Droplet Mixing Using Electrically Tunable Superhydrophobic Nanostructured Surfaces
Soft Matter Key Words
Nanostructures, DNA hybridization, superhydrophobic surface, droplet mixing
Recently there has been a lot of effort towards creating lab-on-a-chip devices. For a lab-on-a-chip device to be effective, it must be able to perform biological assays. Cost, portability, and time are all factors that come into play when designing a lab-on-a-chip device. Most of these devices deal with very small quantities of samples, on the order of a few microliters of fluid. Many biological assays, such as DNA hybridization, require effective mixing of the sample in order to speed up a process. In this vein, there has been a push towards effective mixing techniques for these devices. This paper discusses one method developed in the lab.
The group performs mixing by using electrowetting techniques. First, a superhydrophopic surface is fabricated by etching a lattice of nanoscale pillars into a silicon surface. A layer of thermal oxide was grown on top of these pillars, and then a layer of fluorocarbons was deposited on top of that. The fluorocarbon layer makes the surface extremely hydrophobic, yielding a contact angle as high as 150-160 degrees between a water droplet and the surface. A droplet of water placed on top of this surface sits on top of the pillars to minimize the contact area, as in the right hand side of the cartoon in figure a. The physics behind this unwetted state was described in the 1940's by Cassie and Baxter, and is also called the Cassi-Baxter state.
When there is no voltage applied, the water droplet sits on top of the pillars and is the un-wetted state. However, if a voltage is applied across the water droplet, between the top of the droplet and an electric substrate below the pillars, the droplet will wet the surface and become pinned down. Even when the electric field is removed, the droplet will stay in this wetted state. To restore the droplet to its unwetted state, one can run a large current through the substrate. This will violently throw the droplet off the surface, and when it comes down again for landing it will revert back to its unwetted state. A small amount of water (less than 0.05%) is lost in this hopping procedure, but the turbulence experienced provides for a good mechanism for mixing.