(497d) Microfluidic Mixing of Nonpolar Liquids By Contact Charge Electrophoresis

The manipulation of colloidal scale particles through electrophoresis is a powerful tool for driving basic unit operations such as separation or mixing processes, particularly in microfluidic systems. Contact charge electrophoresis (CCEP) denotes a specific category of electrophoresis where the particle actuated by an electric field acquires charge when in contact with an electrode. This mechanism allows a particle to achieve high frequency oscillations (~100Hz) between parallel electrodes, and can be coupled to simple geometries such as ramps to direct particles transverse to the electric field.  Here we demonstrate a simple microfluidic mixer which uses CCEP to achieve rapid effective stirring of two dielectric liquids by stretching and folding the flowing streams into one another. Interestingly, the simple reciprocal motion of CCEP alone was largely ineffective at mixing two laminar flowing streams due to the kinematic reversibility of low Reynolds number flows. The incorporation of PDMS ramps which guide the particle in an orbital trajectory is critical to achieve rapid mixing. We discuss the optimal operating conditions for CCEP-based mixing, and investigate CCEP in 15 common organic solvents to determine which liquids can be effectively mixed using this approach. As CCEP only requires a low power DC voltage, this mixer is ideally suited for applications in portable microfluidic devices.