Equilibration time, swelling, and the tangential contact-line force on a partially immersed gel thread*

When a liquid contacts a sufficiently soft material, the surface tension of the liquid deforms the substrate at the contact line, forming a ridge. The height of this ridge is on the order of γ/E, known as the elastocapillary length. The precise stress boundary condition at the contact line has a significant effect on this mechanism, but is as yet unresolved. A complicating factor is that liquids also interact with gels by swelling and/or contamination from the uncrosslinked oil in the gel. Hence, it is crucial to distinguish these different mechanisms when conducting an elastocapillary experiment. In our talk, we present experiments with a poly(vinyl siloxane) gel thread partially immersed in liquids of different wettability and swellability. We show that when making stress-strain measurements via fluorescent bead microscopy, the positions of the beads only equilibrate after 12 to 24 hrs. This timescale is incompatible with the use of ethanol as the immersion fluid, since it swells the PVS thread; we find that using FC-40 allows the thread to reach equilibrium without swelling. Once these two effects are resolved, the evidence for a tangential force at the contact line is weak.