The quest for more effective and efficient ways to clean man-made surfaces and fabrics is long standing. Time, money, and environmental cost are always involved. In contrast, many plants seem able to stay clean with nothing more than the occasional rinse by dew or rainwater.
Lotus leaves exemplify this ability. Although they appear smooth, under a microscope they reveal an outer skin of bumpy cells covered by a thin layer of water-repellent wax crystals. These two properties make the leaf self-cleaning. Water falling on the waxy, bumpy surface forms spherical beads that readily roll off; any dirt on the surface clings to the water droplets and is easily carried away with the runoff.
This hydrophobic quality (shedding water) is not unique to the lotus. For example, it enables winged insects like butterflies and dragonflies that are unable to clean their wings with their legs (as many other insects do) to, nevertheless, have clean wings.