Numerical investigation of air entrapment in a molten droplet impacting and solidifying on a cold smooth substrate by 3D lattice Boltzmann method

In this paper, a novel 3D lattice Boltzmann method (LBM) is proposed for simulating a molten droplet impacting and solidifying on a cold smooth substrate surrounded by air. The numerical simulation shows that a small pocket of air is entrapped in the molten droplet adjacent to the cold surface after its impact on the surface. And this trapped air creates a thermal resistance between the droplet and the substrate. It is demonstrated that the no-slip velocity of the air on the solid surface prevents the air being squeezed out completely, and the air gap between the droplet and the substrate is compressed by the falling droplet. The compressed air results in the first contact away from the impact center and an air film is trapped within the droplet. This trapped air pocket eventually forms multiple air bubbles or a single air bubble depending on the surface wettability. Although droplet solidification has an important effect on the number of multiple air bubbles trapped on a surface having a small contact angle, it does not affect the size of the single air bubble trapped on a surface having a large contact angle. Maximum spread factors and dimensionless bubble heights obtained from simulation match well with theoretical values in the literature, validating the accuracy of this numerical model. (C) 2018 Elsevier Ltd. All rights reserved.