Spreading and sliding of viscous droplets on soft substrates under external body forces

The deformability of substrates is known to significantly affect the wetting phenomena. In this experimental work, we investigate the influence of the substrate mechanical properties on the deformation and onset of sliding of a sessile droplet under increasing centrifugal force. The dependence of contact angle at the advancing and the receding edge of the droplet in the direction of the applied force, the drop length and the contour shape on the applied force is analysed. Theoretical models are validated against the obtained results to understand the underlying physical phenomena. The experimental setup used for this study is a custom made state of the art device equipped with a rotating arm, tilting head and an environmental chamber. Three cameras placed at right angles to each other give the side, top and back view of the drop. A custom made Matlab script facilitates the post processing of the video images. Polydimethylsiloxane (PDMS) is used as the substrate and water and 50 wt% glycerine solution as the wetting liquids. Drops on softer surfaces showed large hysteresis as compared to drops on the harder substrates. It is also shown that, by increasing the viscosity of the drop the force required to slide the drop on the softer surfaces is reduced.