Electrowetting as a tool for two phase flow microfluidic operations

摘要

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. 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 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. 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. Guiding of droplets is facilitated by multiple electrodes that create different paths for droplets depending on which electrodes are actuated. 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 chip for sorting droplets at 1200 drops per second is created. 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. By going back to the basics of electrostatics we find 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. Fine-tuning these parameters enables electrowetting dielectric liquids. The combination of electrode actuation with microfluidics enables the accurate control of droplets. 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.