Experimental investigation on spreading and freezing mode of droplets impacting cold wall

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The freezing of droplets upon impact with solid walls is a common phenomenon that significantly impacts various areas of social production. Therefore, it is essential to study the spreading and icing characteristics of droplets impacting solid walls. Utilizing a visualization experimental system, the effects of impact Weber number (We), wall temperature, wall wettability, and wall curvature on the dynamics of droplet impact and icing characteristics were investigated. The temporal evolution of the droplet's icing morphology and spreading coefficient are analyzed. The results indicate that low wall temperatures reduce the extent of droplet spreading and suppress, or even eliminate, retracting and oscillating phenomena. Under low-temperature conditions, the peak spreading coefficient of droplets is generally lower than that at room temperature, exhibiting a spread-freeze mode. As the We increases, the droplet spreads more vigorously, the solid-liquid contact area enlarges, the slope of the spreading coefficient increases, and the freezing rate accelerates. When the wall is hydrophobic, the spreading coefficient undergoes periodic changes, the droplet undergoes rebound behavior, the residence time and contact area decrease, delaying the freezing process. In contrast, when the wall is hydrophilic, the change in the spreading coefficient is singular, the droplet spreads more stably, and freezing is more likely to occur. Compared to horizontal surfaces (We=277), the effect of surface wettability on the morphological evolution and freezing mode of droplets impacting curved surfaces is less significant.