Surface textures suppress visco-elastic braking on soft substrates

A gravity driven droplet will rapidly flow down an inclined substrate, resisted only by stresses inside the liquid. If the substrate is compliant, with an elastic modulus G < 100 kPa, the droplet will markedly slow as a consequence of viscoelastic braking. This phenomenon arises due to deformations of the solid at the moving contact line, enhancing dissipation in the solid phase. Here, we pattern compliant surfaces with textures and probe their interaction with droplets. We show that the superhydrophobic Cassie state, where a droplet is supported atop air-immersed textures, is preserved on soft textured substrates. Confocal microscopy reveals that every texture in contact with the liquid is deformed by capillary stresses. Surprisingly, droplet velocities down inclined soft or hard textured substrates are indistinguishable; the textures thus suppress visco-elastic braking despite substantial fluid-solid contact. High-speed microscopy shows that contact line velocities atop the pillars vastly exceed those associated with visco-elastic braking. This velocity regime involves less deformation, thus less dissipation, in the solid phase. Such rapid motions are only possible because the textures introduce a new scale and contact line geometry.