EFFECT OF SALT CONCENTRATION (NaCl) ON DRYING PATTERN OF FERROFLUID DROPLETS

Droplets leave a coffee ring-like pattern on a solid surface after complete evaporation. This study reports evaporation and drying pattern of a ferrofluid droplet on a PDMS substrate at different concentrations of NaCl using goniometric imaging, confocal microscopy, and optical profilometry. The receding of the contact line of the droplet gets delayed with increase in the salt concentration. Width of the coffee ring pattern decreases with increase in the salt concentration. Crystals are formed at a higher NaCl concentration. The attraction force between a particle and substrate increases with increase in the salt concentration. Zeta potential and thickness of the double layer decrease due to addition of salt leading to reduction in the repulsion force between the particle and substrate surface. Deposition pattern formation gets affected by a combined effect of surface tension force, electrostatic force, and van der Waals force acting on the particle near the contact line.