Material assembly by droplet drying: From mechanics theories to applications

Evaporation of droplets composed of insoluble materials provides a low-cost and facile route for assembling materials and structures in a wide spectrum of functionalities down to the nanoscale and also serves as a basis for innovating ink-solution-based future manufacturing technologies. This review summarizes the fundamental mechanics theories of material assembly by droplet drying both on solid and liquid substrates and in a fully suspended air environment. The evolution of assembly patterns, material deformation, and liquid flow during droplet drying and its response to external stimuli ranging from solution surfactant and pH value, surface geometric pattern and wettability, drying temperature, pressure environment, to electrical field have been highlighted to elucidate the coupling mechanisms between solid materials and liquid solutions and the manipulation strategies for material assembly through an either active or passive means. The recent progresses in ink-based printing technologies with selected examples are also presented to illustrate the immediate applications of droplet drying, with a focus on printing electronic sensors and biomedical devices. The remaining challenges and emerging opportunities are discussed.