Flexible concentration control of Newtonian and non-Newtonian fluids in a microfluidic device via AC electrothermal flow

Microfluid preparation at targeted concentration is crucial for lots of applications such as protein crystallization and drug efficacy evaluation. Herein, we report an efficient method to continuously produce Newtonian and nonNewtonian fluids at desired concentrations via AC electrothermal (ACET) flow. Two miscible fluids with different initial concentrations are first injected into the device via two separate inlets. The demanded concentrations are obtained under ACET flow, which mixes the fluids into specific states first and obtain the targeted concentrations at outlet afterwards via the voltage and fluid velocity control. To elucidate the underlying principles of fluid concentration control through the ACET effect, we have developed a three-dimensional numerical model coupling the electric, thermal, flow and concentration fields to perform analyses. Our findings indicate that fluid concentration modulation is significantly impacted by both intrinsic fluid properties and external control parameters, including the fluid's apparent viscosity, conductivity, velocity, and driving voltage. Specifically, as the fluid's apparent viscosity increases, the flow weakens, resulting in a higher driving voltage being required to achieve the targeted fluid concentration. The Joule effect and temperature gradient in fluids intensify with increased fluid conductivity, enhancing the ACET flow and correspondingly reducing the necessary driving voltage for concentration control. The duration of the ACET flow in the microchannel varies with the fluid velocity, subsequently affecting the concentration adjustment. The intensity of the ACET flow augments with an increase in the driving voltage, leading to diverse fluid mixing performances and outlet concentrations as the voltage changes. Overall, the fluid concentration control is directly influenced by fluid properties and can be flexibly adjusted through external control of fluid velocity and voltage. The simplicity of the chip design and the flexibility of concentration control offered by this approach make it highly appealing for a wide range of applications that require precise fluid concentration control.