commercializing their research, in 

 hopes of helping the rest of the world 

 shed grime as readily as plants do. 



What Barthlott and company dis- 

 covered is a bit counterintuitive. 

 The secret to the self-cleaning prop- 

 erties of a leaf is its extreme ability 

 to shed, not dirt, but water. Such 

 surfaces are described as superhy- 

 drophobic; they are so water repel- 

 lent that H 2 0 just about leaps off 

 them, taking dirt with it. 



A hydrophobic surface, such as a 

 piece of waxed paper, refuses to 

 be wetted by water. On such a sur- 

 face, water molecules have a greater 

 affinity for other water molecules 

 than they do for the wax. 



Wettability can be quantified by 

 placing a drop of water on a surface 

 and measuring the angle between the 

 edge of the drop and the substrate. 

 Try it: Squeeze a drop onto a clean 

 glass surface, and it will spread out 

 nearly flat, with a contact angle of 

 less than twenty degrees. On waxed 

 paper, though, the same-size drop 

 will stand up high and proud, with a 

 contact angle of ninety degrees or 

 more. In theory, on a surface it just 

 couldn't bear to touch, a water 

 droplet would make so little contact 

 that its angle with the surface would 

 be just shy of 180 degrees. Lotus 

 leaves actually approach such sublime 

 levels of water hatred with contact 

 angles of about 140 degrees. 



When a drop of water falls on your 

 skin, clothes, or any other surface, it 

 flattens out from the impact, jostling 

 and lifting dirt as it splats. On a wet- 

 table surface the drop stays flat, and 

 the dirt simply settles back onto the 

 surface. In contrast, when a drop hits 

 an unwettable surface, the cohesive 

 forces between the water molecules 

 in the drop are much greater than the 

 forces between the water molecules 

 and the surface. So, almost immedi- 

 ately after the drop flattens from the 

 impact, it rebounds into a more near- 

 ly spherical shape. Any dirt touching 

 the drop as it flattens becomes sus- 

 pended in the drop or attached to it 



as it rebounds; either way, the dirt 

 doesn't settle back on the surface. 

 Best of all, since the droplet is round, 

 it readily rolls down any slight incline, 

 carrying the dirt away. 



The connection between hydro- 

 phobicity and cleanliness is old 

 news — it's why people wax their cars. 

 No matter what the ads would have 

 you believe, wax is not scuff resistant; 

 instead, wax makes it harder for dirt 

 to stick and easier to wash dirt away. 



But the secret of the superhydro- 

 phobic lotus leaf is more than just a 

 smooth coating of wax. With a scan- 



ning electron microscope, Barthlott 

 and his colleagues discovered that lo- 

 tus leaves (and the leaves of many oth- 

 er plants) are not smooth at all. 

 Rather, their surfaces are covered with 

 microscopic bumps and ridges, ar- 

 ranged in a complex pattern. The 

 bumps, each just ten microns or so 

 across, keep a water droplet up and 

 moving along the contoured surface. 



When a drop falls on such a sur- 

 face, it deforms as it fills in the gul- 

 lies. But the cohesive forces between 

 the water molecules quickly haul the 

 drop out of the microvalleys, along 

 with any resident dirt. Once the drop 

 rebounds, it touches only the peaks of 

 the little wax mountains, leaving such 

 .1 tiny area in contact with the surface 

 that the adhesive forces between the 

 drop and the leaf's contours are van- 



ishingly small. Without these down- 

 ward pulls, the cohesive forces be- 

 tween the droplets water molecules 

 are able to hold the droplet in a near- 

 ly spherical shape as it rolls off the leaf 

 [see illustrations below}. 



Barthlott patented the pattern of 

 bumps on the hydrophobic sur- 

 face and dubbed it "the Lotus effect." 

 A German paint company then li- 

 censed the patent and developed a 

 paint with emulsified waxes that dries 

 into a microscopically rough surface. 

 Introduced in Europe in 1999, the 



paint arrived in North America this 

 past fall. A house painted with Lotus- 

 effect paint can stay clean as long as 

 the surface is regularly washed with 

 water. No scrubbing allowed, 

 though: that would disturb the 

 microscopic pattern of the surface 

 and thus weaken the self-cleaning 

 properties of the paint. 



At this writing, the product is suit- 

 able for exterior use only, because of 

 the need tor regular dousings. So until 

 my office interior can be hosed off. 1 

 may have to take refuge in the idea 

 that the rougher you are around the 

 edges, the cleaner you are likely to be. 



Adam Summers (asummers@uci.edu) is 

 an assistant professor of bioengineering and of 

 ecology diid evolutionary biology at the 

 { University oj California, Irvine. 



Water droplet hits the surface of a lotus leaf and dislodges resident dirt particles 

 (above left). The droplet then rebounds into a nearly spherical form, because it 

 "hates" to make contact with the waxy, bumpy surface of the leaf (above right). 

 The dirt has no strong affinity for the leaf, either, and sticks with the droplet, 

 which can roll down the slightest incline, picking up more dirt as it goes. 



April 2006 NATURAL HISTORY 



41 



