Study of anatase nanofluid droplet impingement on superhydrophobic surface

Abstract

The prospect of Nano Science and Nanotechnology in various emerging fields has led to the design, synthesis, manipulation and optimization of nanoparticles (1nm-100nm) in order to create new opportunities for the utilization of smaller and more regular structures for various applications. Nanofluids are the new class of fluid made by homogeneously suspending nanometre sized particles in the base liquids like water, ethylene glycol, etc. Nanofluid droplet impacting on a sloid surface is a common phenomenon in our everyday life which shows prospects in various applications ranging from assessing the efficacy of a pesticide spray to the clarity of inkjet printing. Different parameters like surface wettability and roughness, surface tension, viscosity and nanoparticle concentration of the liquid, impinging velocity and impacting diameter of the droplet play their respective roles in dictating the outcome of droplet impact on surfaces. It is well-reported in literature that inertia, capillary and viscous forces comes into play as a liquid droplet impacts a solid surface. Akin to the impact of a regular fluid droplet on a flat supehydrophobic surface, a nanofluid droplet would also spread – taking a pancake shape – to a maximum diameter, and then would recede, splash or fragment. Depending upon the impacting droplet momentum, the droplet after impact may again bounce-off. However, several features of nanofluid droplet impact have remained unanswered. For example, the influence of nanofluid particle concentration on the salient impacting behaviors, e.g., maximum droplet spreading diameter, extent of peripheral ring formation, splashing and rebounds of impacted droplets have not been well-investigated. Types of pancake formed after spreading, and its size (diameter) depend on many variables like type and size of nanoparticles, the host fluids, particles volume fraction, nature of surfactant, stabilizing time, impinge velocity, height of impingement, surface wettability, impact of surface roughness, which needs careful study. In this pursuit, the present work focuses on Anatase (Titanium (IV) oxide nanoparticles of <25nm size, dispersed in deionized water) nanofluid droplet impact behavior on superhydrophobic substrates. Upon impact, pancake-shaped spreading of the nanofluid droplet were observed, with the spreading diameter varying with the droplet dispensing needle diameter and the drop impact velocity (the drop release height). Nanoparticle volume fraction ɸ=0.01%, 0.1% and 0.5% were considered in this experiment. The surfactant used here was Cetyl Trimethyl Ammonium Bromide (CTAB). Results of nanofluid droplet impingement are compared with the impact characteristics of a deionized water droplet. Three needles of internal diameter of 0.714 mm , 0.560 mm and 0.231 mm were used for dispensing the droplets from different heights. It was observed that small droplets (dispensed from small size needle) show fully splashing behaviour at lower Weber number. Droplets dispensed from intermediate size nozzle show splashing at higher Weber number, while splashing occurs at intermediate Weber numbers for the large droplets. Similar experiments were performed by varying the nanoparticle loading. It was observed that with increase in particle loading, the smallest-diameter droplets showed the least Weber number for which the nanodroplet splashes, while the droplet with the highest diameter shows the highest Weber number.