Advancing liquid contact line on visco-elastic gel substrates: stick-slip vs. continuous motions

We studied the dynamics of water sessile droplets advancing on hydrophobic and visco-elastic poly(styrene-butadiene-styrene)(SBS)–paraffin gel substrates at various inflation rates. During the advancing process, the droplet contact line exhibits three different regimes of motions. When the contact line advances at a high velocity, it moves continuously with a constant contact angle. As the contact line slows down, it starts a stick-slip motion: the contact line is pinned at a certain position and then suddenly slips forward. With further decrease of the velocity, the contact line stops the stick-slip motion and continuously advances again. The observed threshold values for the transitions of the contact line motions (continuous–stick-slip–continuous) indicate that the rheology of the gel drastically affects the dynamics of liquid on its surface. We suggest that on visco-elastic gels, the moving contact line exhibits both aspects of wetting on elastic solids and wetting on viscous liquids depending on the characteristic frequency of the gel surface deformation. At an intermediate regime, the stick-slip motion of the contact line appears. We also propose a simple geometrical model in the stick-slip regime which allows us to relate the jumps of the droplet radius to the jumps of the apparent contact angle.