Fig. 4: Friction measurements with a cantilever-based force probe.

a Schematic (not-to-scale) showing a droplet attached by capillarity to an elastic cantilever with spring constant k. The friction force acting on the droplet can be measured by moving the substrate and detecting the cantilever deflection Δx with a camera, since F = kΔx. b Photograph of the cantilever-based force probe. Scale bar is 5 mm. c Experimental force data of a water droplet moving on an extremely slippery superhydrophobic etched silicon sample, showing the initial static bump and the subsequent kinetic friction F_fric plateau. d Particle image velocimetry can be incorporated to quantify the flow profile inside the droplet⁸¹. b–d are reprinted with permission from ref. ⁸¹. e Reflection interference contrast microscopy can be used to visualize details of droplet contact, such as contact line pinning on micropillared surface³². Scale bar is 0.1 mm. f Plot showing F_fric as a function of substrate speed U for different liquid-repellent surfaces. Reprinted with permission from ref. ³². Error bars are standard deviations for three or more repeats. g The dimensionless friction force of drops as measured with cantilever-based techniques for different liquid-repellent samples. Reprinted with permission from ref. ⁸¹. Error bars are for different samples and experimental conditions (e.g., droplet speeds).