Nanoparticles deposition patterns in evaporating nanofluid droplets on smooth heated hydrophilic substrates: A 2D immersed boundary-lattice Boltzmann simulation

Abstract

Nanoparticles deposition dynamics and deposition patterns in evaporating nanofluid droplets on a smooth hydrophilic heated substrate in a vapor environment (0.9Tc) under zero gravity are investigated numerically by a 2D immersed boundary-lattice Boltzmann method in combination of the non-isothermal Gong-Cheng liquid-vapor phase change model. The Marangoni flow along droplet interface is found to be the dominated transportation mechanism of nanoparticles inside an evaporating nanofluid droplet when droplet contact line is freely moving. It is demonstrated that the pinning of the triple phase contact line on the substrate having a small contact angle (θ=51°) is a required condition for a ring-like deposition pattern after a nanofluid droplet evaporation. On the other hand, on a substrate with a larger contact angle (θ=74°) where the triple contact line is not pinned, the upward Marangoni flow along droplet interface has a greater strength to resist nanoparticles moving toward the edge of the heated substrate, which result in bump-like deposition patterns after a nanofluid droplet evaporation. A regime map, showing effects of substrate contact angle and substrate temperature on nanoparticle deposition patterns after droplet evaporation, is presented. It is shown that the ring-like deposition pattern occurs when the triple line is strongly pinned (at small contact angle) and the Marangoni flow is weak (small contact angle and low substrate temperature). The bump deposition pattern occurs when the droplet contact line is not pinned (at high contact angle) and the Marangoni flow is strong (large contact angle and high temperature).