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 expended towards creating lab-on-a-chip devices. These devices are meant to carry out biological assays on a smaller scale than conventional lab equipment. Groups working on these lab-on-a-chip devices often focus on making them low-cost, portable, and time effective. 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 certain process. In this vein, there has been a push towards creating effective mixing techniques for small volumes of fluid. This paper discusses one method developed by the authors.
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 mixing mechanism.
The group quantitatively tested this method for droplet mixing by performing a dye-quenching DNA hybridization assay. They used two types of DNA strands. The first single-stranded DNA oligomer is bound to a fluorescent dye, and thus giving off a fluorescent signal. The second single-stranded DNA is the complementary strand to the first, and is functionalized with a broadband absorber. If the two strands hybridize, the second strand will quench the fluorescence of the first, killing the signal. However, if the strands fail to hybridize, the second strand will (at low concentrations) not be able to get close enough to the first strand to quench the signal.
The control experiment was to place a droplet containing the first strand on the surface, and then to add the second strand. The fluorescence as a function of time indicates the amount of mixing that occurs due to diffusion. Next, to test the electrowetting mixing technique, the same experiment is performed except this time the droplet is propelled off the surface as explained above. Again the fluorescence as a function of time is recorded. In the case where the the electrowetting mixing technique is used, the lab saw a much more rapid decrease in fluorescence signal as compared with the control, indicating that the technique was effective in mixing the the two solutions and speeding up the hybridization process. They report that this mixing technique lowered the time it took for complete hybridization to occur from 12 minutes to 10 seconds, a significant achievement. Below the results reported in the paper are shown.