We built up a method that allows the measurement of forces required to slide sessile drops
over surfaces. The forces were measured by means of a deflectable glass capillary stuck
in the drop (schematic image close by). Deflections of this capillary are detected by a laser beam that
is reflected from the glas capillary surface and falls onto a postion sensitive detector.
The drop adhesion force instrument (DAFI) allowed the investigation of the
dynamic lateral adhesion force of water drops of 0.1 to 2 µL volume at defined velocities.
The movement of the drop relative to the surfaces enabled us to resolve the pinning of
the three-phase contact line to individual defects
The DAFI is a suitable tool for characterizing lateral adhesion and thus hydrophobicity. In particular, the relative movement of the drop on surfaces allows us to resolve different wetting phenomena of surfaces spatially. Thus, the DAFI could even be applied for quality control of surfaces made by large-scale industrial processes. We used the DAFI to investigate the "static" and a "kinetic" regime of sliding drops .
We are using the setup to characterize omniphobic surfaces and study the interaction between transport and wetting processes within the collaborative reserach center 1194. Recently, we investigated how contamintations are removed from surfaces by sliding drops. We monitored the removal of individual contaminant particles on the micron scale by confocal microscopy while drops are kept in position by DAFI. We correlate the space- and time-resolved information with measurements of the lateral friction force of the sliding drop .
 Dynamic Measurement of the Force Required to Move a Liquid Drop on a Solid Surface, D. W. Pilat et al., Langmuir 28, 16812-16820 (2012).
 How drops start sliding over solid surfaces, Nan Gao et al., Nature Physics, 14, 191 - 196 (2018).
 When and how self-cleaning of superhydrophobic surfaces works, F. Geyer et al., Science Advances 6, eaaw9727 (2020).