Role of fluid forces and depletion interactions in directing assembly of aqueous gold nanorods on hydrophobic surfaces

The interaction between macroscopic fluid flow and nanoscale forces has resulted in the formation of long-range assemblies through evaporation-induced self-assembly. Anisotropic gold nanorods (AuNR) can form disordered, smectic, or vertically ordered long-range structures, but controlling their assembly remains a challenge and requires a deeper understanding of fundamental interaction mechanisms. In this work, we established a correlation between the in situ drying profiles, measured using an optical tensiometer, and deposit pattern, imaged ex situ using electron microscopy. Increasing particle concentration induced a transition from coffee-ring to uniform deposition at the microscale, while nanoscale structures shifted from isotropic/smectic to vertically aligned crystalline AuNRs. The interplay of capillary and Marangoni flow influences assembly at both macro and nanoscales, with the deposition process and nanoparticle ordering being highly sensitive to interparticle and nanoparticle-substrate interactions. By systematically studying key parameters, we aim to develop a comprehensive framework for the rational design and fabrication of nanomaterials with precisely controlled structure and properties. The schematic shows an overall picture of the role of fluid flows inside an evaporating colloidal droplet at low particle concentration (left) and high concentration (right). Corresponding final dried depositions are shown in the lower panel clearly demonstrating a transition from coffee-ring to uniform deposition as AuNR concentration is increased. • Coffee-ring to uniform deposition pattern in evaporative assembly of gold nanorods. • Interplay of fluid forces at micro-scale and nanoscale interactions are discussed for tunable surface morphology. • Tunable crystalline assembly with smectic-to-vertically aligned AuNR through depletion and particle-substrates interaction.