Coalescence and Particle Self-assembly of Inkjet-printed Colloidal Drops

Abstract

Inkjet printing has received extensive interest as a low-cost and scalable additive manufacturing process for varieties of applications, such as printable electronics, photovoltaics, and microbatteries. The objective of the current research is to advance the fundamental understandings of the transport phenomena in the inkjet printing processes to improve the performance of the inkjet-printed structures and devices. To study the stability of the printed patterns, the recoil and oscillations of single and consecutively printed pure liquid drops with different surface tension and viscosity are studied on substrates with different wettabilities. For both single and combined drops, the oscillations decay faster on more wettable surfaces due to stronger viscous dissipation. When a drop impacts on a sessile drop on hydrophobic substrate, the combined drop recoils twice resulted from the coalescence of the two drops. Whereas no recoil for the single drop impact. A single-degree-of-freedom model is developed for drop oscillation period and duration. Moreover, the particle deposition dynamics in two coalescing drops is examined with different drop spacing and jetting frequencies. The circularity of the deposition decreases with the increasing drop spacing and decreasing jetting delay, and the radius of curvature at the second drop side firstly decreases and then increases with increasing drop spacing. The capillary flow induced by the local curvature variation in the coalescing drops redistributes particles inside a merged drop, causing suppression of the coffee-ring effect for the case of a high jetting frequency while resulting in a region of particle accumulation in the middle of the merged drop at a low jetting frequency. Once colloidal drops are printed on very hydrophilic surfaces, the nanoparticles deposited into various structures, such as concentric multi-rings, radial spokes, spider web, foam, and island-like depositions, as a result of the competition between the receding contact line and particle deposition during drop drying. Based on Marangoni instability, the functional relationship between the characteristic lengths of the deposition and drop drying conditions are established. Finally, using nanoporous templates, the Ag nanotube forests are fabricated. The effect of drying methods on the morphology of nanotube forests is examined.%%%%Ph.D., Mechanical Engineering – Drexel University, 2014