Asymmetric Spreading of a Drop upon Impact onto a Surface.

Study of the spreading of an impacting drop onto a surface has gained importance recently due to applications in printing, coating, and icing. Limited studies are conducted to understand asymmetric spreading of a drop seen upon drop impact onto a moving surface; there is no relation to describe such spreading. Here, we experimentally studied the spreading of a drop over a moving surface; such study also provides insights for systems where a drop impacts at an angle relative to a surface, i.e., drop has both normal and tangential velocities relative to the surface. We developed a model that for the first time allows prediction of time evolution for the asymmetric shape of the lamella during spreading. The developed model is demonstrated to be valid for a range of liquids and surface wettabilities as well as drop and surface velocities, making this study a comprehensive examination of the topic. We also found out how surface wettability can affect the recoil of the drop after spreading and explained the role of contact angle hysteresis and receding contact angle in delaying the recoil process.