Sunday, May 21, 2006

Strange effects

click photo to enlarge
A while ago I heard an interesting BBC Radio programme about "the lotus effect". It seems that in the 1970s Wilhelm Barthlott of the University of Bonn wondered why the leaves of the lotus plant stayed clean, and how dirty water just slipped off them. He examined the leaf surface with a scanning electron microscope and found that at a microscopic level the surface was very bumpy. Further, that each bump had bumps, and that those bumps had bumps, etc. Consequently, drops of water and grime stayed roughly spherical on the surface of the leaf, resting on air as well as the tops of bumps, and, with little consequent friction, readily slid off. This is counter-intuitive, because we would expect that water would most easily slide off a smooth surface!

In subsequent years Barthlott developed a surface coating to mimic the effect of the lotus leaf, realising that it had commercial applications. Industrialists were slow to see the value of the invention so Barthlott designed a special spoon. He used it to attract the attention of the sceptical, dipping it into a jar of honey, and watching their amazement as the honey slid off it, leaving the spoon clean. A plate was also made. When honey was placed on it, the sticky substance behaved very much like mercury! In 1994 a patent was granted, and products are now being made that use the invention, including surgical instruments, glass and paint coatings, and roof tiles. Other products that need to stay clean are doubtless in the pipeline.

I was reminded of this when I was photographing the leaves of Lady's Mantle (Alchemilla mollis) in my garden. One of the attractive features of this plant is the way that, after rain, individual beads of water gather on the surface of the leaves. The way they stay there makes it seem like an "anti-lotus effect" is at work! I don't know why this happens, but I suspect it has something to do with the hairs on the leaves. I used a macro lens to take my shot, and stayed far enough back to capture both the drops of rain, the shapes of the open and opening leaves, and the shadows between the leaves.
photograph & text (c) T. Boughen