Insights into Drying of Noncircular Sessile Nanofluid Droplets toward Multiscale Surface Patterning Using a Wall-Less...

Surface patterning with functional colloids is an important research area because of its widespread applicability in domains such as nanoelectronics, pharmaceutics, semiconductors, and photovoltaics among others. In this endeavor, we propose a low-cost patterning technique that aspires to eliminate the more expensive methodologies that are presently in practice. Using a simple document stamp on which patterns of any geometry can be embossed, we are able to print 2D millimeter-scale "wall-less confinement" using an ink-based hydrophobic fence on any plasma-treated superhydrophilic surface. The confinement is subsequently filled with nanocolloidal liquid(s). Using confinement geometry, we are able to control the 3D shape of the droplet to exhibit multiple interfacial curvatures. The droplet in the "wall-less confinements" evaporates naturally, exhibiting unique geometry (curvature)-induced flow structures that induce the nanoparticles to self-assemble into functional patterns. We have also shown that by modifying the geometry of the pattern, evaporation, flow, and particle deposition dynamics get altered, leading to precipitate topologies from macro- to microscales. We present two such geometrical designs that demonstrate the capability of modifying both macroscopic and microscopic features of the final precipitate. We have also provided a description of the physical mechanisms of the drying process by resolving the unique flow pattern using a combination of imaging and microparticle image velocimetry. These provide insights into the coupled dynamics of evaporation and flow responsible for the evolution of particle deposition pattern. Precipitate characterization using scanning electron microscopy and dark-field microscopy highlights the transformation in the deposit morphology.