Colloidal Transport, Self-Assembly, and Deposition in Evaporating Droplets

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From beverage marks on the kitchen table to salt deposits on windows of the family car, we are all familiar with these stains that plague our homes and vehicles. However, such residues are not in random nor uniform patterns, and they often appear as circular spots with a bright center and a dark rim. A closer inspection of this phenomenon reveals that the accumulation of the particles at the edge is facilitated by the fluid flow inside the droplet, which is, in turn, driven by the liquid leaving the free surface of the drop due to evaporation. Apart from its everyday relevance, the evaporation-driven deposition of solutes offers a simple, inexpensive method for assembling complex miniature structures. This method can also be used as a diagnostic tool for forensic analysis, disease examination, and biodetection. The goal of this award is to examine the motion of particles suspended in evaporating colloidal droplets that rest on a substrate, and to link the transport of the colloids to where they finally reside when the droplet is desiccated. The planned studies are coupled with a range of educational activities that involve outreach to underrepresented middle and high school students, mentorship of diverse community college and graduate students, and curriculum development.This award aims to investigate the transport and aggregation of nonvolatile particles in drying colloidal sessile drops with the ultimate goal of understanding the mechanisms underlying various deposition patterns. Specifically, high-fidelity numerical simulations and theoretical analyses will be used to study the (i) role of the liquid-gas interface in the evolution of the evaporative self-assembly process, (ii) impact of shape and polydispersity on transport and deposition of solutes, and (iii) sensitivity of particle transport and deposition to droplet geometry and shielding effect. The findings of this award will narrow the gap in our current knowledge and fundamental understanding of the evaporation-induced transport, self-assembly, and deposition of particles initially suspended in a sessile drop of a simple liquid. They will also establish a transformative physics-based regime map for deposition patterns that can serve as an engineering guideline for tailoring the system parameters in order to elicit the desired self-assembled structure for a variety of high-tech applications. The map can also be utilized as a diagnostic tool for pinpointing the properties of the suspended particles and the nature of their interactions with themselves, with the liquid-gas interface, and with the substrate.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.