Impact of surfactant characteristics and particle aspect ratios on the dried deposit patterns of gold nanofluid droplets.

Abstract

The evaporation of a sessile colloidal suspension droplet on a substrate yields various drying-induced patterns, which are crucial for both technological advancements and fundamental understandings of colloidal droplet evaporation. Several factors influence the formation of various drying-induced patterns on the substrate. In this study, the variation in dried deposit patterns formed after complete evaporation of a gold nanorod (Au-NR) dispersion droplet is investigated by simultaneously varying the characteristics of the surfactant stabilizing the particles and the particle aspect ratio. Here, cetyltrimethylammonium bromide (CTAB) is used to prepare Au-NRs of aspect ratios 3.5 and lower, and the combination of CTAB and benzyl-dodecyldimethylammonium chloride (BDAC) is used to prepare Au-NRs of aspect ratios 5.4 and higher. As the Au-NR dispersion evaporates, coffee-stain patterns with distinct microstructures form. Changes in the characteristics of the surfactant stabilizing the Au-NR dispersion droplets influence their adsorption at the three interfaces during droplet drying and alter the depinning rate of the droplet during the initial stages of evaporation. This, in turn, affects the deposition of Au-NRs outside the coffee stain and the diameter of the coffee stain. Additionally, as the aspect ratios of the Au-NR in the droplet increase, the width of the outer coffee-stain edges and the extent of Au-NR self-assemblies within it significantly increase, as supported by interaction energy calculations. This study demonstrates that changing the surfactant type used to stabilize colloidal particles can significantly impact the drying kinetics and the resultant dried patterns of colloidal suspension droplets.