Three-Dimensional Microfluidic Focusing With Surface Charge Patterning

Electrokinetically-driven flow circulations resulting from heterogeneous surface patches have previously been employed to improve mixing in microchannels. Here, numerical simulations demonstrate local in-channel hydrodynamic focusing through the use of strategically-patterned surface charge. Presented first is the case of a single straight channel with an axially-localized cross-sectional surface patch (ring). The surface patch exhibits a zeta potential equal in magnitude to the native microchannel surface but opposite in sign. The unsteady species transport in the presence of the electrokinetically-induced circulations is modelled, and a mean residence time is quantified. In general, residence times indicate the potential application of these circulations to microfluidic-based memory storage. Next, an improved focusing process for pinched-injection is demonstrated that exploits non-uniform surface patches. Lastly, surface patches are applied to enhance stream focusing in the microfluidic cross geometry. It is demonstrated that with this technique three-dimensional hydrodynamic focusing can be achieved in a single planar microfluidic structure. In one case, the microfluidic fluid stream was constrained to the centre of the channel and focused to 12% of its original cross-sectional area. Extensions of this work are discussed, as are the microfabrication and surface modification processes required for lab-on-chip implementation of these numerically simulated processes.