Spin Dewetting of Ultrathin Polymer Films

Spin coating is a very well-established technique for creating ultrathin films of polymers, colloids, sol-gel devices, inorganic materials, and so on. However, it is often observed that when a very dilute polymer solution over a defect-free, flat substrate is spin-coated, a continuous film does not form. Generally, this is attributed to defects on the surface, which are likely to result in patchy deposition. However, we show that an isotropic array of nearly equal-sized polymer droplets spanning over the entire substrate forms, right after spin coating of an extremely dilute solution, in a reproducible manner, which cannot be due to the existence of any surface defects. We argue that this happens due to spontaneous instability and subsequent dewetting of the casting solution during rotation due to the Marangoni effect and term the phenomenon as spin dewetting. We report the effect of parametric variation on spin dewetting and show, among other results, that the feature size (d_D) and periodicity (λ_F) of the droplets reduce with an increase in the concentration of the casting solution (C_n). Based on our observations, we argue that spin dewetting is strongly influenced by evaporation-mediated Marangoni stress, which is in clear contrast to disjoining pressure-mediated dewetting of thin polymer films. As the patterns form during spin coating itself, the technique can be used as a rapid, ultrafast meso-patterning technique with excellent reproducibility.