Short time wetting dynamics on soft surfaces

The wetting of solid surfaces by fluids has been studied for more than two centuries. However, it was only in recent years that investigations of the first milliseconds of spontaneous drop spreading on solid surfaces started. For non-deformable surfaces, this fast dynamic wetting process is known to be dominated by inertia and controlled by surface wettability. In this work we studied spontaneous spreading of liquids on soft, viscoelastic rubber films with shear moduli |G| between 0.2 and 510 kPa and thickness d between 30 and 160 mm. We found that the early stage of fast wetting of soft surfaces is also dominated by inertia and that the wetting dynamics follows a power law which mainly depends on wettability, but not on softness. This finding allows us to apply fast dynamic wetting measurements for inferring the equilibrium contact angle theta(eq) on soft surfaces. On such surfaces static contact angle measurements with sessile drops would not yield univocal results and Young's equation is not directly applicable. On the other hand, the duration of the fast inertial wetting is controlled by surface softness. This is an indication of a viscoelastic dissipation process occurring during wetting that starts after some characteristic time dependent on the surface tension of the liquid, on the viscosity of the surface, and on the speed of wetting.