Flexible modulation of droplet concentrations via AC electrothermal flow for fabrication of crystalline drug particles

Controllable fluid concentration regulation is significant for numerous applications, including cell culture, material synthesis and protein crystallization. However, current methodologies are confined by intricate fluidic networks, specialized chip fabrication and manipulation equipment, as well as limited and inflexible manipulation capabilities. In this study, we introduce a novel approach for efficient droplet concentration modulation utilizing AC electrothermal (ACET) flow. The designed microfluidic device comprises two functional units: a fluid mixing zone and a droplet generation region. The fluid mixing zone features an electrode array positioned at the microchannel's bottom, configured in an asymmetrical layout to induce robust ACET flow. When two aqueous solutions with different concentrations are introduced into the device through separate inlets, they undergo mixing to achieve the desired concentrations in the main channel due to the stirring of ACET flow first and encapsulated into dispersed droplets by the shear force of the oil phase afterwards. The droplet concentrations can be dynamically adjusted by modulating the driving voltage, signal frequency and inlet flow rates. The versatility of this technique is validated through the successful fabrication of phenolphthalein crystal particles under different fluid concentrations. Therefore, this research holds significant promise for a wide range of applications requiring droplet concentration manipulation.