Collision versus collapse of droplets in coarsening of dewetting thin films

Thin films of viscous fluids coating solid surfaces can become unstable due to intermolecular forces, leading to break-up of the film into arrays of droplets. The long-time dynamics of the system can be represented in terms of coupled equations for the masses and positions of the droplets. Analysis of the decrease of energy of the system shows that coarsening, decreasing the number of droplets with increasing time, is favored. Here we describe the two coarsening mechanisms present in dewetting films: (i) mass exchange leading to collapse of individual drops, and (ii) spatial motion leading to droplet collisions and merging events. Regimes where each of mechanisms are dominant are identified, and the statistics of the coarsening process are explained.