Slip effects in dewetting polymer microdroplets

Spherical caps on a substrate with less than equilibrium contact angles contract as a result of capillary forces. Applying the classical no-slip condition at the liquid-substrate interface results in diverging stress at the contact line. This divergence can be alleviated, however, by allowing finite flow velocity at the substrate, corresponding to the slip boundary condition. Experiments have been conducted in which glassy polystyrene microdroplets are placed upon, as substrates, different self-assembled monolayers (SAMs). The spherical caps are prepared such that initial contact angles are much less than the equilibrium contact angle. Above the glass transition temperature, a capillary induced flow is observed; the droplet radii shrink while their heights grow. Furthermore, the intermediate height profiles are highly non-spherical. Different SAMs give rise to differing slip lengths, resulting in dramatic changes to the temporal and morphological path these tiny droplets take toward their equilibrium spherical cap shapes.