All terrain droplet actuation
Original Entry: Ian Bruce Burgess Fall 2009
M. Abdelgawad, S.L.S. Freire, H. Yang and A.R. Wheeler, Lab on a Chip 8, 672-677 (2008).
This paper introduces a new platform for droplet-based microfluidic manipulation on flexible substrates. Electrowetting forces are applied to drops via copper electrodes embedded in a flexible polyimide film. The electrodes are protected with coatings of poly(dimethylsiloxane) (PDMS) and teflon. Performance properties and applications of these devices are explored. Electrowetting forces were sufficiently strong to drive droplets with sufficiently small volume (<7.3<math>\mu L</math>) up a vertical incline. Droplet actuation in various configurations of the flexible substrate are shown in the figure below.
A large significance of having a flexible platform for microfluidic droplet manipulation is that the device can easily be integrated across different environments. As an example, the authors demonstrate the transport of a water droplet containing the dye methylene blue and glucose across an air-oil interface into a bath of silicone oil. In an anaerobic environment, the dye is reduced and loses its color. They also demonstrate how the temperature of a droplet can be controlled easily using deformation of the chip. Using the same geometry, they then demonstrate effective purification of a water droplet containing oligonucleotides through liquid-liquid extraction of histones. This is accomplished by driving the droplet into a bath of a water-immiscible phenol solution and then returning it to the air after allowing diffusion of the histones into the phenol solution.
The significance of this paper from the point of view of soft-matter and our class material is that it demonstrates how electrowetting can be used to make practical devices. Fig. 1a shows how the voltages on the substrate are manipulated to drive the droplet form one pad to the next. The voltage gradient across the drop causes the contact angle to be different in the front and rear sides of the drop. This asymmetry in the drop curvature causes an effective attractive force in the direction facing the high voltage electrode, causing the drop to move. In this way, the authors demonstrate that electrowetting can be used to move droplets conformally around a flexible electrode array in arbitrary conformation. This type of droplet-based digital microfluidics has the advantage over channel microfluidics that it is much easier to manipulate small amounts of fluid in a controlled environment. A droplet can be an ideal isolated reaction vessel. Digital microfluidics is an ideal platform for conducting controlled chemical reactions on the microlitre scale. The use of a flexible substrate allows control of the droplet's surroundings during a reaction. The authors have demonstrated controlled immersion of their drop in another liquid, only requiring that the liquid is immiscible with the liquid in the drop. This allows controlled experiments using osmosis and diffusion from other liquids.