No abstract

The liquid layer generators enable excellent dynamic anti-icing performance and show great potential at temperature of −60 °C.

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.

Hydrophobic surfaces like Lotus leaves show amazing self-cleaning properties with the apparent water contact angle above 150° and contact angle hysteresis below 10°. Thus, at low inclination angles, millimeter drops can roll-off easily. This effect can be a consequence of the air trapped below the drop, which allows the droplet to reach a superhydrophobic Cassie-Baxter state. However, the superhydrophobic state can be accompanied by very different adhesive properties due to the pinning of the droplet to the microstructures, implying that even in a hydrophobic or superhydrophobic state, the droplet might not roll-off easily. A superhydrophobic state with minimum adhesion to the surface has been the pursuit in many applications where a super-repellent state is highly desired. Many microstructures have been shown to be able to reach a superhydrophobic state, but only a few have been shown to be capable of achieving a super-repellent state without the help of more complex hierarchical structures. Here, we show that conical structures provide a template for designing super-repellent surfaces where the wetting characteristics look to be invariant in the microscale range. The conical structures can maintain a super-repellent state for all intrinsic contact angles larger than 90°, and the transition from the Cassie-Baxter to the Wenzel state is controlled by the apex angle of the conical structures. This finding advances the understanding of why conical structures can show a superhydrophobic state, which will be beneficial for the design of super-repellent surfaces with a wider intrinsic contact angle range.