Water-Repellent Surfaces Consisting of Nanowires on Micropyramidal Structures

Abstract

Super-repellent surfaces are relevant for several practical applications, such as water collection and self-cleaning and anti-icing surfaces. However, designing surfaces that can maintain their super-repellency when the surface is subjected to a humid environment is still a challenge. Here, we present a two-tier roughness surface consisting of nanowires on micro-pyramidal structures. We compare the wetting properties of this surface with other single-level roughness surfaces and surfaces with nanowires on micro-pillars, so as to investigate the role of the two-tier roughness with micro-pyramidal structures.Surfaces are characterised by both the static contact angle and sliding angle of a water droplet on the surfaces. The characterization is performed also for the surfaces after these ones have been subjected to condensation conditions. Compared to the single-level roughness surfaces and surfaces with nanowires on pillars, the surface with nanowires on pyramidal structures shows no degradation of water repellency properties during condensation, and shows better performance in terms of low droplet adhesion than similar surfaces comprised of the more commonly used pillar structures. This is thanks to the nanowires roughness that minimizes the contact area of the droplets with the base surface and the V-shaped cavities between the pyramids that provide the droplets with an upward driving force due to Laplace pressure. Furthermore, this study shows the importance of characterising surface wetting properties not only on dry but on wet conditions as well. The combination of a nano-scale roughness with micro-pyramidal structures appears as an attractive solution for super-repellent substrates under humid and wet conditions.

Copper-Based Superhydrophobic Nanostructures for Heat Transfer in Flow Condensation

“…Smaller droplet departure diameter and higher droplet departure frequency on the SAM-coated plain surface were reported compared to the nanotextured surface by Zhong et al which was attributed to condensate and surface interfacial interactions. A comparative study between a two-tier roughness surface consisting of nanowires on micropyramids and single-level structured surfaces for condensation under the ambient condition showed that the two-tier surface with nanowires on micropyramids yielded superior droplet mobility under both wet (condensation) and dry conditions . Due to the structures on micropyramids of the two-tier surface, droplets showed lower adhesion to the pyramid side, and the transition from the Cassie to Wenzel state was avoided.…”

Superhydrophobic MoS2 Nanosheet–Cu2O Nanoparticle Antiwear Coatings

“…Besides, after humid treatment, the CA of MoS 2 –Cu 2 O coating changed from ∼164 to 152°. This is due to the extra adhesion produced by the liquid already existing below the water droplet. , On a dry surface of MoS 2 nanosheet–Cu 2 O nanoparticle coating, a water droplet may remain in the Cassie state, but during condensation conditions, vapors are nucleated in the grooves of MoS 2 nanosheet–Cu 2 O nanoparticle coating. Subsequently, when a water droplet is deposited on this surface, the water droplet combines with the nucleated vapor.…”

Coalescence-Induced Droplet Jumping

“…The shape of the microcone was reported by Sharma et al (Figure c1) and Zhang et al (Figure d), and the cone array was reported by Zhao et al (Figure a1). The columnar graded surfaces reported by Vandadi et al (Figure b1) all promoted droplet coalescence jumping by the spontaneous dewetting transition of condensate droplets. − During the growth process of droplets at the bottom of the structure, the droplets were in the Wenzel state or the partial wetting state. The Laplace pressure difference between the upper and lower surfaces of the droplet would cause the droplet to move upward in forming the Cassie state.…”

“…(d) Oblique view and cross-section view of pyramids with nanowires. Reproduced from ref , copyright 2019, American Chemical Society.…”