Advection kinetics induced self-assembly of colloidal nanoflakes into microscale floral structures

The article explores the governing role of the internal soluto-thermal hydrodynamics and advective transport within sessile colloidal droplets on the self-assembly of nanostructures to form floral patterns. Water–acetone mixture and Bi₂O₃ nanoflakes based complex fluids are used as the experimental liquids. Micro-liter sessile droplets are allowed to vaporize and the dry-out patterns are examined using scanning electron microscopy. The presence of distributed self-assembled rose-like structures is observed and is postulated to be formed by the hydrodynamic interactions within the drying droplet. The population density, structure and shape of the floral structures are noted to be dependent on the binary fluid composition and nanomaterial concentration. Detailed microscopic particle image velocimetry and infrared thermography analysis is undertaken to qualitatively and quantitatively describe the solutal Marangoni advection within the evaporating droplets. It has been shown that the kinetics, regime and spatial distribution of the internal flows are dominantly responsible factors towards the advection influenced clustering, aggregation and self-assembly of the nanoflakes. In addition, the size of the nanostructures and the viscous character of the complex fluid are also noted to play dominant roles. The resulting interplay of hydrodynamic behavior, adhesion and cohesion forces during the droplet dry-out phase, and thermodynamic energy minimization leads to formation of such floral structures. The findings may have strong implications towards modulating micro-hydrodynamics induced self-assembly in complex fluids. (paper)