Evolution of Leidenfrost Layer Studied by Ultrafast X-ray Imaging

Abstract

When a liquid drop gently impinges on a heated surface well above the boiling point of the liquid, fast evaporation of the liquid generates a vapor layer, called Leidenfrost layer, under the drop. Acting as an insulator, the vapor layer significantly reduces the evaporation rate of the drop, known as Leidenfrost effect, which is applicable for cooling or heat transfer process involved in fuel combustion or spray cooling. One of the key aspects to understand Leidenfrost effect is the evolution of Leidenfrost layer. Direct visualization of liquid/air and air/solid interfaces in high temporal and spatial resolutions, which is absolutely required for Leidenfrost effect, has been hampered in conventional optical imaging, mostly due to severe light scattering and short depth of focus. The purpose of this research is to investigate the evolution of Leidenfrost layer during drop impact based on using ultrafast X-ray phase contrast imaging, which is capable of significantly enhanced boundary contrast. The temporal evolution of Leidenfrost layers during drop impact has been successfully visualized using ultrafast phase-contrast X-ray imaging. It is for the first time revealed that a vapor rim is formed after lifting off of a spreading edge during drop spreading. Then the vapor rim propagates undergoing capillary wave during drop recoiling.