Disk to dual ring deposition transformation in evaporating nanofluid droplets from substrate cooling to heating†
Substrate temperature plays an important role in deposited morphologies formed after the evaporation of nanofluid droplets. The deposited patterns are shown to vary from a uniform disk-like profile to a dual ring from cooling to heating of the substrate. The droplet on the substrate with a relatively low temperature reveals three primary stages. Stage I features an outward transport of nanoparticles along the liquid–air interface near the droplet edge. Meanwhile some nanoparticles form sediment on the solid surface with a certain distance to the contact line. In the central region nanoparticles are dominated by Brownian motion so they fluctuate around their positions. Stage II is characterized by an increasing outward transport of nanoparticles in the bulk so the coffee ring is gradually enhanced. Most particles in Stages I and II are central-concentrated, leaving an annular gap sparsely covered adjacent to the outer ring. In Stage III, the pattern is homogenized by filling the gap with the arrival of the interior nanoparticles. Upon increasing the substrate temperature, the accompanied flow pattern exhibits a transition when the substrate still remains cooler than the atmosphere. It is resulted from the evaporative cooling at the droplet apex counteractive to the applied temperature gradient by substrate cooling. Above the transition temperature, the induced inward Marangoni flow takes place earlier at a higher substrate temperature, and in conjunction with the outward radial flow a dual ring pattern is formed.