Trapping of Micro- and Nanoparticles within Microfluidic Constrictions in AC Electric Fields

Trapping and separation of particles near microfluidic constrictions are efficiently achieved using electric fields. The phenomenon has been attributed to the dielectrophoretic (DEP) force arising from the nonhomogeneous electric field within the constrictions, which predicts particle trapping at or away from the constriction tip. In this work, we provide a more insightful description of the particle behavior around constrictions when subjected to ac electric fields. We demonstrate that, at low frequencies (below 10 kHz) and for solutions with conductivities lower than 0.1 S/m, new trapping positions close to the tips occur which cannot be explained using DEP forces only. We use the term extraordinary trapping position (ETP) to distinguish them from the trapping positions due to DEP. These trapping positions are explained when considering the action of, at least, two different phenomena: the hydrodynamic wall-repulsion induced by concentration-polarization electroosmosis (CPEO) on the particle surface and the fluid flow vortices due to CPEO on the constriction walls. Correctly interpreting these observations is crucial for experiments, such as those aiming to measure the electrical polarizability of proteins by trapping them in microfluidic constrictions.