Coalescence of sessile droplets driven by electric field in the jetting-based 3D printing framework

The recent developments revealed droplet jetting technologies, as applied to 3D printing (additive manufacturing), to be a strategic tool in creating biological sensors and wearable, flexible three-dimensional electronic devices. While the typical discretely formed droplets tend to limit throughput, several highlights to the jetting process include an ample choice of ink/substrate combinations and printing with nearly zero waste. From a functional manufacturing perspective, it is important to understand how these discretely formed droplets can be interconnected into digitally patterned lines and films within the limitations of the physics and hardware involved. Here, we investigate the effectiveness of a Coulomb force created by charged electrodes placed either below the substrate or on the printhead. From the physical point of view, the phenomenon of dynamic electrowetting-on-dielectric (DEWOD) is used. It is demonstrated that sessile droplets, placed initially separately with little chance of natural coalescence, can be selectively coerced by the added electric field into the electrically enhanced forced coalescence. Positive results were recorded for both electrode configurations at spacing distances greater than those achieved in literature. These results reveal novel manifestations of electrically driven coalescence, which hold great promise for new manufacturing design opportunities, reduction in raw material use, operation on extremely rough surfaces, and continuous narrow prints in situations where the previous approaches failed. In addition to droplet-into-line coalescence, the first-approximation potential to merge 2D droplet arrays into films is also demonstrated.