Evaporation dynamics of colloidal sessile droplets on smooth and nanorough surfaces

Abstract

The phenomenon of evaporation-induced self-assembly inspires the co ee-ring phenomena, in which a drying co ee droplet creates a ring-like pattern on the surface. On the nano or microscale, the majority of surfaces, regardless of their fabrication processes, will have some nite surface roughness. In this thesis, the e ect of substrate roughness on CRE has been thoroughly investigated. When the various inter playing surface forces that in uence ring formation are taken into account, roughness becomes a much more relevant factor. Our studies demonstrate that the wetting property and van der Waals interaction decrease as the nanoscale roughness increases. Furthermore, cracks occur on the rims during the latter phases of the drying process. On a rough surface, the crack density decreases signi cantly. On surfaces with positive skewness and a smaller small scale asperity radius, a ring with inner deposit appears, whereas on surfaces with negative skewness and a larger small scale asperity radius, a distinct ring deposit forms. To further acquire a better understanding of droplet drying dynamics, a study was conducted to investigate the combined in uence of random roughness, particle size, and concentration. For an evaporating colloidal droplet on a smooth substrate, the constant contact angle mode is dominant, but for a rough substrate, the constant contact radius mode dominates. A particle size and concentration dependent threshold concentration was linked to the transition from ring-like annular deposits to patterns with both annular and interior deposits. Individual dependence on either the volatile liquid or the nanoparticle content was examined in binary mixture droplets. The volume of the binary droplets reduced nonlinearly with time as the ethanol content in the binary mixture increased due to faster evaporation of ethanol. Droplets with lower nanoparticle concentrations and greater ethanol concentrations were shown to have better wettability. Faster ethanol evaporation causes nanoparticle agglomeration during the early stages of evaporation, resulting in a particle de cit at the rim. In circumstances where uniform deposition is required, however, co ee ring deposit patterns provide a problem. A pure ellipsoidal nanoparticle droplet creates a co ee ring, as well as cracks after complete evaporation. The addition of polymer to the air-water interface increases particle adsorption and creates a network structure that prevents outward capillary movement, resulting in uniform deposition. The crack formation process is also inhibited after a speci c amount of PVA is introduced to the ellipsoidal nanoparticle droplet. Altogether, the current studies add to our understanding of colloidal particle self-assembly and its link to evaporation kinetics, deposit patterns, particle size and concentration, and substrate nanoscale roughness.