Ultra-durable superhydrophobic cellular coatings

Abstract

Developing versatile, scalable, and durable coatings that resist the accretion of matters (liquid, vapor, and solid phases) in various operating environments is important to industrial applications, yet has proven challenging. Here, we report a cellular coating that imparts liquid-repellence, vapor-imperviousness, and solid-shedding capabilities without the need for complicated structures and fabrication processes. The key lies in designing basic cells consisting of rigid microshells and releasable nanoseeds, which together serve as a rigid shield and a bridge that chemically bonds with matrix and substrate. The durability and strong resistance to accretion of different matters of our cellular coating are evidenced by strong anti-abrasion, enhanced anti-corrosion against saltwater over 1000 h, and maintaining dry in complicated phase change conditions. The cells can be impregnated into diverse matrixes for facile mass production through scalable spraying. Our strategy provides a generic design blueprint for engineering ultra-durable coatings for a wide range of applications.

Fig. 1. Design and characterization of cellular coatings.

a Schematic illustration of cellular design. The rigid cells serve as a shield to protect the nanoseeds from mechanical abrasion and a chemical bridge to bond with the matrix and substrate. b SEM image of the cell. c A zoom-in SEM image of the coating surface. The diatomite shell, epoxy matrix, and silica nanoseeds are rendered blue, yellow, and red, respectively. d Simultaneous exhibition of high fracture strength σ_c and water contact angle θ* by cellular coatings, which is in contrast to the tradeoff facing the controls (i.e., coatings with seed alone, shell alone). The horizontal and vertical dotted lines refer to the matrix strength and superhydrophobic boundary, respectively. e, f Change of the coating fracture strength σ_c and water repellence (θ* and θ_roll-off) of cellular coatings as a function of the cell content α (e) and chemical bond density β (f). The errors represent the standard deviations from at least three independent experiments.

Fig. 2. Mechanical properties of the cell.

a Mechanical characterization of the cell (in red) and nanoseeds (in blue) by nanoindentation. The apparent breakpoint shows the fracture strength of the cell. The inset shows the SEM image of the fractured cell under 4-mN load. Scale bar: 10 μm. b Variation of the scratch depth as a function of the indenter displacement under 0.4-mN load, in which the red line refers to the cellular coating and the blue line refers to the nanoseed-alone coating. As shown in the inset images, the cellular coating was kept almost intact whereas a deep groove was created on the nanoseed-alone coating by the indenter. Scale bar: 10 μm. c Hardness and elastic modulus of different coatings. The errors represent the standard deviations from at least three independent experiments. d, e SEM images of the cellular coating (d) and zoom-in view of the released nanoseeds (e) after micro-scratching under a load of 4 mN. f, Comparison of roughness change of different coatings after abrasion. The errors represent the standard deviations from at least three independent experiments.

Fig. 3. Mechanical robustness of cellular coatings (coating thickness ~80 μm).

a, b Evolution of water contact angles (a) and roll-off angles (b) of different coatings during Taber abrasion under 1-kg load. For comparative study, cellular coatings in different matrixes (i.e., epoxy, acrylic, polyurethane, and ceramic), shell-alone coating, hybrid seeds-alone coating, nanoseed-alone coating, and Ultra-ever Dry coating were also tested. The inset image in (b) highlights the distinct worn scars on the cellular coating and the other coatings after abrasion. c Comparison of the wear resistance of the cellular coating with that of the existing reports. See Supplementary Fig. 15 for details. d, e Evolution of water contact angles (d) and roll-off angles (e) of different coatings along with the water jet impact time (jet velocity ~40 m s⁻¹, Weber number ~44,444). f Summary of the roll-off angles on the cellular coatings after diverse standard durability tests. The errors represent the standard deviations from at least three independent experiments.

Fig. 4. Multiphase repellence of cellular coatings for sustainable applications (coating thickness ~80 μm).

a, b Anti-corrosion. a SEM and optical images of the Q235 steel plates with the cellular coatings and control coating during salt spraying corrosion. Note the cross notch on the coating surfaces. b Comparison of the corrosion resistance in 3.5 wt.% NaCl solution of the cellular coatings with the conventional coatings. The insets are the optical images of the coatings after 60-day immersion. See Supplementary Fig. 25 for details. c Anti mortar adhesion. Evolution of mortar adhesion with different coatings during Taber abrasion under 1-kg load. The inset image shows the mortar self-removal from the cellular coating at a tilt angle of ~30°, which highlights the solid phase resistance of the cellular coating. The errors represent the standard deviations from at least five independent experiments. d–f Thermal management through anti-frosting. For comparison, the cellular coating after 10-min sandblasting and a commercial hydrophilic coating were tested. d Defrosting behaviors on heat-exchangers with the cellular coating and commercial hydrophilic coating. The frost sheet is rendered blue. e Energy consumption of the heat exchangers with different coatings during defrosting. Inset shows the relationship between the defrosting power and time. f Heat-transfer improvement by cellular coatings in comparison with the commercial hydrophilic coating. The errors represent the standard deviations from at least three independent experiments.