Pattern formation by rim instability in dewetting polymer thin films

The dewetting process of thin polystyrene films from 5 to 50nm in thickness on a silicon substrate is investigated. A spreading dry patch formed on dewetting is surrounded with a circular rim, which stores the liquid matter expelled from the dewetted region. When the diameter of the patch exceeds a thickness-dependent critical value, the rim is deformed by morphological instability; the instability gives rise to the diversity of self-organization processes of liquid ridges or droplets formed eventually on partially wettable substrates. A morphological phase diagram of the deformed rim is given in the diameter of dry patch and the initial film thickness. In the final stage of dewetting, liquid droplets arrange to form a "polygon network" in thick films and "polygons with random droplets" in thin films. The origin of the instability is discussed in terms of surface free energy.