Wetting transition of sessile and condensate droplets on copper-based superhydrophobic surfaces

Abstract

Superhydrophobic state on natural materials and synthesized surfaces has been exploited in a broad range of technologies including thermal management, water harvesting, anti-icing, and flow control. However, under certain circumstances wetting transition from Cassie’s mode to Wenzel’s mode becomes inevitable. Such wetting transition degrades the performance of superhydrophobic surfaces and limits their applicability. Here, we report distinct wetting stabilities of two copper-based superhydrophobic surfaces which are with nano-asperities (diameter ∼70 nm) of different packing density. Both the static (sessile droplet) and dynamic (dropwise condensation) wetting stabilities of the two surfaces are characterized. We show both theoretically and experimentally that sessile droplets on the surfaces of densely packed nano-asperities (pitch ∼120 nm) can remain in stable Cassie’s mode, while the wetting transition from Cassie’s mode to Wenzel’s mode occurs spontaneously on the surfaces of coarsely packed nano-asperities (pitch ∼300 nm). The apparent contact angle on the surfaces of coarsely packed nano-asperities reduces from over 150° to around 110°, and the sliding angle increases from less than 5° to over 60° within 200 s, whereas the changes of both angles on the surfaces of densely packed nano-asperities are not noticeable. We also find that in dropwise condensation, condensed droplets on the surfaces of densely packed nano-asperities maintain a stable Cassie’s mode, while condensate droplets on the surfaces of coarsely packed nano-asperities are in Wenzel’s mode. Exploiting the coupling effects of surface topography and wetting behaviors can open up existing vistas on surface engineering, leading to durable and sustainable surface design for diverse applications such as dropwise condensation and boiling heat transfer.