Evaporation of Multicomponent
Inkjet Printed Droplets

Abstract

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.