Yating Hu

Superhydrophobic photothermal anti-icing coatings represent a crucial strategy for addressing outdoor icing challenges. However, they still face significant challenges regarding long-term durability and large-scale production. In this study, a facile and cost-effective one-step spraying strategy is proposed to construct a robust photothermal superhydrophobic composite coating based on a modified epoxy resin system. By introducing an epoxy oligomer grafted with perfluorodecyltrimethoxysilane and precisely regulating the synergistic ratio of 14 nm nanoscale silicon dioxide (SiO2) and carbon black, a dual-scale hierarchical superhydrophobic structure was successfully fabricated. The SiO2 nanoparticles act as physical spacers to effectively suppress carbon black agglomeration, which not only induces a light-trapping effect with an optical absorptance exceeding 96% to enhance solar energy harvesting, but also endows the coating with excellent superhydrophobicity (water contact angle of 169.2°, sliding angle of 1.7°). Owing to the overall homogeneity and structural self-similarity of the composite coating, it exhibits exceptional mechanical robustness and weatherability, maintaining its anti-icing and liquid-repellent functions even after 180 tape peeling cycles, 45 sandpaper abrasion cycles, and 288 hours of intense UV aging. Notably, the coating demonstrates superior thermal stability, retaining excellent superhydrophobic characteristics after continuous annealing in a muffle furnace at 200 °C for 7 days, thereby effectively overcoming the industrial bottleneck of performance degradation in photothermal coatings under severe summer sun exposure. In extreme anti-icing tests at -30 °C, The coating significantly reduced the adhesion strength of ice on the aluminum substrate from 80.4 kPa to 21.5 kPa; concurrently, the complete freezing time of water droplets was extended from 27 s on aluminum to 93 s. Under xenon lamp irradiation at 25 mW/cm2, its freezing delay time was prolonged from 46 s on aluminum to 214 s. Under an irradiation intensity of 50 mW/cm2, the coating relies on an efficient interfacial melting mechanism to shed a large 1.5 cm3 ice block in merely 73.3 s. This research provides a novel perspective for the fabrication of low-cost, long-lifespan coatings with active-passive synergistic anti-icing performance for harsh extreme environments.

We consider the classical problem of droplet impact and droplet spread on a smooth surface in the case of an ideal inviscid fluid. We revisit the rim–lamella model of Roisman et al. (Roisman et al . 2002 Proc. R. Soc. Lond. A 458 , 1411–1430 ( doi:10.1098/rspa.2001.0923 )). This model comprises a system of ordinary differential equations (ODEs); we present a rigorous theoretical analysis of these ODEs, and derive upper and lower bounds for the maximum spreading radius. Both bounds possess a W e 1 / 2 scaling behaviour, and by a sandwich result, the spreading radius itself also possesses this scaling. We demonstrate rigorously that the rim–lamella model is self-consistent: once a rim forms, its height will invariably exceed that of the lamella. We introduce a rational procedure to obtain initial conditions for the rim–lamella model. Our approach to solving the rim–lamella model gives predictions for the maximum droplet spread that are in close agreement with existing experimental studies and direct numerical simulations.

While the simplest outcome of a normal impact on a flat stationary solid surface is radially symmetric spreading, it is important to note that asymmetric spreading can intrinsically occur with a tangential velocity along the surface. However, no previous attempt has been made to restore the symmetry of a lamella that intrinsically spreads asymmetrically. Adjusting the lamella's asymmetric shape to a symmetric one is achieved in this work by varying wettability to affect the receding velocity of the contact line, according to the Taylor-Culick theory. Here we theoretically and practically show how restoring the symmetry can be achieved. Theoretically we built a framework to map the needed receding velocity at every given point of the contact line to allow for symmetry to be restored, and then this framework was applied to generate a wetting map that shows how at each local the wettability of the surface needs to be defined. Simulated results confirmed the effectiveness of our framework and identified the envelope of its applicability. Next, to apply the idea experimentally, the wetting map was transformed to a single wettability contrast area dubbed the "patch". Experimental results showed the effectiveness of the patch design in correcting the asymmetric spreading lamella for water droplets impacting a surface for the following Weber number conditions: We_n ≤ 300, We_t ≤ 300, and 0.51 ≤ We_n/We_t ≤ 2.04.