Solvent evaporation and particle deposition of colloidal drops in inkjet printing process

Inkjet printing has garnered much attention due to its ability to dispense precise amounts of functional materials onto targeted areas. It is a low-cost and scalable manufacturing process for a wide range of applications including thin film solar cells, electronic circuits and flexible displays. To realize the potential of inkjet printing, the evaporation, contact line (CL) dynamics, and particle deposition of inkjet-printed aqueous colloidal drops have been investigated by using fluorescence microscopy and a high resolution goniometer. The results show that the relative humidity affects the extent to which the drop is able to spread, the evaporation rate at the drop surface, the evaporation-driven flow inside the drop, and finally the particle deposition morphology. The deposition behavior of inkjet-printed colloidal mixture of micro and nanoparticles on a substrate with varying wettability is investigated. It is found that particles inside an evaporating drop rearrange themselves near the CL according to their sizes, where smaller particles deposit closer to the CL. By increasing substrate wettability, particles in the bi-dispersed mixture can be further separated. Forces acting on the particles determine the role of particles in enhancing CL pinning, which ultimately determines the final deposition morphology of a bi-dispersed colloidal mixture. The dynamics of drop coalescence, evaporation, and particle deposition of two inkjet-printed consecutive drops have been studied by jetting the second drop on the prewetted region of the first drop with various temporal delays and drop spacings. As drop separation distance increases, the circularity of the deposition decreases. Moreover, a longer delay between two drops causes the formation of two separate coffee rings as more particles are pulled and deposited in the first drop region due to capillary pressure. The effect of particle shape on the deposition morphology is also studied to suppress the coffee ring effect for spherical particles. Finally, applications of inkjet printing for conducting lines and organic solar cells are explored. The conditions under which stable lines and uniform films are formed have been examined. The knowledge obtained will benefit a wide range of applications that involve evaporation-driven assembly of complex fluids ranging from printable electronics to tissue engineering.