When a thin layer of polystyrene of about 100 nanometres is dewetting from a PDMS layer we observe the appearance of nucleated holes at the surface. The polystyrene that is removed from the dry centre of the hole accumulates at a dewetting rime surrounding the latter. These dewetting rims have characteristic shapes and grow with time accumulating more and more of the dewetted polystyrene. For the dewetting dynamics we can distinguish three regimes according to the elasticities of the PDMS substrates by spanning a range of three orders of magnitude (few MPa to few kPa).
The first regime concerns elastic moduli of few MPa to few hundreds kPa where we could observe dewetting velocity decreases for decreasing elastic module. The second regime (from few hundreds kPa to few ten kPa) is a regime where the dewetting velocity is very low (tends to zero); and the last regime concerns elasticities in the range of only a few kPa where we could observe an increasing dewetting velocities for decreasing elastic moduli. A comparison of the rim shapes, heights, and contact angles for these three regimes gives valuable hints about the influencing parameters involved on the dewetting dynamics and therefore would lead for a better theoretical description.
In the latter stage of the dewetting process, the dewetting rims coalesce and form a pattern of straight ribbons that decay by Rayleigh-Plateau instability into isolated droplets which are the equilibrium state of dewetting and enable to characterize E-module and size dependent contact angles that can be correlated to the dewetting dynamics in the three different regimes.
Our experiments consist of polystyrene 18kg/mol layers with a typical thickness of 120 nm dewetting from PDMS rubber substrates with typical thicknesses of 6-10 microns and variable E-module. The polystyrene layers are prepared in a glassy state and dewetting is started when heating the samples above the glass transition temperature of the polystyrene. When a desired dewetting situation is reached the sample can be quenched down to room temperature and the rim or droplet shape can be obtained by atomic force microscopy. Lifting the glassy polystyrene layer off from the rubber PDMS layer using a UV-curable glue enables additionally to image the formerly buried polystyrene/PDMS interface and thus to obtain the full three dimensional shape of the dewetting morphologies.