Phase diagram for the spreading behavior of water drops impacting hot walls observed via high-speed IR imaging

Abstract

We aimed to derive a phase diagram in the parameter space of wall temperature T_w and Weber number We for the spreading behaviors of water drops impacting a heated sapphire plate. Focusing on hydrodynamic instabilities, we performed drop impact experiments to investigate spreading behaviors with varying wall temperatures (T_w = 60 to 150 ℃) and Weber numbers (We = 29.5 to 443) using high-speed infrared (IR) imaging. A phase diagram was established accommodating three different regimes: the gentle spreading regime, rim instability regime, and contact-line instability regime. The gentle spreading regime occurs under low-We and low-T_w conditions. In this regime, the drop spreads while maintaining a round shape with no perturbations. With an increasing Weber number at any wall temperature, the spreading behavior undergoes a transition to the rim instability regime, where a finger like perturbation can be observed along the lamella rim of the spreading drop. For low-We and high-T_w conditions, instead of rim instability, we observed a transition to the contact-line instability regime, where finger like perturbations form along the contact line. Additionally, we explored the temperature dependency of the maximum spreading ratio, dimensionless perimeters, and number of fingers for the contact-line instability and rim instability regimes. The experimental results indicated that intense evaporation around the contact line has a significant impact on the growth of the observed hydrodynamic instabilities, leading to changes in spreading behaviors. To the best of our knowledge, ours is the first study to observe the contact line instability induced by thermal effects. Considering the occurrence of contact line instability only at high T_w values, we expect that intense evaporation around the contact line is a key factor in the underlying mechanism of contact line instability.