Trapping of Nanoparticle in Direct Current Insulator Microfluidic Device: Dielectrophoresis Vs Nonlinear Electrophoresis.

Electrokinetically driven insulator-based microfluidic devices have emerged as a powerful platform for enabling the filtration, concentration, separation, and characterization of micro- and nano-sized particles. The concept of insulator-based dielectrophoresis (iDEP), inspired by traditional electrode-based dielectrophoresis, was introduced two decades ago. These devices use insulating structures, such as posts, membranes, obstacles, or constrictions, to reshape the spatial distribution of an applied electric field. This deformation induces dielectrophoretic (DEP) forces that act on particles within the fluid. For years, it has been widely accepted that DEP governs particle trapping in iDEP devices, regardless of whether the electric field is generated by direct current (DC). However, recent studies suggest that DEP may not be a primary influence in these devices. Instead, effects previously attributed to DEP appear to stem from the deformation of the ion cloud surrounding charged particles, resulting in a non-linear relationship between electrophoretic velocity and the applied electric referred as nonlinear electrophoresis (NLEP). By conducting finite element simulations of a recurrent iDEP device across various particle and environmental properties, we show that DEP is overshadowed by NLEP as particle sizes and ionic strengths decrease. However, this relationship is not consistent; at approximately 0.2 M concentrations and particle sizes around 50 nm, both forces are comparable, with DEP dominating at larger concentrations and particle sizes. Understanding the forces involved allows us to define a novel microfludic platform which exploits NLEP to trap particles with different properties in strategic regions of the device.