The Green Geek

Another step towards self-cleaning surfaces

Japanese scientists have made an important step towards self-cleaning surfaces with the development of a material which can switch from water absorbent to water repellant on command. The inspiration? A lotus blossom. Water drops bead up and roll off of the leaves water-repellent surface, washing away every speck of dust. This type of self-cleaning surface would be very useful to us as well: No more car washes, no more dirty windows, no more signs obscured by mud or dust, all as an inherent ability of the object.

The secret behind the ability of the lotus is is in the tiny surface features, consisting of tiny nubs, on the leaves. These tiny protrusions don’t provide a surface for water drops to form, so the surface doesn’t get wet. Instead, the drops form into beads and roll off the surface carrying away any particles in their path. On a normal surface, water drops form hemispherical shapes and instead of rolling, glide over the surface. This spreads and smears dirt particles but does not remove them.

The Japanese researchers have now synthesized a special substance, a member of the group of compounds known as diarylethenes, and produced a microcrystalline film of this substance on a support. Electron microscopy images show that the surface of this film is initially smooth. When the diarylethene film is irradiated with UV light, the previously colorless surface turns blue—and is no longer smooth.

Instead it is covered with a fine down of tiny fibers that have a diameter of about 1 µm. This down has a similar effect to the micronodules on the lotus blossom, resulting in a super-water-repellent surface. If the surface is irradiated again, this time with visible light, the fibers and color vanish, leaving a colorless, smooth, and wettable surface.

This is a step towards making self-cleaning surfaces which could not only reduce some dangerous jobs but will also result in fewer chemical cleaning agents being used as well. This could also have important applications for vehicle tires, as a way to reduce accidents associated with loss of traction on wet road surfaces. GE has also been doing research on this topic, and has posted videos on their blog, they’re amazing to see!
Scientists synthesize new compound – Surface becomes super-water-repellent on command, via What’s Next In Science & Technology