Continuous detection of trace level concentration of oil droplets in water using microfluidic AC electroosmosis (ACEO)

Detection of oil micro-droplets in water is important to environmental oil spill monitoring agencies. We present a microfluidic device for high-throughput trapping of oil droplets in water by using an AC electric field in a continuous flow. The electric field is applied through optically transparent indium tin oxide (ITO) electrodes fabricated on a glass substrate to enable direct visualization of the trapped oil droplets in water. The continuously flowing oil droplets in water are trapped by the opposing AC-driven electroosmotic (ACEO) flow due to the formation of electric double layers because of the deposition of polyelectrolytes. The deposition of these polyelectrolytes is necessary for the hydrophilization of the polydimethylsiloxane (PDMS) reactor. An array of SU8 micro-pillars was fabricated within the oil droplet entrapment zone to reduce the hydrodynamic drag force of the incoming fluids. The entrapment efficiency of oil droplets in water was studied as a function of the applied frequency of the electric field. We also observed the presence of a size-dependent negative dielectrophoretic (DEP) force as the droplets exit the entrapment zone. This can improve the entrapment efficiency for larger sized droplets as the strength of the DEP force would be higher.