Pattern formation by dewetting and evaporating sedimenting suspensions

Pattern formation from drying droplets containing sedimenting particles and dewetting of thin films of such suspensions was studied. The dewetting causes the formation of finger-like patterns near the contact line which leave behind a deposit of branches. We find that the strikingly low speed of dewetting is due to the high particle concentration in the contact line region, leading to a strongly enhanced viscosity. For pattern formation from drying droplets (containing particles), evaporation also causes dewetting. In both cases, we find a similar relationship between the size of the patterns and the dewetting speed. The coefficient of this relationship gives us the effective viscosity at the contact line. We present a simple model that accounts for this, and that shows that the size of the particles is the relevant length scale in both problems.