Late-stage coarsening of an unstable structured liquid film

Thin films of poly(styrene-b-methyl methacrylate) diblock copolymers above the bulk order-disorder transition temperature, within a certain thickness range, are structurally unstable on SiO(x)/Si substrates. They dewet autophobically, forming droplets on a self-assembled brush. We investigated the late-stage evolution, coarsening, of the droplets on the brush. The average droplet cross-sectional area increased with time, proportional, variant t(gamma). This was accompanied by a decrease in the number of droplets per unit area with time, N(t) proportional, variant t(-gamma). We analyzed the droplet size distribution, F(S/) vs S/, and found that the shape of the curve was virtually identical at different stages throughout the process. This suggests that a structural self-similarity is associated with the process. A comparison of F(S/) vs S/ data with distributions based on Ostwald ripening and coalescence cluster coarsening (dynamic and static) mechanisms strongly indicates that the dominant coarsening mechanism involved motion of droplets across the brush and subsequent coalescence, i.e., dynamic coalescence coarsening, not Ostwald ripening.