Coupled effect of concentration, particle size and substrate morphology on the formation of coffee rings

Abstract

Our study reports the coupled effects of particle size, concentration and the substrate roughness on the evaporation process of an aqueous sessile droplet containing silica nanoparticles. The patterns formed after complete drying of the sessile drops have also been studied. On smooth substrates the droplets evaporate in constant contact angle (CCA) and mixed modes (MM) with CCA mode as the predominant mode of evaporation. For substrates with nanoscale roughness all the three, namely CCA, constant contact radius (CCR) and MM modes exist with CCR mode as the predominant mode of evaporation. On nanorough substrates, droplets leave circular ring-shaped stains due to strong pinning along the droplet perimeter whereas irregular shape stains are formed on smooth substrates. We found that the rim width of the ring-like particulate deposits scales with the particle concentration following a power law behavior. Furthermore, we compared crack density and uniformity of cracks in the deposits formed on both substrates obtained by drying aqueous drops containing particles of different sizes and concentrations. For a given roughness, delamination is found to occur in the deposits formed by drying drops with higher particle concentration and smaller particle size. Finally, we summarize our study by presenting a phase diagram to classify the particulate deposits and the crack patterns as a function of particle concentration and substrate roughness.