MATERIALS PROCESSING: Light moves liquids

Sept. 1, 2000
Scientists at the Tokyo Institute of Technology have succeeded in moving a liquid droplet using photoirradiation by controlling the wavelength and intensity of incident light. A liquid droplet placed on a solid surface is known to move along the plane due to the surface-tension gradient caused by the temperature difference of the solid surface.

Incorporating news from O plus E magazine, Tokyo

TOKYO—Scientists at the Tokyo Institute of Technology have succeeded in moving a liquid droplet using photoirradiation by controlling the wavelength and intensity of incident light. A liquid droplet placed on a solid surface is known to move along the plane due to the surface-tension gradient caused by the temperature difference of the solid surface. The laboratory has extended this idea to move a liquid droplet by creating a free-energy gradient on the solid surface via photoirradiation.

An optically responsive adsorption monolayer on top of a solid surface was made using a crown conformer of 0-carboxymethylated calix[4]resorcinarene (CRA-CM) bearing four azobenzene residues at one of the rims of the cyclic skeleton. To prepare the surface, an aminosilylated silica plate was submerged in CRA-CM solution.

Azobenzene undergoes optical isomerization reactions, so light irradiation causes the molecular structure and polarization of the film surface to change. Thus the free energy of the surface can be adjusted. The desired free-energy gradient is created by adjusting the intensity, location, and wavelength of incident light.

A drop of liquid is placed on the prepared surface and then photoirradiated using a xenon mercury lamp at either 365 or 436 nm. First, a dose of 365-nm light with an intensity gradient across the droplet causes the droplet to expand in one direction; next, intensity-graded 436-nm light causes the droplet to shrink in the same direction, resulting in net movement.

It was discovered that both olive oil and liquid-crystal droplets can be controlled by changing the wavelength and the intensity gradient of the light. The liquids that were successfully controlled have only a small degree of hysteresis between the contact angle of the advancing front of the droplet and the contact angle of the receding front.

When no light strikes the plate, the monolayer molecules are in the trans state and the hydrophobic alkyl radicals are on the surface. When ultraviolet light is incident upon the plate, photoisomerization occurs and the molecules change to the hydrophilic cis state. From this result it was discovered that to move the liquid droplets, it is not sufficient to understand thermodynamic phenomena such as surface tension. It is also necessary to understand the molecular-level dynamics at the interface between the monolayer on the plate and the liquid droplet.

The researchers also have successfully moved liquid-crystal material up and down a 2.3-mm inside-diameter glass tube by preparing the inner surface of the tube. With this method it should be possible to create miniature chemical-reaction devices.

Courtesy O plus E magazine, Tokyo

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