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Moving Droplets of Liquid with Light

Photonics Spectra
Nov 2007
Alternating UV and visible light can propel droplets on surfaces functionalized
with a photochromic substance.


Hank Hogan

Pumps are fine for moving everyday volumes of liquid, but something different is needed to move nanoliter quantities of fluids on lab-on-a-chip analyzers. Now researchers have demonstrated a technique that could direct such small volumes wherever they are wanted.

TWLiquid_Fig1.jpg

Visible and UV light switch azobenzene between two forms, so that shining UV light on one end of a droplet and visible light on the other causes one end of the droplet to have an advancing contact angle lower than the receding angle of the other side. If the difference is large enough to overcome forces that pin the droplet in place, the liquid moves. Image reprinted with permission of the American Chemical Society.


A group from Arizona State University in Tempe, from Sandia National Laboratories in Albuquerque, N.M., and from Los Alamos National Laboratory, also in New Mexico, has moved droplets across a prepared flat surface using light. It even has merged miniature droplets.

“Our motivation is partly scientific fascination, but mostly the motivation is to develop methods to handle increasingly smaller volumes of liquid,” said S. Tom Picraux, chief scientist at the Center for Integrated Nanotechnologies at Los Alamos and a research professor at Arizona State.

TWLiquid_Fig2.jpg
This sequence of images — showing the side view of a benzonitrile droplet on a smooth, azobenzene-coated surface — shows the liquid moving in response to UV light shining on only half the droplet. In response, the droplet moves toward the illumination.


Nanoliter droplets have a high surface-to-volume ratio as compared with larger droplets. As a result, surface effects dominate, and miniaturized versions of standard pumps do not work.

Scientists have been investigating ways to manipulate small amounts of liquid, but because of the advantages of no moving parts, no contact and easy miniaturization, the use of light is particularly interesting, and some early research already has been done in this area. Picraux noted that his team’s work extends these earlier investigations.

In their study, led by graduate student Dongqing Yang, the researchers used azobenzene, a photochromic organic molecule that switches from one form to another when exposed to ultraviolet radiation and that changes back when exposed to visible light. The two forms have different geometries that result in different surface wettability.

They synthesized azobenzene and attached it via a tether molecule to create a molecular monolayer coating on the surface of smooth silicon chips. They placed droplets of water, benzonitrile and other liquids on the functionalized silicon and, by adding and removing liquid, measured the advancing and receding contact angles, respectively, of the droplets with the surface. They performed these measurements and irradiated the surface for 5 min with 366-nm light using a lamp from UVP LLC of Upland, Calif. Having changed the azobenzene from one form to the other, they remeasured the contact angles.

TWLiquid_Fig3.jpg
Two droplets of benzonitrile merge as they slide toward the UV-illuminated center area, with the outside illuminated by visible light. The azobenzene-coated surface changes its surface wettability because of the difference in illumination, which propels the droplets together.


They exposed the silicon chips to visible light using an illuminator from Dolan-Jenner Industries of Boxborough, Mass., employing a 410-nm long-wave pass filter to ensure that only visible light reached the surface. Doing so switched the azobenzene back to its other form, enabling the researchers to check on the repeatability of the contact angle change.

From this data they determined that it would be possible to move some liquids across the surface by selectively irradiating half of a droplet with ultraviolet and half with visible light. They could only do so for those liquids in which they could create enough of a surface tension gradient to overcome the forces that wanted to pin the droplet in place. One liquid that they could achieve this with was benzonitrile, and they demonstrated the technique by moving a 5-μl drop of the liquid. They used the same technique to merge two droplets of benzonitrile that were initially separated by ~1 mm.

The investigators plan to include the implementation of light-controlled fluid switches and valves. Other research aims to extend the technique to water, which will be a challenge. They also intend to investigate the use of lithographic patterning and other techniques to create controlled surface roughness. Doing so, Picraux noted, is expected to bring benefits.

“We can greatly amplify the photocontrolled change in contact angles and even extend the optical switching on a surface to reversibly change between the extreme cases of superhydrophilic and superhydrophobic wetting.”

Langmuir, Oct. 9, 2007, pp. 10864-10872.


GLOSSARY
photonics
The technology of generating and harnessing light and other forms of radiant energy whose quantum unit is the photon. The science includes light emission, transmission, deflection, amplification and detection by optical components and instruments, lasers and other light sources, fiber optics, electro-optical instrumentation, related hardware and electronics, and sophisticated systems. The range of applications of photonics extends from energy generation to detection to communications and...
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