Electrowetting as a tool for two phase flow microfluidic operation
Motivated by the desire to analyze individual cells, the objective of this research is the design, fabrication and implementation of a microfluidic chip capable of manipulating small water droplets in oil flow. In this work droplets are manipulated by electric forces that arise when applying an electric potential over electrodes embedded in the microchannel substrate. Different electrode geometries allow for different actuations. While in oil flow droplets can be guided along a rail, trapped at a specific location, split in two, merged to form larger droplets, and sorted at high speed based on content.
The thesis gives an introduction to electrowetting theory and its applications. It puts the technical developments in perspective to other droplet manipulation techniques in a broad literature review. The theory and application of electric potential wells is discussed. The principle is based on a contrast in conductivity between the drop and the continuous ambient phase, which ensures successful operation even for drops of highly conductive biological media like phosphate buffered saline. Moreover, since the electric field does not penetrate the drop, its content is protected from electrical currents and Joule heating. A simple capacitive model allows quantitative prediction of the electrostatic forces exerted on drops.
These electric potential wells are used for the guiding and trapping of droplets in oil flow. Guiding is facilitated by an electrode geometry consisting of multiple electrodes that create different paths for droplets depending on which electrodes are actuated. In this case the electrodes laterally guide droplets to 6 lanes in the microchannel. Consecutively, 36 guided droplets are trapped against the oil flow at arrays of trapping electrodes. At these locations the drop content can be analyzed.
The capability to trap and release droplets is extended by the introduction of a hydrophilic patch, which resembles antibodies printed on the substrate needed for analyzing cancer cells. A novel fabrication process is developed to create a dual-sided microchip with actuation electrodes in the bottom substrate, and hydrophilic gold patches on a hydrophobic top substrate. We focus on the trapping and release of droplets in the presence of a hydrophilic patch. Two problems needed to be solved: an oil film between the drop and patch prevents interaction, and a droplet interacting with the patch gets ‘stuck’. Eventually we were capable of electrically trapping a droplet long enough for the oil film to break up. By using a secondary pair of electrodes the droplet can be pulled from the patch.
Also, a chip for sorting droplets at high speeds is created. The maximum achievable sorting rate is determined by a competition between electrostatic and hydrodynamic forces. Sorting speeds up to 1200 per second are demonstrated for conductive drops of 160 pL in low viscosity oil. Finally, together with the Radboud university, experiments are performed to sort droplets containing fluorescently labeled cells.
In traditional electrowetting setups the electric forces strongly decrease when going to high AC frequencies. Our method used for manipulating droplets electrically turns out to be – not only very versatile, but also – applicable even beyond the usual AC frequencies for electrowetting, as evidenced by successful high speed sorting experiments using liquids in this so-called ‘dielectric regime’. By going back to the basics of electrostatics the parameters that dictate the magnitude of electric energy and electric force are described. It turns out that the decrease in electric energy in the dielectric regime lies in relative geometric length scales of the droplet and insulating layer and the dielectric contrast between these two materials.
Ultimately, we demonstrated a novel combination of techniques for aqueous droplet manipulations in two phase microfluidics. The control by electronics makes our system very accurate and manageable. The relatively large and predictable forces acting on the droplet are advantageous, but the complex fabrication method can be costly. Whether the downsides outweigh the advantages for practical applications remains to be seen.
