Researchers at the University of California at Los Angeles (UCLA) have demonstrated that liquid droplets can be moved, injected, mixed, and split using laser light rather than electronic control. The new technique should bring microlaboratories a step closer by making them less complex electronically, eliminating the need for thousands and potentially millions of electronic interconnections. Other advantages include speed—droplets can be moved at 7 to 8 mm/s—and the fact that a single beam of light can be used to move several droplets at a time. Though devices have been fabricated, as yet there has been no attempt to integrate the laser into the system.

Aided by electrowetting on a photoconductive layer, a light spot moves a droplet from place to place.

Optoelectrowetting, as its name suggests, is an extension of an existing technique. A polarizable liquid is held between two electrodes that have nonstick insulating layers (the UCLA team uses Teflon). By changing the voltage across the droplet, the wetting ability of the surface (that is, whether it is hydrophilic or hydrophobic) is altered through a change in surface tension. If the electrodes are made so they are pixelated, then the droplet can be attracted to the surface selectively, with the leading edge being pulled down while the rear is not. Thus, the droplet can be made to move.

The UCLA team has extended this electrowetting technique by including a photoconductive layer in the electrode structure. Now, instead of needing to address individual electrode pixels electrically, a voltage can be applied across them optically.^{1, 2} This not only makes the electrode circuits much simpler than they would otherwise be, but it also prevents a potentially enormous communications bottleneck: an all-electronic 1 × 1-cm device contains tens of thousands of electrode pixels. Researchers hope to build systems with much larger active areas; the fact that only two bias wires are required for this device makes it unusually scaleable.

Another advantage of the optical technique is that, because the device is transparent at the 532-nm laser line, both top and bottom electrodes can be activated using the same beam of light. Further, the size of the droplet that is manipulated can be varied by changing the size of the beam; otherwise, the limit is set by the pixel size, which determines the size of the smallest controllable droplet. The researchers are currently able to work with volumes from 10 nl to 1 µl.

If several droplets have to be moved simultaneously, a single beam can be moved from place to place pulling each in turn (time multiplexing). And researchers say the light can perform functions other than straight actuation. For instance, droplets can be pulled from a larger block of liquid and held in a reservoir using the light beam. At the trailing edge of the droplet, the surface becomes hydrophobic, thinning the droplet until it eventually breaks off. A similar technique can be used when a large droplet (perhaps one that has been the product of two droplets optically brought together for mixing) has to be separated into two. In this case two beams are necessary, but otherwise the process is the same. Another potential use of the laser beam, this time requiring much more careful control, would be to use it as an optical tweezer (holding a droplet) or wrench (turning a droplet). These latter functions have yet to be integrated into the UCLA system.

One thing that researchers would like to understand better is the theoretical limitations of electrowetting. Though their device works well, the droplet contact angle (a measure of how much it is being pulled) saturates at around 75°; increasing the light intensity produces no increase in pull after this point. Several groups are currently studying this phenomenon, but as yet they report different mechanisms for this saturation taking place.

1. Pei Yu Chiou et al., Sensors and Actuators A 3104, 222 (May 15, 2003).
2. Pei Yu Chiou et al., Proc. MEMS '03, Kyoto, Japan (Jan. 19-23, 2003).