Fig. 2: Micropipette force sensor measurements of the friction of water droplets moving on superhydrophobic surfaces.

a Photograph of the MFS setup with a millimetric water droplet on a superhydrophobic surface. The droplet is attached through capillary forces to a force-calibrated micropipette cantilever (pulled from a 1 mm thick glass capillary). The experiment is recorded with a camera from the side. Scale bar 5 mm. b Photograph of water droplet (radius \(R = 620 \pm 8\,\mu\)m, contact region diameter \(D = 210 \pm 20\,\mu\)m) on the spikes surface (sample A). During the experiment (see Supplementary Movie 1), the droplet is initially pulled along the substrate (resting on a motorized xyz-translational stage that starts moving to the left at time ~5 s with a constant speed of \(v = 0.1\) mm s⁻¹) until the elastic force (\(F = -k_{\mathrm{p}}{\mathrm{{\Delta}}}x\), where \(k_{\mathrm{p}}\) is the spring constant of the pipette obtained through calibration) from the deflected (\({\mathrm{{\Delta}}}x\)) micropipette matches the kinetic friction force (\(F_{\upmu}\)) of the substrate. At this point, the micropipette deflection remains nearly constant while the droplet slides along the surface. Scale bar 200 μm. c Force as a function of time from a typical experiment (same as in b). The average equilibrium (zero-force) position of the micropipette was first recorded for ~5 s after which the surface started moving. The difference between the average zero and kinetic plateau force gives the kinetic friction force (\(F_{\upmu}\)). The error includes the standard deviation of both averages.