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.

固体表面的形貌与化学缺陷往往会钉扎运动液滴的三相接触线（three-phase contact line）。然而，当前学界对该钉扎过程的定量认知仍较为有限。本研究借助原子力显微镜（atomic force microscope），在疏水表面上滑动约100皮升的水甘油混合液滴并测量其摩擦力。通过采用皮升尺度液滴，我们提升了对接触线处相关过程的检测灵敏度。本研究发现，仅接触线周围200纳米以内的区域会对摩擦力产生贡献。通过对孤立纳米球形缺陷进行成像，我们得以量化液滴前后缘经过该缺陷时的受力与能量耗散情况，并将实验结果与理论模型进行对比。