further increases. The threshold of the normal load depends on the initial velocity, and their relationship is similar to the relationship between frictional force and initial velocity. The temperature dependence of viscosity-dominated friction is minimal under low normal loads. The primary source of viscosity is the interparticle interaction, indirectly highlighting the role of these interactions in friction.