Abstract

Droplet impact and its flattening on a surface play an essential role in many industrial applications such as inkjet printing, agriculture, and plasma spraying. Understanding the physics of droplet spreading is the key to maintaining the mass transfer process in all relevant applications. Two different problems are investigated in this study. In the first parts, the behavior of a hollow droplet after impact on a surface is considered, while in the last chapter, the effect of gas desorption on the flattening and solidification of a molten particle in investigated.
Most of the droplets observed in nature are dense droplets. Nevertheless, special droplets have been observed in several industrial applications such as aerosol transfer from the sea, oxygen dissolution, controllable biomedicine, and thermal spray coating which are called hollow droplets. In plasma spraying process, the accumulation of these flatten droplets (splats) on top of each other forms a coated layer. Due to their embedded medium, the cavity-containing droplets (hollow droplets) trigger cavitation when they reach the target and influence the splat properties by releasing the inner substance. However, it is difficult to study the impact and flattening of hollow droplets in thermal spraying, regarding the harsh environments, and phenomena small size and high velocity.
To better understand the flattening process of a hollow droplet, in this work, a comprehensive experimental, numerical, and theoretical study is performed on water and glycerol droplets impacting on a rigid surface. The experiments are repeated on different surfaces, including aluminum, sand-blasted steel, and superhydrophobic. The results show that the mechanisms of the post-impact process of hollow droplets are different from those of dense droplets in several aspects. We study the role of surface wettability, liquid properties, impact velocity, surface angle, and bubble size and location on the droplet flattening process. In the numerical part, compressible Navier-Stokes equations are solved using the volume of fluid (VOF) method. A theoretical model is developed to analyze the maximum spreading diameter of the hollow droplet impact analytically. Its prediction is in good agreement with the experimental and numerical results.
The comparison of simulation results with the experimental photographs shows that the numerical solver can correctly predict the hollow droplet shape evolution. It is demonstrated that flattening a hollow droplet has two significant distinctions compared to a dense droplet flattening. The first distinction is the formation of a counter-jet following the collision of a hollow droplet impact perpendicular to the surface. It is proven that the formation of the counter-jet is an inherent phenomenon of hollow droplet flattening and is unaffected by impact velocity or substrate angle. Nevertheless, it is revealed that the counter-jet length depends on droplet velocity or liquid viscosity. The second distinction is the ultimate shape of the flattened droplet. After contact on a hydrophobic surface, the dense droplet partially recoils toward the center and produces a dome shape. However, upon impact on a hydrophobic surface, the hollow droplet takes the form of a donut. This is owing to the perturbations caused by bubble rupture on the spreading droplet surface. As a result of these perturbations, the spreading liquid sheet is fragmented and, the droplet is unable to recoil toward the center, forming a donut shape. The results show that the spreading diameter and the counter-jet height formed after the hollow droplet impact grows with impact velocity increasing. Investigating the size and location of the entrapped bubble shows an optimum bubble size that facilitates the hollow droplet flattening. It is also demonstrated that the ripples on splats produced by the hollow droplets with a larger bubble size are higher than those of small bubbles.
In the end, the effects of surface gas desorption on the splat formation are studied. In plasma spray, the splats resulting from the impact, spreading, and solidification of molten particles are the building blocks of the spray coatings. Fragmented splats are formed on substrates held at room temperature and atmospheric pressure. Although the formation of a fragmented splat is attributed to adsorbates on the substrate surface, its dynamics have not been adequately addressed. In this study, a numerical model is developed to investigate the formation of fragmented splats during droplet flattening and solidification in plasma spraying conditions. Compressible Navier—Stokes equations are solved, and the volume of fluid (VOF) method is used to capture the liquid and gas interface. In addition, the source term method is used to capture the solidification process during droplet flattening.
Moreover, a new boundary condition is defined to consider the effect of gas desorption on the substrate surface after droplet impact. The numerical results show that gas desorption from the surface produces a barrier layer between the droplet and the substrate. This high-pressure region detaches the edge of the spreading droplet from the surface and forms a liquid sheet. The liquid sheet rises above the substrate and spreads up to 2 times more than droplets impacting surfaces without gas desorption. The fragmentation of the liquid film follows the overspreading of the droplet. As a result, only a portion of the initial droplet remains at the location of the impact, which forms a small solidified splat.