Simultaneous Spreading and Imbibition of Droplets on Crosslinked PDMS Films

A liquid droplet spreads on a completely wetting substrate according to Tanner’s Law, where the radius, R, increases with time, t, as R ∼ t^1/10. This behavior is altered when a droplet is simultaneously spreading and imbibing into a substrate. While imbibition into rigid porous media is well understood, less attention has been given to imbibition into soft, elastomeric media. We investigate the simultaneous spreading and imbibition of a silicone oil droplet on an ideal crosslinked poly(dimethylsiloxane) (PDMS) thin film. By varying the film thickness, we alter both the effective stiffness of the substrate film, and the droplet penetration depth. We characterize the spreading dynamics and find that droplets spread faster on thinner films. A modified Tanner’s Law helps explain these wetting dynamics.