Role of retraction dynamics in bouncing to pinning transition during drop impact on cold superhydrophobic surfaces

A deeper understanding of the post-impact phenomenology of droplets on cold surfaces is crucial for comprehending and developing anti-icing surfaces for various applications. In the present study, a systematic experimental investigation has been done in a controlled environment with a wide range of subcooled surface temperatures (T-s similar to 1 degrees C -25 degrees C), slightly over the freezing point of water. The inertia force dominates during the spreading phase, and the time for maximal spreading is independent of the surface temperature. However, surface temperature has a major impact on the recoiling phase and governs the post-impact outcome. During the receding phase, the dynamic receding angle varies drastically and is also found to be strongly dependent on surface temperature. It is proposed that the micro-cavity condensation induced water bridge formation and viscous dissipation critically influences the receding dynamics. The retraction becomes partial retraction and finally pins at low temperatures with an enhanced retraction time, thus aiding the proposed mechanism. An empirical relationship is found for the average receding contact angle as a function of surface temperature. A scaling relation for retraction time is proposed that takes into account both the transient and surface temperature dependent variation of receding contact angle variation and the changes in thermophysical properties of the fluid. A theoretical framework has been proposed to predict the pinning to bouncing regimes for drop impact over subcooled superhydrophobic surfaces. The postulated scaling relation and prediction models are in good agreement with the experimental results.