Retraction dynamics of a water droplet impacting onto a microgrooved hydrophobic surface at different velocities and surface temperatures

A grooved hydrophobic structure is one of the most commonly adopted surface structures inspired by nature, such as rice leaves and butterfly wings, to achieve anisotropic wetting/dewetting. The retraction dynamics of a liquid droplet after impact on a solid surface is less explored than its counterpart, the spreading dynamics, even though it is equally crucial for determining the performance in actual applications. Therefore, in this study, the effects of groove orientation, impact velocity, and surface temperature on the retraction dynamics of a water droplet were investigated through high-speed imaging. The impact velocity and surface temperature were systematically changed while keeping the size of the grooved structures the same. As the impact velocity of the droplet was increased, the enhanced spreading along the groove structures delayed the initiation of droplet retraction. This resulted in anisotropic shapes of the droplet during retraction. During the early stage of droplet retraction, the retraction rate of the droplet was found to be independent of both the groove orientation and impact velocity. The anisotropic retraction of the droplet was also observed by increasing the surface temperature. At a high surface temperature (250°C), there was significant reduction in the retraction rate (~20%) in the direction perpendicular to the grooves, resulting in the formation of an ellipsoidal droplet. It is proposed that the oscillating vapour flow underneath the droplet caused by the groove structures is responsible for the temperature-induced anisotropic retraction.