Learning from "Coffee Rings": Ordered Structures Enabled by Controlled Evaporative Self-Assembly
Drying droplets containing nonvolatile solutes (polymers, microspheres, nanoparticles, single-walled carbon nanotubes, DNA, etc.) on a solid surface have been utilized to yield self-assembled, dissipative structures, possessing dimensions of a few hundred submicrons and beyond. However, these dissipative structures created via evaporation are often irregular and far from equilibrium. Yet for many applications in microelectronics, data storage devices, and biotechnology, it is highly desirable to achieve surface patterns that have a well-controlled spatial arrangement. To date, only a few elegant studies have centered on establishing a means of harnessing the drying process of an evaporating droplet to produce highly regular structures. Among them, controlled evaporative self-assembly (CESA) in a restricted geometry stands out as an extremely simple route to creating intriguing one- or two-dimensional structures. In this geometry, the evaporation flux, the solution concentration and the interfacial interaction between the solute and substrate are precisely controlled, thereby producing intriguing, well-ordered structures with high fidelity and regularity. When compared with conventional lithography techniques, surface patterning by controlled solvent evaporation is simple and cost-effective, offering a lithography- and external field-free means of organizing nonvolatile materials into ordered microscopic structures over large surface areas.
Over the past several years, I have crafted a wide range of intriguing and highly regular micro- and nanostructures composed of conjugated polymers, block copolymers, and latex particles, metallocene-containing polymers, etc. enabled by CESA in rationally designed "curve-on-flat" geometries. The mechanism of structure formation is elucidated both experimentally and theoretically. Moreover, by applying external magnetic field in conjunction with the solvent evaporative field, CESA of magnetic nanoparticles is promoted to yield intriguing asymmetry patterns. Finally, hierarchically structured wrinkles formed within the gradient stripes prepared by CESA are systematically scrutinized. As such, CESA represents a state-of-art strategy for crafting highly structured, multifunctional materials and devices for potential applications in optoelectronics, photonics, and biosensors.