When and how particles are removed by drops

Particulate contaminants decrease the power output of solar panels and the transparency of windows and are detrimental to microelectronics, where even a single particle can induce a short circuit. Despite substantial research on particle adhesion and self-cleaning, it remains unclear when and how a drop can remove a particle from a surface, thus efficiently cleaning the surface. Here, by combining lattice Boltzmann simulations and confocal microscopy experiments, we show that at least six different scenarios arise from the complex interplay between capillary and friction forces when a drop collides with a particle on a surface. Notably, the capillary force plays a dual role in particle removal: While its tangential component always drives removal, its normal component can also hinder it. We introduce a dimensionless capillary capture parameter that successfully predicts particle removal across a wide range of particle and surface properties. Our results reveal that the design of easy-to-clean surfaces should focus not only on maximizing hydrophobicity but also on minimizing the particle-surface friction.