Microfluidic Concentration Manipulation via Controllable AC Electroosmotic Flow.

The ability to precisely prepare microfluids with targeted concentrations is critical for numerous applications, including protein crystallization and drug efficacy evaluation. This study presents an efficient microfluidic method for the continuous preparation of fluids at desired concentrations utilizing AC electroosmosis (ACEO). Two miscible fluids of different initial concentrations are introduced through separate inlets. Target concentrations are achieved through ACEO-driven mixing, where fluid manipulation via electric signal and flow velocity control enables precise concentration adjustment at the outlet. To elucidate the concentration control mechanism via ACEO, we develop a three-dimensional numerical model coupling electric, flow, and concentration fields. Our results demonstrate that concentration modulation is significantly influenced by intrinsic fluid properties and external control parameters, including fluid viscosity, conductivity, axial fluid velocity, driving voltage, and signal frequency. Specifically, higher fluid viscosity and conductivity dampen electroosmotic flow, necessitating increased voltage to achieve target concentration. Axial fluid velocity determines the residence time in the mixing zone, directly affecting mixing efficiency and concentration control effect. The intensity of ACEO flow increases with applied voltage, enabling tunable mixing performance and outlet concentration. Overall, the simplicity of device design combined with precise concentration manipulation makes this method particularly attractive for applications requiring accurate fluid preparation.