Search

For the impacting-freezing process of the droplet on cold surfaces, more studies have focused on stationary surfaces, while moving surfaces have received less attention. Here, a three-dimensional numerical model based on the VOF model and the Solidification/Melting model is established to numerically study the impacting-freezing process of the droplet on a lateral moving cold superhydrophobic surface, and focusing on the morphology evolution of the droplet and the initial freezing location. For the lateral moving surface, five different morphology distributions were observed during the impacting-freezing process of the droplet: full rebound, near full rebound, partial rebound, near full adhesion and full adhesion. The initial freezing location of the droplet preferentially appears on the upstream side of the droplet. The freezing on the downstream side of the droplet lags behind that of the upstream side due to the cooling effect of the cold surface, and the lagging effect is strongly influenced by the cold surface temperature. Moreover, the effects of cold surface temperature, supercooling degree and impacting Weber number on the spreading factor during the impacting-freezing process of the droplet are also discussed. The effect of the cold surface temperature on the spreading factor is greater than that of the supercooling degree. And a unified morphology distribution map of rebound and adhesion of the droplet is plotted based on the weighted average temperature and the impacting Weber number, which can show the competitive relationship between droplet impact hydrodynamics and droplet solidification.

A two-dimensional Lattice Boltzmann model was established to study the dynamic physical process of frost growth on a cold flat surface. It is verified that the theoretical results are in good agreement with the experiment data with better accuracy. The frost morphology is explored by showing two-dimension density distribution with different cold surface temperatures. The results also include dynamic characteristics of frost properties such as thickness, deposition weight, volume fraction, density, temperature. According to the results, it is found that the nearer it is from the cold flat plate, the larger of frost crystal volume fraction and density are. The frost surface temperature increases rapidly at the early period of frosting and then increases gradually towards the triple point temperature of the water. The frost internal temperatures rise in a linear tendency with the frost thickness and decrease at the same frost layer location with frost growth. Super-saturation degree of moist air has non-negligible effect on frost growth parameters such as frost thickness, frost crystal deposition mass, frost deposition rate and frost crystal volume fraction.

A droplet can bounce in a short contact time after impacting on superhydrophobic surfaces (SHSs), which is of benefit to the applications like anti-icing, anti-frosting and self-cleaning. In this study, we numerically investigate the fast droplet bouncing induced by asymmetric spreading on the concave SHS with a trapezoidal cross-section and focus on the droplet morphology evolution and contact time variation. A preferential transport at the both sides of the droplet from the inclined part to the flat part will occur because of the concave structure feature. The droplet will exhibit asymmetric spreading behavior. Moreover, the effects of structure parameters and impact velocity on the droplet contact time are explored. Within this concave structure, the droplet contact time is less influenced by the impact velocity and more influenced by the structural parameters. The presence of the tangential momentum can enhance the droplet asymmetric spreading, but it is not always beneficial to the droplet contact time reduction. Therefore, there will be an optimal structure to achieve the minimum droplet contact time. A concave SHS structure with the structure width w = 1–1.2r and the inclination angle of 45° is a more desirable choice, which is expected to reduce the droplet contact time by 40~45%.