Digital Microfluidics

Micro/Bio/Nano Fluidics

Electrowetting is the process of changing the surface wetting properties using electric fields. Digital microfluidics is a field of microfluidics where electrowetting is used to control and manipulate discrete fluid droplets. This idea is inspired by digital microelectronics but instead of electric current, discrete (or digitized) droplets are used to move a certain quantity of fluid or a reactant contained within over a certain distance in a certain time. Digital microfluidics finds applications in various biochip designs because of their high re-configurability and ability to speed up the process through massive parallelization.

The most important surface wetting property is the contact angle between the fluids and the surface. FLOW-3D’s powerful surface tension model, in conjunction with the electrokinetic model, is used to capture the wetting dynamics in digital microfluidics processes such as di-electrophoresis, thermocapillary actuation (actuation through temperature dependent surface tensions) and electrowetting itself.

Electrokinetics

Dielectrophoresis

Dielectrophoresis involves the creation of forces on polarizable particles to induce movement in non-uniform electric fields (usually AC electric fields). Dielectrophoretic forces can be used to characterize, handle and/or manipulate microscale and nanoscale bioparticles. This can include sorting, trapping and separating cells, viruses, bacteria, DNA, and the like. Dielectrophoresis can be fully accounted for in FLOW-3D and can be activated along with all other fluid flow options available in the code, such as one-fluid or two-fluid flow, with or without sharp interfaces.

Electrowetting

When a conducting liquid drop is placed on an electrode having a thin dielectric coating and an electric potential is then applied between the liquid and the electrode, the drop flattens and spreads over the electrode surface. This phenomenon is often referred to as electro-wetting. Because the phenomenon is associated with the development of an electrical charge layer, an external electric field may be used to manipulate the drops causing them to move, coalesce or break apart.

Electrowetting is a technique used to change the apparent contact angle of a dielectric fluid under the influence of electric potential. In this example, the fluid is originally hydrophobic and beads on the surface. However, when a 50V potential difference is applied, the fluid is forced to wet the surface becoming hydrophilic.

Lab-On-Chip Electro-wetting Applications

An electrowetting based Lab-on-chip that can manipulate discrete droplets allows designers to perform complex procedures similar to traditional lab apparatus but with much smaller volumes. These devices are required to efficiently transport, merge and split droplets. FLOW-3D can be a useful tool in the design process by allowing the user to simulate the effects of geometric parameters and voltages used to operate these devices.

The animations below demonstrate FLOW-3D‘s capability to simulate transport, merge and split droplets. The Lab-on-a-chip consists of two parallel plates separated by about 300μm. The bottom plate has electrodes inserted in it that are used for manipulating the droplets. The droplets are water (slightly conductive) surrounded by silicone oil. The volume of the droplet is about 800nl.

This lab-on-a-chip electrowetting simulation demonstrates an electric field being applied in order to split a small droplet.

Here an electric field is being applied in order to merge two small droplets.

This simulation shows an electric field being applied to a small droplet to control its motion.

Thermocapillary Actuation

Temperature dependence of surface tension can be used for the routing of fluid droplets over patterned surfaces. Surfaces are patterned with either hydrophilic or hydrophobic contact angles such that the fluid droplet is restricted to follow a channel formed by the hydrophilic-hydrophobic interface. Additionally an array of micro-heaters, heated in a programmable fashion, cause the thermocapillary actuation, propelling the fluid droplet away from the hotter region towards the colder region. The images below show the top and cross-section views of the problem setup (Anton A. Darhuber et al.) followed by the setup in FLOW-3D.

Liquid droplet moving along hydrophilic microstripe
Top-view of a liquid droplet moving along a hydrophilic microstripe. The array of Ti-resistors (shown in light gray) beneath the hydrophilic stripes locally heat the droplet thereby modifying the surface tension and propelling the liquid toward the colder regions of the device surface. The dark gray stripes represent the leads and contacts (Au) for the heating resistors.
Cross sectional view of device
Cross-sectional view of a portion of the device containing two micro-heaters and an overlying droplet.

A region of cooler surface temperature maintains a higher surface tension than the neighboring warm spots, exerting a tangential surface force pulling the liquid. Partially wetting (contact angle > 0) surfaces are a desirable option because they allow the fluid transport with lesser volume loss compared to a full-wetting surface (contact angle = 0).

FLOW-3D setup of three microheaters
Top view of the setup in FLOW-3D showing three microheaters in pink, yellow and blue respectively. The central hydrophilic strip is shown in black with a fluid (water) droplet in sky blue.

The animation below shows a comparison between a fully-wetted and a partially-wetted surface. As expected, the fully-wetted surface makes the droplet flatter (and more spread out) than in the partially-wetted surface. As the heaters are activated one at a time, the droplet is propelled towards a colder area. It can be seen that the fully-wetting surface loses more fluid volume by the end of the simulation as more fluid is left behind. Therefore, partially-wetting surface is a more preferable option to reduce fluid loss. In both cases, the droplet is forced to stay in the middle because of the central hydrophilic strip surrounded by a hydrophobic surface.

References

Anton A. Darhuber, Joseph P. Valentino, Sandra M. Trian and Sigurd Wagner, Thermocapillary Actuation of Droplets on Chemically Patterned Surfaces by Programmable Microheater Arrays, Journal of Microelectrochemical Systems, Vol. 12, No. 6, December 2003

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