A few years ago, Joanna Aizenberg (Harvard), David Quéré (ESPCI Paris) and Kripa Varanasi (MIT) realized that textured substrates which are infiltrated by a lubricant form a new class of functional surfaces, called slippery surfaces. Slippery surfaces have several promising applications based on the fact that drops slide on these surfaces with almost zero static friction. Thus far, slippery surfaces have been characterised via video microscopy, macroscopic contact angle measurements and, by measuring the sliding angle. However, the physics of slippery surfaces is still poorly understood. More specifically: it is unknown what happens on the microscale when drops slide and how microscopic processes are related to macroscopic observations. Does the lubricant influence the shape of the drop? How does a drop move on a slippery surface? How do slippery surfaces compare with superhydrophobic surfaces?
Droplet receding on a slippery surface
To optically resolve the shape and the dynamics of drops on slippery surfaces with micrometre resolution, laser scanning confocal microscopy was used. To induce a slow and controlled recession of the solid-TPCL, we let water drops evaporate and monitored the receding contact line. As an example, one sequence of a water drop evaporating from an FC70-infiltrated microarray is shown. For better resolution of the receding motion, the lubricant did not contain a dye. At time t = 0, the drop was pinned at its outer-right edge. The apparent contact angle was ≈160°. During evaporation, the apparent receding contact angle decreased until it reached its apparent receding contact angle. Within less than 3 s, the contact line moved to the left-hand side of the pillar and depinned. Depinning was accompanied by an abrupt increase in the apparent contact angle. Immediately following depinning, the apparent contact angle increased and the process repeated.
Droplet advancing on a slippery surface
To cause the drop to advance, we pushed it forward by applying a weak air flow. After the drop was deposited, the rightmost contact line was pinned at the right-hand side of the pillar. As the drop advanced, the contact line bent downward. This was accompanied by a continuous increase in the contact angle, which gradually approached 180°. As soon as the drop–lubricant interface touched the top face of the rightmost pillar, the contact line jumped forward and the contact angle decreased by up to 20°.