Sliding viscoelastic drops on slippery surfaces

We study the sliding of drops of constant-viscosity dilute elastic liquids (Boger fluids) on various surfaces caused by sudden surface inclination. For smooth or roughened hydrophilic surfaces, such as glass or acrylic, there is essentially no difference between these elastic liquids and a Newtonian comparator fluid (with identical shear viscosity, surface tension, and static contact angle). In contrast for embossed polytetrafluoroethylene superhydrophobic surfaces, profound differences are observed: the elastic drops slide at a significantly reduced rate and complex branch-like patterns are left on the surface by the drop's wake including, on various scales, beads-on-a-string like phenomena. Microscopy images indicate that the strong viscoelastic effect is caused by stretching filaments of fluid from isolated islands, residing at pinning sites on the surface pillars, of the order ∼30 μm in size. On this scale, the local strain rates are sufficient to extend the polymer chains, locally increasing the extensional viscosity of the solution, retarding the drop and leaving behind striking branch-like structures on much larger scales.