Liquid droplet mops

Surface contamination is a common challenge in both natural and industrial contexts, often reducing the efficiency of various processes. Water is widely used for cleaning due to its low cost and natural abundance, but conventional methods consume large amounts of water because of low utilization efficiency—especially in solar farm maintenance. Here, we report a counter-intuitive phenomenon: cleaning efficiency depends non-monotonically on droplet energy, reaching an optimum at intermediate values. Notably, a raindrop-sized droplet can effectively remove particles of various densities from superhydrophobic surfaces. Through experiments and theoretical modeling, we show that droplet impact velocity and particle-droplet interactions govern removal behavior. We construct regime maps outlining four outcomes, from no lift-off to complete particle removal. Leveraging this mechanism, we introduce "droplet mops" as an efficient cleaning method for solar panels, achieving 99.9% removal efficiency while using ~10 times less water than traditional liquid jets. This approach offers a simple, scalable, and water-saving alternative. Based on our estimates, adopting this method at just 10% of global solar farms could save up to 1 billion gallons of water annually.