Synchronous oscillatory electro-inertial focusing for particle manipulation

The manipulation of suspended colloidal particles by external fields or hydrodynamic forces has applications ranging from cell sorting to water purification. Often, field methods such as electrophoresis are used for the precise manipulation of smaller particles (<1 um) while hydrodynamic methods based on fluid inertia such as inertial focusing are used for the rapid manipulation of relatively large particles (>10 um). Here, we present an experimental study on the non-linear coupling between electrophoretic and inertial forces for particle manipulation in microfluidic devices. This was realized by the application of an oscillatory electric field and a synchronous oscillatory flow at frequencies ranging from 10 to 1000 Hz. We investigated the effects of oscillatory amplitude, frequency, and phase difference between the electric and velocity fields on the rectified motion of colloidal particles. For certain combinations of oscillation frequencies and phase differences, we observed focusing positions and equilibrium states that were inexplicable by electrophoresis and inertial focusing alone. We demonstrate that this approach can overcome the limitations of a purely field-based or flow-based approach by manipulating nanoparticle suspensions at high-throughput. We hope to motivate the non-linear interaction between multiple synchronous oscillatory fields as a general strategy for particle control.