In recent times, the utilization of three-dimensional (3D) printing technology, particularly a variant using digital light processing (DLP), has gained increasing fascination in the realm of microfluidic research because it has proven advantageous and expedient for constructing microscale 3D structures. The surface wetting characteristics (e.g., contact angle and contact angle hysteresis) of 3D-printed microstructures are crucial factors influencing the operational effectiveness of 3D-printed microfluidic devices. Therefore, this study systematically examines the surface wetting characteristics of DLP-based 3D printing objects, focusing on various printing conditions such as lamination (or layer) thickness and direction. We preferentially examine the impact of lamination thickness on the surface roughness of 3D-printed structures through a quantitative assessment using a confocal laser scanning microscope. The influence of lamination thicknesses and lamination direction on the contact angle and contact angle hysteresis of both aqueous and oil droplets on the surfaces of 3D-printed outputs is then quantified. Finally, the performance of a DLP 3D-printed microfluidic device under various printing conditions is assessed. Current research indicates a connection between printing parameters, surface roughness, wetting properties, and capillary movement in 3D-printed microchannels. This correlation will greatly aid in the progress of microfluidic devices produced using DLP-based 3D printing technology.

The dispensing of tiny droplets is a basic and crucial process in a myriad of applications, such as DNA/protein microarray, cell cultures, chemical synthesis of microparticles and digital microfluidics. This work systematically demonstrates droplet dispensing into immiscible fluids through electric charge concentration (ECC) method. It exhibits three main modes (i.e., attaching, uniform and bursting modes) as a function of flow rates, applied voltages and gap distances between the nozzle and the oil surface. Through a conventional nozzle with diameter of a few millimeters, charged droplets with volumes ranging from a few μL to a few tens of nL can be uniformly dispensed into the oil chamber without reduction in nozzle size. Based on the features of the proposed method (e.g., formation of droplets with controllable polarity and amount of electric charge in water and oil system), a simple and straightforward method is developed for microparticle synthesis, including preparation of colloidosomes and fabrication of Janus microparticles with anisotropic internal structures. Finally, a combined system consisting of ECC-induced droplet dispensing and electrophoresis of charged droplet (ECD)-driven manipulation systems is constructed. This integrated platform will provide increased utility and flexibility in microfluidic applications because a charged droplet can be delivered toward the intended position by programmable electric control.

Contact charge electrophoresis (CCEP) is an electrically controllable manipulation technique of conductive droplets and particles by charging and discharging when in contact with the electrode. Given its straightforward operation mechanism, low cost, and ease of system construction, it has gained traction as a versatile and potential strategy for the realistic establishment of lab-on-a-chip (LOC) in various engineering applications. We present a CCEP-based digital microfluidics (DMF) platform with two parallel electrode modules comprising assembled conventional pin header sockets, allowing for efficient mixing through horizontal and vertical shaking via droplet reciprocating motions. The temporal chromic change caused by the chemical reaction between the pH indicator and base solutions within the shaking droplets is quantitatively analyzed under various CCEP actuation conditions to evaluate the mixing performance in shaking droplets by vertical and horizontal reciprocating motions on the DMF platform. Furthermore, mixing flow patterns within shaking droplets are successfully visualized by a high-speed camera system. The suggested techniques can mix samples and reagents rapidly and efficiently in droplet-based microreactors for DMF applications, such as biochemical analysis and medical diagnostics.