Direct friction force measurements of water droplets rolling on superhydrophobic surfaces

Extremely water repellent surfaces are common in nature and widely mimicked for technological applications such as anti-wetting, anti-fouling, and anti-fogging coatings. As the quality of these artificial superhydrophobic surfaces reach new, improved levels, the existing characterisation techniques of their slipperiness have become inadequate. In our work, we have applied micropipette force sensors to directly quantify droplet friction forces at nanonewton resolution (Backholm, et al., Commun. Mater. 1, 64 (2020)). We have studied the physics and fluid dynamics of water droplets moving on extremely slippery superhydrophobic surfaces and report a surprising rolling motion of slowly moving droplets. Our findings enable the search for even more slippery surfaces as the wetting community approaches the extreme limit of slipperiness.