Drying and Deposition of Picolitre Droplets of Colloidal Suspensions in Binary Solvent Mixtures

Abstract

Picolitre droplets of colloidal suspensions in a mixture of two solvents are emitted by a drop-on-demand ink jet print head onto coated and uncoated glass substrates. The evaporation rate and internal flows inside the drying droplets are investigated and the deposit pattern formed is related to the drying dynamics. High-speed imaging of the droplet profile from the side al- lows the droplet diameter, height, contact angle and volume to be measured during evaporation. The internal flows throughout dry- ing are visualized by following tracer particles within the fluid with an inverted microscope. The resulting deposits at higher solid content are imaged by scanning electron microscopy in or- der to relate the morphology and fine structure to the internal flows within the droplet. The evaporation of binary solvent mixtures can cause a gra- dient in the surface tension at the liquid-air interface, resulting in a Marangoni flow. The ratio of solvent composition and the surface tension of the more volatile solvent relative to the less volatile solvent is varied, to manipulate the direction and mag- nitude of any introduced Marangoni flow. Pure solvent droplets are compared to mixed binary solvent systems, to determine the importance of Marangoni flows on the morphology of the final deposit.