Spreading characteristics of water droplets impacting onto a moving hydrophilic surface

The impingement of water droplets onto solid moving surfaces is a ubiquitous phenomenon in nature and industry, making it fundamentally important to understand the droplet spreading dynamics. Here, the influence of surface movement on the asymmetric spreading characteristics of water droplets on horizontally moving hydrophilic surfaces is experimentally investigated. The spreading process and liquid film morphology, maximum spreading factor and time, and liquid film centroid are analyzed under different tangential moving and normal impact Weber numbers (i.e., We_t and We_n). In the moving direction, the spreading is stretched by the surface movement, increasing the maximum spreading diameter. Ellipse and tail patterns are observed in the regions of We_t < 0.72We_n and We_t > 0.72We_n. For both patterns, the ratios of the maximum spreading factor in the moving direction to that perpendicular to the moving direction could be expressed as functions of $W{e}_{\text{t}}^{0.5}W{e}_{\text{n}}^{-0.5}$. The spreading time perpendicular to the moving direction is reduced by the surface movement and this reduction is normalized by a modified correlation. The liquid film centroid travels slower than the moving surface in the early stage and travels as fast as the moving surface in the final stage. The relative displacement of the liquid film centroid can be scaled as $W{e}_{\text{t}}^{0.5}W{e}_{\text{n}}^{-0.5}$. This study deepens our understanding of the droplet impact behaviors on moving surfaces and the findings help analyze the dynamics on more compilated moving surfaces.