Friction Measurements with Picoliter Droplets Using Scanning Probe Microscopy

Topographical and chemical defects on solid surfaces tend to pin three-phase contact lines of moving liquid drops. Our quantitative understanding of the pinning process is, however, still poor. Here we use an atomic force microscope to slide ≈100  pL droplets of water-glycerol mixtures over hydrophobic surfaces and measure friction forces. By using picoliter droplets, the sensitivity for detecting processes at the contact line is enhanced. We have found that only a region <200  nm around the contact line contributes to friction. By imaging isolated nanospherical defects, we could quantify the force and energy dissipation when the front and rear of the droplet passes the defect and compare it to theory.