Evaporation of sessile nanofluid droplet
Date of Issue2017
School of Mechanical and Aerospace Engineering
Sessile nanofluid droplet evaporation is receiving increasing attention lately due to its potential in various applications requiring controlling of the solutes morphology after liquid drying. Despite extensive progress, the understanding still remains elusive about how the deposited morphology is affected and produced owing to the numerous interrelated factors of a nanofluid droplet system. I investigate sessile nanofluid droplet drying in respect to the domains of colloidal suspensions, base solutions and solid substrates. In respect to suspensions, the addition of mono graphite nanoparticles is found to reduce the droplet wettability, enhance the contact line pinning effect, and induce a slight spreading at the initial stage of evaporation. By using proper combinations of dual-sized alumina nanoparticles, the heterogeneities of the two single-sized nanoparticle patterns are eliminated, and a uniform mixture pattern is produced. Besides, the mixture pattern assembled by two nanoparticle species either exhibits both the features of the single-species patterns or resembles one of them. In respect to the base solution, we probe the flow paradigms during evaporation from two aspects. One is employing water-ethanol binary droplets to examine the effect of ethanol concentration. The binary droplets display three distinct flow regimes. The relative weighings of Regimes I and II are enhanced and Regime III is shortened upon raising the ethanol component. The ethanol dependent weighings of the three regimes determine the congregation and trajectories of the nanoparticles and thus the final deposition. The other study for base solution is probing the autophobic effect induced by cationic surfactant cetyltrimethylammonium bromide in sessile droplets. For the pure droplet, below the critical micelle concentration of the surfactant, the droplet dominated by the autophobic effect exhibits two distinct phases of depinning: Phase 1 featuring rapid droplet shrinkage, and Phase 2 characterized by slower droplet receding. The velocity of the three-phase contact line in Phase 1 shows a transition as the surfactant concentration increases above 0.042 mM, while such a transition is absent for Phase 2. Besides, the spreading of the sessile droplets as they form before the retraction and the maximum contact angle led by dewetting are found regularly dependent on the surfactant concentration. For graphite nanofluid droplets, the autophobic effect is enhanced upon increasing the nanoparticle concentration. The deposited morphology evolves to a distinct coffee ring with a loading of the surfactant, owing to the surfactant adsorption at the nanoparticles surface and the liquid-solid interface, which repels the nanoparticles from the liquid-vapor and the liquid-solid interfaces. In respect to the solid surface, the substrate temperature was varied to find its role in controlling the flow patterns and the stain morphology. The deposited patterns transforms from a disk-like profile to a dual ring from cooling to heating of the substrate. The droplet on the substrate at low a temperature reveals three primary stages. Stage I features the outward transports of nanoparticles along the liquid-vapor interface near the droplet edge. Meanwhile some nanoparticles deposit on the solid surface with a distance to the contact line. In the central region, nanoparticles are dominated by Brownian motion, so they fluctuate irregularly around their positions. Stage II is characterized by the enhanced outward travellings of the nanoparticles in the bulk, leading to a pronounced coffee ring. Most nanoparticles 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 of the interior nanoparticles. Upon increasing the substrate temperature, the accompanied flow pattern displays a transition when the substrate is still remained cooler than the atmosphere. It is attributed to 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. Recommended future works comprise the employments of dual-sized fluorescent spheres to examine the size-dependent motion of spheres, textured substrates for distinct droplet dynamics, and the infrared thermographic technique for detecting the temperature profile of droplet free surface.