Droplet impact-solidification behavior on cold surfaces: Role of wettability, undercooling, and impact inertia

Droplet impact on cold substrates involves coupled inertial spreading, capillary recoil, and solidification, yet the combined influence of these processes remains unclear. In this work, we experimentally investigate the impact dynamics of succinonitrile droplets on substrates with varying wettability and thermal properties over a range of Weber numbers (5–100) and undercooling ΔT = 23–43 °C. The results show that the early-stage spreading is inertia-dominated and largely independent of thermal conditions, whereas the post-impact evolution is governed by the competition between recoil and solidification. High-wettability and thermally conductive surfaces promote rapid heat removal, leading to early contact-line arrest and suppressed recoil, while low-wettability and thermally insulating surfaces allow significant retraction and oscillations. A unified framework based on the ratio of the characteristic freezing and spreading timescales, Π=tfreeze/tspread, captures the transition between impact regimes. When Π<1, the droplet reaches its maximum spreading before significant solidification, resulting in hydrodynamically dominated spreading. For Π∼1, solidification and spreading occur over comparable timescales, leading to partial retraction and damped oscillations. When Π>1, rapid solidification during the advancing stage arrests the contact line and reduces the maximum spreading factor. These findings demonstrate that droplet impact with phase change is governed by a coupled hydrodynamic-thermal balance and provide a physically based framework for predicting impact outcomes on cold surfaces, supporting applications involving controlled deposition and solidification.