Evaporation of Multicomponent Inkjet Printed Droplets

Inkjet printing allows for the controlled, contactless deposition of functional picolitre droplets. Industrial applications of droplets are varied and use complex formulations, the motivation of this thesis was to gain an understanding of how different solutes change the evaporation behaviour of drying droplets. To that end I studied solvent mixtures of ethanol, water and ethylene glycol (with and without ethanol vapour) as well as solutions of sucrose, lactose, sodium chloride, sodium nitrate and ammonium sulfate. Thus my experiments spanned volatile, low volatility and involatile solutes with and without crystallisation. In each case I measured the change in droplet profile over time and how that compared to a volume-averaged model of droplet evaporation, the direction and speed of internal (solutal Marangoni) flows by adding tracer particles, and the final deposits. Using my model I found that all but ethanol-water mixtures developed large concentration gradients, with involatile solutes accumulating at the liquid-vapour interface and suppressing evaporation. Sugar droplets approached a high-viscosity glassy state while salt droplets became supersaturated, leading to fast growing crystalline structures nucleating. By using picolitre rather than microlitre sessile drops I uncovered aerosol-like behaviour where salt droplets would stop nucleating above a certain efflorescence relative humidity. To directly measure concentration gradients we developed a novel reflectometry experiment, printing droplets on a silica hemisphere and using the evolving refractive index difference between droplet fluid and the substrate to determine the composition. This allowed me to verify the presence of an ethanol residue persisting after the end of solutal Marangoni flows in ethanol-water droplets. During the solutal Marangoni flows we observed particles migrating across flow streamlines to either the centre or liquid-vapour interface depending on the system in question. From the reversal of migration direction in response to a flipped ethanol concentration gradient we suggest diffusiophoresis as the cause of migration, as a number of other mechanisms were ruled out.