Exploring the gap between theory and experiment at the three-phase contact line of polystyrene droplets on soft PDMS

The shape of liquid polystyrene (PS) droplets obtained via the dewetting of nanometer thin PS films from soft viscoelastic polydimethylsiloxane (PDMS) substrates are investigated. For a range of droplet sizes and substrate elasticities we measure the profiles of all the interfaces by combining lift-off techniques with atomic force microscopy and compare them to the predictions of fully time-dependent sharp-interface models for the PS/PDMS system, that are derived through energy minimization methods and allow to follow the dewetting dynamics towards their equilibrium states. Our analysis shows that there is a thin layer of uncrosslinked PDMS molecules that cloaks the PS droplets. By incorporating the effect of cloaking into the surface energies of our theoretical model, the experimental droplet and substrate profiles are shown to be in excellent quantitative agreement for all considered droplet sizes and substrate elasticities. Interestingly, our comparisons also establish small but systematic discrepancies between the experimental results and the theoretical predictions in the vicinity of the three-phase contact line. These discrepancies tend to increase for softer substrates and smaller droplets. Our analysis shows that global variations in system parameters, such as surface tension and elastic shear modulus, cannot account for these differences but instead point to a locally larger elastocapillary length, whose possible origins we investigate in detail.