Fast drop detachment on superhydrophobic surfaces

Abstract

Non-wetting (superhydrophobic) surface, which is widely observed in nature, for example, on plant leaves, insect wings, bird features and animal furs, has received considerable attention due to its unique properties rendered by its large contact angle and small contact angle hysteresis. Especially, the spectacular complete rebound of an impacting droplet from the superhydrophobic surface is of relevance for many practical applications including anti-icing, dropwise condensation, anti-fouling. Because the mass, momentum and energy exchange between liquid drop and solids are all related with the time that the drop is in close contact with the substrate, thus the minimization of such a contact time is highly desirable. To date, it is well accepted that drop impacting on superhydrophobic surfaces usually undergoes drop spreading and retraction stages before its complete rebound, and the contact time is a constant. The objective of this thesis is to design and fabricate novel structured surfaces that can significantly reduce the liquid-solid contact. The main results are as follows. In the first part of this thesis, we fabricated a surface with straight post arrays at sub-millimeter scale. Water drop impact experiments were then conducted on the as-fabricated surface and a new drop bouncing regime was identified: an impacting drop spreads on impact and leaves the surface directly in a pancake-like shape without retraction. The contact time is extremely reduced compared to conventional case. We demonstrate that the pancake bouncing results from the rectification of capillary energy stored in the penetrated liquid in a localized region into upward motion adequate to lift the drop. During the drop penetrating and emptying the posts, we hypothesized an capillary energy storage and release process, similar to Hooke’s law spring whose force is proportional to the displacement. Thus, tapered post arrays were fabricated with the radius of single post that increases continuously and linearly with depth in the vertical direction, which renders the acceleration of the penetrated liquid increases with penetration depth. This permits modelling of the capillary force as a harmonic spring, allowing the occurrence of pancake bouncing and rapid drop detachment over a large window of impact velocities. We also showed that the pancake bouncing is a robust phenomenon which could be observed on other surfaces. Second, based on drop impacting on the tapered post arrays fabricated, contact time reduction due to the acceleration of retraction process was investigated at relatively small impact velocity compared to that for pancake bouncing. Experimental results showed that, liquid penetrated and emptied the posts twice during the spreading and retraction process, respectively. In comparison to drop impacting on flat surface, drop impacts on post arrays could achieve a larger maximum spreading after the first emptying. This larger maximal lateral extension endows the drop a fast retraction speed later. Besides, the second liquid penetration into the posts during the retraction process helps to keep this fast retraction. The integrated effect makes the overall contact time smaller compared to that in conventional drop impacting on superhydrophobic surfaces. Third, a series of curved surfaces were developed to shorten the liquid-solid contact time during the drop impacting process. We showed that the contact time reduction was induced by the asymmetric surfaces which leads to asymmetric drop spreading and retractions in two orthometric directions. Simulation results further verified that this asymmetric dynamics is ascribed to asymmetric momentum distribution of the impacting drop. Finally, we conducted drop impact experiments to demonstrate that asymmetric surfaces induced contact time reduction is also a robust phenomenon, which could be seen on other non-holosymmetric surfaces. In summary, several surfaces with different structures were fabricated to tailor the impacting process to promote fast drop detachment. The dynamic process of drop impact on these superhydrophobic features was explored in detail using experiments and theoretical analysis. We believe that the discovery of the new impact pathways featuring short contact time through manipulating the surface structure will provide insight to deep understanding of liquid-solid interactions and stimulate new applications.