Liquid Contact Line on Visco-Elastic Gels: Stick-Slip vs Continuous Motions

We studied the dynamics of water sessile drops being inflated on hydrophobic and visco-elastic Poly(styrene-butadiene-styrene)-paraffin gel substrates. While advancing, the droplet contact line exhibits three different motions. When the contact line advances at a high velocity, the contact line moves continuously with a constant contact angle. As the contact line slows down, it starts stick-slip: the contact line stays at a certain position then suddenly slips forward. With further decrease of the velocity, the contact line stops stick-slip and continuously advances again. The type of motions (continuous - stick-slip - continuous) depends on drop radius and mean velocity only through their ratio, $f = v/R$, which is a typical frequency of the contact line motion. The observed transitions of the contact line motions indicate that the rheology of gel drastically contributes to the wetting dynamics on its surface. On visco-elastic gels, the moving contact line exhibits both aspects of wetting on elastic solids and wetting on viscous liquids depending on $f$. At an intermediate regime, the stick-slip motion appears. In this conference, we will present our latest experimental results and suggest possible mechanisms explaining the present phenomena.