Direct Current Electrokinetic Particle Trapping in Insulator-Based Microfluidics: Theory and Experiments.

The classic theory of direct-current (DC) insulator-based dielectrophoresis (iDEP) considers that, in order to elicit particle trapping, dielectrophoretic (DEP) velocity counterbalances electrokinetic (EK) motion, that is, electrophoresis (EP) and electro-osmotic flow (EOF). However, the particle velocity DEP component requires empirical correction factors (sometimes as high as 600) to account for experimental observations, suggesting the need for a refined model. Here, we show that, when applied to particle suspensions, a high-magnitude DC uniform electric field induces nonlinear particle velocities, leading to particle flow reversal beyond a critical field magnitude, referred to as the EK equilibrium condition. We further demonstrate that this particle motion can be described through an exploratory induced-charge EP nonlinear model. The model predictions were validated under an insulator-based microfluidic platform demonstrating predictive particle trapping for three different particle sizes (with an estimation error < 10%, not using correction factors). Our findings suggest that particle motion and trapping in "DC-iDEP" devices are dominated by EP and EOF, rather than by DEP effects.