All-perfluoropolymer, nonlinear stability-assisted monolithic surface combines topology-specific superwettability with ultradurability

Abstract

Developing versatile and robust surfaces that mimic the skins of living beings to regulate air/liquid/solid matter is critical for many bioinspired applications. Despite notable achievements, such as in the case of developing robust superhydrophobic surfaces, it remains elusive to realize simultaneously topology-specific superwettability and multipronged durability owing to their inherent tradeoff and the lack of a scalable fabrication method. Here, we present a largely unexplored strategy of preparing an all-perfluoropolymer (Teflon), nonlinear stability-assisted monolithic surface for efficient regulating matters. The key to achieving topology-specific superwettability and multilevel durability is the geometric-material mechanics design coupling superwettability stability and mechanical strength. The versatility of the surface is evidenced by its manufacturing feasibility, multiple-use modes (coating, membrane, and adhesive tape), long-term air trapping in 9-m-deep water, low-fouling droplet transportation, and self-cleaning of nanodirt. We also demonstrate its multilevel durability, including strong substrate adhesion, mechanical robustness, and chemical stability, all of which are needed for real-world applications.

Keywords: 3D structure; biomimetic materials; nonlinear stability; robustness; superwettability.

Graphical abstract

Figure 1

The MPS strategy coupling wetting and mechanical stability (A) A pillar model showing the MPS design for promoting outstanding chemical resistance, substrate adhesion, and eco-friendliness. (B) Change of theoretical wetting stability and influence of theoretical strength against buckling and bending of the pillar array as a function of the pillar slenderness λ. The light blue region indicates the optimal range for achieving both wetting and mechanical stability. (C and D) Experimental validation of the pillar strength based on a pillar array (λ₀ = 8) by plotting the change of the slenderness λ and liquid-solid contact fraction f_ls of the pillar array during repeated abrasion (C), and pressing under different apparent pressures (D), respectively. Data are mean ± SD from at least five independent measurements.

Figure 2

MPS Fabrication (A) Schematic illustration of fabrication method consisting of thermal fusion and high-temperature 3D soft imprinting. The method enables outstanding 3D topological controllability, substrate adhesion, and scale-up capability. (B–D) Photographs of MPS biomimetic materials, including coatings on diverse substrates (eg, glass, textile, PI, and aluminum) (B), a small shed made of a self-supporting superhydrophobic MPS membrane (C), and a composite tape stuck on a bent surface (D).

Figure 3

Topology-specific functionalities (A–D) MPS comprising cavities with doubly reentrants mimicking the skin structure of a springtail beetle for effective air trapping. (A) Optical image of a springtail. Image courtesy of Jan J. van Duinen (photographer). (B and C) SEM image of the cavity array (B) and the doubly reentrants (C). (D) The relationship between the fraction of entrapped air in the cavities and the immersion time. The scheme illustrates two models of liquid suspending at the first and second reentrant, respectively. (E–H) MPS comprising micro protuberances mimicking the rose petal for low-fouling liquid droplet transportation. (E) Optical image of a rose flower with a high adhesion to water droplets. (F) SEM images of the MPS micro protuberances. (G) Optical images showing a droplet of serum transported between two MPS surfaces. (H) Measurement of the sample loss during transport. The MPS surface is free of observable fouling, while the counterparts, surfaces made of PP and fluorinated PS, suffer from serious fouling by bioanalytes. (I–M) Hierarchical MPS pillar array mimicking the lotus effect. (I) Image of a superhydrophobic lotus leaf. (J and K) SEM images showing the hierarchical topology of the pillar array (J) and the hierarchical unit (K). (L) Optical image showing the bouncing of a water droplet on the lotus-mimicking surface. (M) Representative images show the dirt removal tests. Only MPS surface showed self-cleaning property against nano-sized dirt (∼200-nm in diameter), as compared with control surfaces (Video S2). Data are mean ± SD from at least five independent measurements.

Figure 4

Multipronged robustness (A and B) Substrate adhesion. (A) Adhesion strength of MPS with various substrates compared with the commercial glue 3M PR1500. (B) XPS spectrum of the substrate surfaces after the perfluoropolymer layers were detached in the tests. The spectra are identical to that of MFA F1540. (C and D) Mechanical robustness. (C) Relationship between water repellence and apparent pressure applied to the surface in Figure 2E. For comparison, the ground pressure of an adult man, a military tank, and an elephant, respectively, are denoted in the insets. (D) Relationship between the water repellence of the surface in Figure 2E and abrasion cycles. (E and F) Chemical and aging robustness. (E) Water repellence of the MPS before and after treatment with strong gaseous chemicals (acid, alkali, oxidant, and organic solvents) for 1 week. (F) Evolution of the water repellence of three superhydrophobic materials during accelerated aging. In (C)–(F), the black and blue dash lines denote the boundaries of superhydrophobicity (θ∗ >150° and θ_roll-off <10°). In (E) and (F), a commercial NeverWet spray and water-repellence textile were also tested to provide reference index of the chemical and weathering stability. Data are mean ± SD from at least five independent measurements.

Figure 5

MPS combines topological controllability and multilevel durability (A) Illustration of two major groups of existing strategies (lower schematics), i.e., synthetic coatings and micromachined/3D printed surface, as well as MPS strategy (upper schematic) for preparing biomimetic surfaces. (B) A radar map showing the qualitative indexes of the characters of biomimetic coatings produced using different strategies (see references and rubrics in Tables S2 and S3). The potential applications, e.g., anti-icing and drag reduction, require multipronged merits of biomimetic coatings (light green area). MPS biomimetic surfaces (light blue line) can fulfill the multipronged requirements, addressing the limitations of synthetic coatings (gray line) and micromachined/3D printed coatings (dark yellow line).