Papers by Keyword: Droplet Impact

Abstract: The following study investigates splashing of impinging water droplets on superhydrophobic surfaces with and without the presence of a stagnation flow. Droplets were accelerated by either gravity or gravity and co-flow. By changing the height and the air flow velocity different combinations of stagnation flow and droplet velocity were created. The spreading diameter, spreading velocity and contact time were studied for different air and droplet speeds. It was clearly observed that for a fixed impact velocity (i.e. constant Weber number), the presence of the stagnation flow promotes splashing and formation of satellite droplets. Consequently, for the co-flow droplet impact experiments, the mass of the recoiled droplet is significantly smaller than that of the impinging droplet in still air.

Abstract: In this paper, a Finite Element Analysis is carried out in order to simulate the process of spreading and solidification of a micrometric molten droplet impinging onto a cold substrate. This process is a crucial key to have a good understanding of coatings obtained by means of thermal spraying. The effect of thermal contact resistance (TCR) on the droplet spreading and solidification was investigated using different values of TCR and different droplet sizes. The solidification time was found to be a linear function of the droplet diameter square. Viscous dissipation, wettability and surface tension effects are taken into account. The Level Set method was employed to explicitly track the free surface of molten droplets.

Abstract: In this paper, the effect of material’s wettability on the droplet impact has been investigated by numerical apporach. The unsteady flow behaviors of liquid droplet impacting against the rough solid surface with different wettabilities have been simulated based on lattice Boltzmann method. The spreading and bounding characterisitcs of droplet have been discussed. For the hydrophilic material, the droplet will sink into the grooves among roughness bumps, and its apparent contact angle in steady stead will be smaller than its corresponding intrinsic contact angle; while for the hydrophilic material, droplet will flow into the grooves but suspend on the top of roughness elements without any contacting with the bottom surface, and the apparent contact angle is larger than its intrinsic contact angle.

Abstract: A numerical computation and theoretical model are presented on spreading of a single droplet impacting on a solid surface at low Weber number. The numerical simulation uses combined Level Set-VOF method and a precise wetting model. The singularity at the moving contact line was analyzed and removed by present wetting model. A theoretical model based on the energy balance was developed to predict the maximum spreading ratio, accounting for wetting effect by a correction factor. The droplets shapes, the dimensionless spreading radius and the dimensionless height of droplet calculated by present numerical method were compared with the experimental data. The theoretical model is used to predict the maximum spreading ratio. The numerical and theoretical results agree well with the experimental data. Present theoretical model indicates that capillary effects may be neglected if 6We(We/3+4)>>Re.

Abstract: The impact behavior of a liquid droplet on solid surface is a complex phenomenon and yet is a basic component of various industrial processes particularly in the pharmaceutical industry. In this industry, film coating technique is used in tablet coating, in which coating uniformity is important especially if the coating is for functional purposes. Coating uniformity on a tablet could be affected by several factors, one of which is the impingements of droplets on its surface. In this work, the maximum spreading diameter and the initial impact behavior of a single droplet on pharmaceutical tablet surfaces and metal surfaces having different surface properties are investigated. A Charged-Coupled-Device (CCD) high-speed camera with framing rate of 2,000, attached to a 10X microscope, was used to capture the phenomena. The results show that the initial impact behavior of a droplet is not affected by the porosity of a surface. The results on the pharmaceutical tablets, stainless steel and etched silicon surfaces show that the rougher the surface the lower the spreading factor. The droplets on all surfaces demonstrate that a droplet that produces higher spreading factor gives a lower bouncing factor.