Formation and development of distinct deposit patterns by drying Polyelectrolyte-stabilized colloidal droplets at different surfactant concentrations

Abstract

Understanding and controlling the formation of various deposit patterns such as coffee stains, multiple ring-like morphologies, and uniform deposition are essential for developing various applications. The presence of polymer and surfactant in a drying colloidal droplet can alter the droplet evaporation mode and deposit structure. In this work, we study the effect of Marangoni stresses on the evaporative dynamics and the structure and alignment of nanoparticles in the dried deposit patterns when a droplet of electro-sterically stabilized (poly(styrene sulfonate)) gold nanorod dispersion (PSS Au-NR) dries on a hydrophilic silicon substrate at low and high concentration regimes of an added surfactant (CTAB). The evaporative kinetics study shows that the evaporating droplets of PSS Au-NR dispersion exhibit rapid pinning and quick evaporation at zero and high CTAB concentrations, which transforms to three stages of droplet evaporation with an initial droplet depinning stage at low CTAB concentration. Pinning of the PSS Au-NR dispersion droplet in the absence of CTAB results in a coffee stain with densely and loosely packed PSS Au-NRs without any nanorod ordering in the outer and inner coffee stain edges. The formation and systematic evolution of various new deposit structures such as depinned region, depletion region, and fingering pattern are noted both outside and inside the coffee stain at different CTAB concentrations. Our study demonstrates that the kinetics of droplet drying and the deposit pattern of an evaporating polyelectrolyte-colloidal droplet can be tailored systematically by controlling the internal flows in the droplet dispersion medium through the addition of surfactant.