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.
