Creating, Transporting, Cutting, and Merging Liquid Droplets by Electrowetting-Based Actuation for Digital Microfluidic Circuits

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Original entry: Warren Lloyd Ung, APPHY 225, Fall 2009

"Creating, Transporting, Cutting, and Merging Liquid Droplets by Electrowetting-Based Actuation for Digital Microfluidic Circuits"
Sung Kwon Cho, Hyejin Moon, and Chang-Jin Kim.
Journal of Microelectromechanical Systems (2003).


Soft Matter Keywords

Contact angles, Electrowetting, Electrowetting on dielectric (EWOD), Digital microfluidics

Figure 1: (a) Schematic of Electrowetting on Dielectric; (b) Demonstration of Electrowetting on Dielectric.

Summary

In brief, electrowetting is a phenomenon by which the contact angle of a liquid resting on a solid interface can be modulated by an applied voltage. Regardless of the sign of the voltage, the electric fields cause a change in the energy of the solid-liquid interface, which leads to a change in contact angle. Cho et al are specifically interested in a type of electrowetting, where the liquid phase is separated from the electrodes, which apply the fields, by a thin layer of dielectric material (see Figure 1). This is known as electrowetting on dielectric (EWOD).

By assembling arrays of independent electrodes, it is possible to create devices in which small liquid drops can be controlled with a high degree of precision. Drops can be dispensed from a large reservoir of fluid, moved about the device, merged, split, and mixed. Given the discrete nature of the devices and their roots in electronic actuation, this is often given the name digital microfluidics.

The system described in this paper uses EWOD to manipulate small drops of liquid in air. Although other systems exist which manipulate droplets in oil [1], it turns out that it is a more difficult problem to actuate droplets in air. Cho et al have demonstrated digital microfluidic devices based on the principles of EWOD that can perform all the basic operations necessary to make complex devices, which could meet the ideal of a full lab-on-a-chip. In addition, they are able to perform these operations at voltages as low as 25V; values lower than previously reported for the actuation of droplets in air.

Soft Matter Discussion

Electrowetting on dielectric, as described previously, causes a decrease in contact angle due to an applied electric field. This decrease in contact angle causes a decrease in the Laplace pressure within a drop, since for a fixed height, the drop's radius of curvature decreases. In order to maximize this effect, the surfaces of the device are coated with Teflon, making them hydrophobic. As a result, under no applied voltage, the drops have a high contact angle with the dielectric surface.

If the contact angle is increased non-uniformly throughout the drop, for instance, if an electric field is applied to a region of the drop, the Laplace pressure within that region will decrease. Since the Laplace pressure within regions with no electric field remains high, fluid will move towards the area of low pressure. Thus, a drop placed on an array of electrodes will migrate towards an energized electrode, provided the field from that electrode overlaps the drop of interest. Note, that although the motion of a droplet can be explained by electrowetting, others believe that there should be a distinction between the term electrowetting, and the motion of liquid due to an applied field [2].

Figure 2: Droplet configuration for cutting: (a) top view, (b) cross section along B-B', and (c) cross section along A-A'. Dotted lines show the intial droplet shape with no electric actuation.

This principle can be extended to splitting a drop in two; a process referred to as cutting by Cho et al. Liquids in this device are suspended between the bottom dielectric layer and the upper ground plane. In contrast with jets of water emerging into air or columns of water sitting on a surface, which exhibit dynamic instability, a column of water within the device is stable. Drops, thus, cannot form spontaneously. In order to create drops in this device, a droplet must be actively pulled apart.

Fortunately, the an arrangement of three electrodes is sufficient to perform this splitting under the right conditions. Figure 2 on the right depicts the apparatus used to split a drop. The two electrodes on either side of the drop are energized, while the electrode in the middle is grounded. Using Laplace pressure arguments, Cho et al show that the following expression governs whether a drop can split. The result is as follows:

<math>\frac{R_2}{R_1} = 1 - \frac{R_2}{d}(cos \theta_{b2} - cos\theta_{b1} )</math>

where <math>R_1</math> is the radius of the drop above the grounded electrode, <math>R_2</math> is the radius of the drop above the energized electrodes, <math>\theta_{b1}</math> is the contact angle above the grounded electrode, <math>\theta_{b2}</math> is the contact angle above the energized electrodes, and <math>d</math> is the separation between the ground electrode at the top and the dielectric at the bottom (refer to Figure 2). The drop splits, when the Laplace pressure moving the fluid into either drop is enough to create a neck above the grounded electrode (Figure 2a); this corresponds with a negative radius of curvature above the grounded electrode (<math>R_1 < 0 </math>). In order to increase the likelihood of splitting, the following parameters are important: large change in contact angle, large droplet size (related to <math>R_2</math>) and small channel gap, <math>d</math>. A series of devices with different dimensions were built in order to confirm this hypothesis. Cutting was achieved with small channel heights (<math>d=70\mu m</math>) and square electrodes of <math>700\mu m</math> side length.

Although splitting is a complex process, which depends heavily on the geometry and design of the device, merging drops is relatively simple. It turns out that merging two droplets together can be accomplished by simply bringing the two drops into contact. On the device, the easiest way to do this is to move drops onto the same electrode simultaneously.

The same principles used to split drops apart can be used to dispense droplets from a reservoir of fluid. By subsequently energizing a set of electrodes adjacent to a reservoir, fluid can be pulled out of the reservoir along the electrodes. This column of fluid can be severed by grounding the electrodes between the drop of fluid at one end, and the fluid in the reservoir. Different electrode geometries can be produced which yield reproducible droplet dispensing.

With these tools, the digital microfluidic device developed by Cho et al demonstrates that it can perform the operations necessary for complex applications.

Applications

Electrowetting in general has a variety of applications beyond those discussed here. It can, for instance, find uses in spin-coating and printing [2]. Electrowetting is even used to create displays[3].

Digital microfluidics using the principles outlined in this article are useful in finding solutions to the minitiarization of biological and medical research. For instance, lab-on-a-chip devices are being developed by a number of companies and research groups including Advanced liquid logic[4], and have uses in DNA sequencing, biochemical assays, and other areas of clinical diagnostics[5].


References

  1. "Electrowetting-based actuation of liquid droplets for microfluidic applications." Michael G. Pollack, Richard B. Fair, & Alexander D. Shenderov. Applied Physics Letters 2000, 77 (11), 1725-1726.
  2. "On the Relationship of Dielectrophoresis and Electrowetting." Thomas B. Jones. Langmuir 2002, 18, 4437-4443.
  3. "Electronic Paper." http://en.wikipedia.org/wiki/Electronic_paper.
  4. Advanced Liquid Logic.
  5. "Chemical and Biological Applications of Digital-Microfluidic Devices." Richard B. Fair, Andrey Khlystov, Tina D. Tailor, Vladislav Ivanov, Randall D. Evans, Peter B. Griffin, Vijay Srinivasan, Vamsee K. Pamula, Michael G. Pollack, and Jack Zhou. IEEE Design & Test of Computers 2007, pp. 10-24.