Colloidal droplets often leave behind ring-shaped depositions of material after they evaporate called “coffee ring” patterns that are undesirable in many medical diagnostic and printing applications. By applying electric fields to a droplet, additional electrowetting and electrophoretic forces can be applied on the droplet’s contact line and particles, respectively, to suppress this phenomenon. Better understanding of the effects of electrowetting and electrophoresis may lead to novel techniques for nanoparticle self-assembly in evaporating colloidal droplets that are subjected to electric fields.
This work experimentally examines the effect of particle selection on nanoparticle deposition in evaporating droplets. Droplets consist of deionized water seeded with polystyrene or titanium oxide nanoparticles on the order of 20 nm. Colloidal droplets are evaporated on substrates coated with an SU-8 photoresist. Before fundamentally understanding the effects of an applied electric field, the evaporative dynamics and resultant colloidal transport in similar unactuated systems must first be understood. Current trials involve studying these unactuated control cases, but future trials will involve droplets actuated on the same material with applied AC or DC electric fields. Droplet interface shapes during evaporation are recorded and compared between trials. Final deposition patterns and their particle distributions are also qualitatively examined. Polystyrene droplets pinned for approximately the first 30% of the total evaporation time and then receded at a constant contact angle and produced ring depositions. Titanium oxide droplets pinned for approximately the first 60% of the total evaporation time and then receded in a slip-stick pattern. These produced more uniform depositions with a less-distinct outer ring, despite pinning for more of the evaporation time. The variations in transient behavior suggest that differences in particle characteristics may be impacting the contact line dynamics and resulting in different final deposition pattern.