Journal of Flow Visualization & Image Processing, 25(3–4):245–258 (2018) EFFECT OF SALT CONCENTRATION (NaCl) ON DRYING PATTERN OF FERROFLUID DROPLETS S.K. Saroj & P.K. Panigrahi * Department of Mechanical Engineering, IIT Kanpur, Kanpur, India, 208016 *Address all correspondence to: P.K. Panigrahi, Department of Mechanical Engineering, IIT Kanpur, Kanpur, India, 208016, E-mail: panig@iitk.ac.in 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. KEY WORDS: ferrofluid, deposition pattern, salt concentration 1. INTRODUCTION A droplet containing micro/nanoparticles leaves the coffee ring-like pattern on the substrate surface after complete drying. Solutes move toward the periphery during evaporation of solvent and form a ring-like structure known as a coffee ring. The deposition pattern of evaporating droplet has several applications such as inkjet printing, DNA sequencing, painting, biological assays, photonics, sensing devices, separation of different size of particles and biological cells (Jung and Kwak, 2007; Wong et al., 2011). Several methods have been reported in literature to control the coffee stain of droplet such as electrowetting (Mampallil et al., 2012), DC potential (Orejon et al., 2013), contact angle hysteresis (Li et al., 2013), shape of particle (Yunker et al., 2011) and preventing contact line pinning by coating of silicon oil on substrate (Das et al., 2017). The pinning or depinning of the contact line depends on the properties of substrate, fluid, size and shape of the colloid particles, ambient conditions (humidity and temperature) and additives. Deegan et al. (1997) suggested that pinning of the contact line is due to accumulation of particles near the contact line. The evaporation rate at the contact line is higher compared to the air- liquid interface, and particles move toward the contact line along with the fluid movement for replenishing the loss of fluid. Stick–slip behavior increases with increase in TiO 2 nanoparticle concentration (Moffat et al., 2009). The unbalance of the interfacial forces and colloidal forces (van der Waals and electrostatic) at the contact line are responsible for pinning or depinning of contact line. The presence of the additives such as surfactant (Still et al., 2012), salt, and polymer 1065–3090/18/$35.00 © 2018 by Begell House, Inc. www.begellhouse.com 245