Non-expert summaryThe authors use confocal microscopy to directly image how drops (approx. 1 mm diameter) capture and remove particles (approx. 250 µm diameter) from flat hydrophobic surfaces at a range of speeds (10-500 µm/s). The force acting on the drop during the removal process is also measured by imaging the deflection of a flexible cantilever with a known spring constant.
At low speeds (50 µm/s), the particle attaches to the drop/air interface due to surface tension, and the particle is successfully removed. At high speeds (500 µm/s), the particle crosses the interface because surface tension is insufficient to overcome the friction force acting on the particle as it rolls on the surface. Consequently, the particle detaches from the drop and remains on the surface. In both cases, the viscous stress acting on the particle is small compared to surface tension and can be neglected when the liquid is water.
These results suggest that two main forces govern the removal of particles from surfaces by drops: surface tension of the drop and friction force between the particle and the surface. These findings will help guide the design of self-cleaning surfaces.