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