Tunable Droplet Manipulation and Characterization by ac-DEP

A novel ac-dielectrophoretic (DEP) device for tunable manipulation and characterization of particles and droplets is presented in this work. To induce DEP forces, the ac electric field is applied via two embedded microelectrodes to generate a local nonuniform electric field perpendicular to the channel length through a pair of asymmetric orifices on the opposite microchannel walls. The droplets experience the DEP effects only when passing through the vicinity of the small orifice, where the strongest gradient of the nonuniform electric field exists. In this study, the ac-DEP manipulation of the particles in the microchannel under different strengths of electrical field was demonstrated first. Then, the separation of particles by size, separation of mixtures of ionic liquid (IL) droplets and oil droplets with the same size by types, and movement of the particles and IL droplets with different frequencies of the applied ac electric field were investigated, respectively. The experimental results match well with the theoretical simulation. In addition, the lateral migration of an IL droplet as a function of the ac frequency was measured, which shows a trend similar to the corresponding Clausius-Mossotti factor. The experimental results demonstrate that with this method, the separation of target particles/droplets with specific size and type can be accomplished by simply adjusting the strength and the frequency of the ac field applied to the microchannels. This paper, for the first time, measured the ac-DEP lateral migration of the particles and IL-in-water emulsion droplets varying with the frequency of the applied ac electric field in the microfluidic chip, providing a method to identify the critical frequency of the droplet and the fingerprint to characterize the droplet.