Contact line motion in dewetted polymer droplets on anisotropic nanopatterned surfaces

Abstract

Thin polymer films can become unstable and dewet on a non-wettable substrate leading to the formation of an array of droplets. This instability-mediated drop formation lacks spatial order on flat substrates, but it can be ordered for better combinatorial studies using patterned substrates. In this work, we studied the process of dewetting polystyrene (PS) films on grating patterned substrates upon solvent vapor exposure. The PS film thickness was commensurate with the grating pattern height (h_P). Our findings show that the initial dewetting stages follow the direction of the underlying grating pattern with the formation of directional holes in the film. During the later stages of dewetting, there was a lateral coalescence of polymer threads across the grating stripes. The final morphology comprised smaller droplets or threads confined within the pattern grooves and anisotropic large drops covering several pattern stripes. Furthermore, the larger drops show a unique behavior of shape change from anisotropic to isotropic as a function of solvent vapor concentration (C_n) inside the dewetting chamber. The drop regained its anisotropic shape with an increase in C_n, and this transition continued, with the movement of the three-phase contact line (TPCL). While the capillary flow of the polymer causes anisotropy during high C_n, the local orientation of the contact line and a mismatch in the value of the equilibrium contact angle can drive the drop back into an isotropic shape as C_n reduces and capillary forces weaken. We also observed that the extent of anisotropy quantified as droplet distortion ratio (D_r) not only depends on the C_n during dewetting but also on solvent type and h_P. This new-found dynamic behavior of dewetted polymeric drops can be studied in greater detail and potentially leveraged for applications in sensing and microfluidics.