= Lotus effect
The <Lotus effect> is self-cleaning by water droplets on a strongly water-repellent, textured surface, exemplified by lotus <leaves>. The surface structure reduces the contact of both water and contaminating particles with the solid. Mobile droplets can collect the particles and carry them away, connecting <wetting>, particle adhesion and surface microstructure.<Purity of the sacred lotus, or escape from contamination in biological surfaces by W. Barthlott and C. Neinhuis>[Publisher abstract, relation of surface microrelief, water repellency and reduced particle adhesion]{ref}
The motivating experiment was more specific than simply noticing that leaves look clean. <W. Barthlott> and <C. Neinhuis> artificially contaminated smooth and rough water-repellent plant surfaces with particles, then rinsed them with a sprinkler or fog generator. Rolling droplets removed the particles from the water-repellent surfaces. Comparing surface types linked cleaning performance to the combination of roughness and water repellency.<Purity of the sacred lotus, or escape from contamination in biological surfaces by W. Barthlott and C. Neinhuis>[Primary abstract, contamination and artificial-rinsing experiment]{ref}
A <Cassie–Baxter state> provides an idealized picture of a droplet supported partly by solid asperities and partly over trapped air. This reduces the effective solid contact area. Real cleaning additionally requires the drop to move: <contact-line pinning> and <contact-angle hysteresis> can hold a droplet even when its static <contact angle> is large. Thus measuring a <contact angle> and observing particle removal are different tests.
A force estimate explains why a cleaning experiment should specify drop size and tilt. On a surface inclined by $\alpha$, a droplet of mass $m$ has a downslope component of <gravitational weight>
$$
F_{\parallel}=mg\sin\alpha.
$$
For a $10\,\mathrm{\mu L}$ water drop, taking <density> $1000\,\mathrm{kg\,m^{-3}}$ gives $m=10^{-5}\,\mathrm{kg}$. At $\alpha=10^\circ$, the driving force is about $17\,\mathrm{\mu N}$. A surface that requires more force to overcome <contact-line pinning> can retain that drop. This is an illustrative force balance, not a measured lotus-leaf threshold; different drop volumes supply different driving forces.
For a <biomimetics> experiment, compare contaminated surfaces under the same droplet volume, inclination and rinsing protocol, and record both droplet motion and particles remaining. Wear or a change in surface chemistry can alter the result even if the surface still looks rough. The useful mechanism is the coupled behavior of surface texture, <wetting>, adhesion and droplet motion.
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