Transition from conventional to pancake and breakup bouncing of droplets impacting on cold post-array...

The bouncing dynamics of droplets impacting solid cold surfaces play a critical role in the development of anti-icing technologies. We hypothesize that, non-conventional bouncing modes (pancake and breakup) persist and transition on cold post-array surface under specific conditions, and these transitions can be controlled by post spacing, surface temperature, impact velocity and rationalized via competing time scales.

The combined effects of impact velocities (1.43 m/s-2.39 m/s), surface temperatures (-9.8 ^∘C- -35.3 ^∘C), and surface structures on the dynamic behaviors of water droplets on cold post-array aluminum surfaces have been experimentally investigated. In particular, the dependence of different bouncing behaviors on surface textures is analyzed via high-speed imaging and laser manufacturing.

Besides conventional bouncing, two new bouncing behaviors are identified on the cold textured surfaces due to liquid penetration, namely pancake bouncing and breakup bouncing. On the surface with smaller post spacing, the impacting droplets can detach from the surface in a pancake shape within a relatively high surface temperature range. The absence of a horizontal retraction process leads to a 57%-76% reduction in contact time compared with conventional bouncing behavior. In contrast, increased viscous dissipation induced by a lower surface temperature would cause the transition from pancake bouncing to conventional bouncing. As the post spacing or impact velocity increases, the liquid film ruptures internally to produce several secondary droplets, and these droplets rebound to form breakup bouncing. The breakup bouncing mode results in a 28%-55% reduction of contact time compared with conventional bouncing. To explain the transitions among these bouncing modes, we develop a physical model that couples the impalement process with the emptying process of the penetrated liquid. This model provides a scaling relationship between several critical timescales to predict distinct bouncing behaviors. A regime map based on the dimensionless contact time and temperature, and the Weber number, is proposed to reveal the conditions under which a specific bouncing behavior dominates.