When the apolar (LW) and polar (P) components of the spreading pressure (S(LW), S(P)) are of opposite signs, thin non-equilibrium films can undergo a morphological phase separation (MPS) manifested as a periodic structure of nanodrops in stable coexistence with thinner flat films. The morphology, film pressure, and equilibrium contact angles of the resulting stable microscopic drops are investigated based on the nonlinear dynamics of the growth and saturation of the film-instability. The equilibrium contact angles of microscopic drops are substantially smaller than those of their macroscopic counterparts, which are only attained for relatively large drops. Simulations show that the equilibrium film pressure of the microdrop is largely independent of the drop size, and may be assumed to be the same as for the corresponding large drop in calculations of the contact angle. The reasons microdrop angles are smaller are: that (a) a significant portion of the drop (and not just the contact zone) experiences excess intermolecular interactions, and (b) due to the large curvature of the drop, the capillary pressure is not always negligible compared to the disjoining/conjoining pressure. The contact angles of type II systems (S(LW) < 0, S(P) > 0) are very small compared to those of type IV systems (S(LW) > 0, S(P) < 0) for the same value of the total spreading coefficient, S, because of significant film pressure, II --> S, for type II systems. Finally, thick films of type IV systems remain stable (wetting), but thin films evolve into microdrops of finite contact angles, regardless of the sign of the spreading pressure. (C) 1994 Academic Press, Inc.
