Probing the physical origins of droplet friction using a critically damped cantilever.

Previously, we and others have used cantilever-based techniques to measure droplet friction on various surfaces, but typically at low speeds U < 1 mm s -1 ; at higher speeds, friction measurements become inaccurate because of ringing artefacts. Here, we are able to eliminate the ringing noise using a critically damped cantilever. We measured droplet friction on a superhydrophobic surface over a wide range of speeds U = 10 -5 -10 -1 m s -1 and identified two regimes corresponding to two different physical origins of droplet friction. At low speeds U < 1 cm s -1 , the droplet is in contact with the top-most solid (Cassie-Baxter), and friction is dominated by contact-line pinning with F fric force that is independent of U . In contrast, at high speeds U > 1 cm s -1 , the droplet lifts off the surface, and friction is dominated by viscous dissipation in the air layer with F fric ∝ U 2/3 consistent with Landau-Levich-Derjaguin predictions. The same scaling applies for superhydrophobic and underwater superoleophobic surfaces despite their very different surface topographies and chemistries, i.e. , the friction scaling law derived here is universal.