Durable and highly absorptive ant-nest structured superhydrophobic sponge for efficient de-icing and interfacial evaporation in polar environments

Abstract

The Arctic plays a crucial role in the Earth's climate system. However, the unique geography and climate of the Polar Regions present significant challenges for anti-icing/de-icing and clean water production in the Polar Regions, and there is an urgent need for innovative materials to help personnel and instrumentation address these issues. In this work, a composite structure with both micro- and nano-rough surfaces, excellent vapour escape channels and superhydrophobic properties is developed with the design concept of an anthill delicate cross-scale multi-stacked void structure. The light absorption reaches 98% across wavelengths from 200 to 2500 nm. It also has a hydrophobicity angle of 154.5°. It de-ices within 540 s at low solar intensities and delays icing up to 5400 s at −20 °C. A vapor escape channel enables efficient interfacial evaporation, achieving a rate of 2.76 kg m⁻² h in Arctic seawater. Notably, the study achieved the integrated exploration of interfacial evaporation and de-icing, converting 0.5 cm of Arctic ice into fresh water in 7200 s. Additionally, PMOS (PDA@MWCNTs@MnO₂@CuO@MS) shows high durability, retaining superhydrophobicity after 200 tape strips, friction tests, and 50 icing–deicing cycles—offering a reliable solution for polar de-icing and interfacial evaporation.

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New concepts

Our study presents a novel concept for the design of highly efficient anti-icing and water production materials by integrating superhydrophobic surfaces with multiscale porous structures. This composite material significantly enhances light absorption and vapor escape, leading to unprecedented de-icing performance and interfacial evaporation rates in Arctic environments. This concept paves the way for new solutions in polar de-icing and clean water production, addressing the critical challenges posed by extreme climates.

1. Introduction

The Arctic is a unique and extremely important region of the Earth, and its geo-environmental influence, geo-advantages, resource reserves, scientific research value, military potential and other characteristics make the Arctic an extremely important strategic position.^1–3 Various countries have accelerated the pace of scientific research on the Arctic and the exploitation of Arctic resources. However, ice on the surface of infrastructure such as instrumentation, transmission lines, and infrastructure platforms often causes significant economic damage and safety hazards.^4–8 The rapid formation and accumulation of ice, especially at low winter temperatures, poses a serious threat to de-icing and freshwater production, for which the development of long-term and effective anti-icing and freshwater production technologies is key to advancing the polar development strategy.^2,9 Although early and widely used active de-icing methods such as hot air, electrical heating and mechanical de-icing provided solutions to the icing problem,^10–13their high energy consumption, cost and environmental impact are not feasible in the polar environment.¹⁴ In recent years, with the advancement of nanotechnology, the passive de-icing technology of surface design has received extensive attention from the scientific community, and a variety of efficient de-icing structures have been gradually developed, such as superhydrophobic surfaces,^15–17 smooth liquid infused porous surfaces (SLIPS),¹⁸ low interfacial toughness surfaces,¹⁹etc. Compared with active de-icing, passive de-icing has the advantages of energy saving, environmental protection, low maintenance cost and strong continuity, and has shown great potential for application in a variety of infrastructure applications.²⁰

In recent years, the design of superhydrophobic surfaces in passive deicing technology has received much attention. Superhydrophobic surfaces can trap air to form an air cushion and reduce the solid–liquid contact area, thus delaying icing.^21,22 Superhydrophobic surfaces inhibit ice nucleation and significantly reduce ice adhesion (down to 5–20% of that on a normal surface), allowing ice to be easily removed by slight mechanical action or airflow.²³ However, in high humidity and low temperature environments, droplets may puncture air pockets and penetrate into the micro- and nanostructures, transforming the surface from a Cassie–Baxter state into a Wenzel state, leading to firm embedding of ice droplets into the structure and creating an interlocking effect,^24–26 which results in a firm attachment of the ice layer and reduces the anti-icing efficiency of the material.²⁷ In addition, although SLIPS can reduce ice adhesion, it has poor abrasion resistance and has limited effectiveness in low-temperature environments.^28–30 Recently, researchers have successfully achieved photothermal deicing of superhydrophobic surfaces and SLIPS under sunlight irradiation.^31,32 Superhydrophobic surfaces and the photothermal effect work synergistically to form a closed-loop mechanism from icing inhibition to active ice melting. Photothermal superhydrophobic surfaces can remain ice-free at low temperatures with a single exposure to sunlight, effectively overcoming the de-icing drawbacks of conventional superhydrophobic surfaces. And it has been demonstrated that the microstructures of photothermal superhydrophobic surfaces and photothermal materials play a significant role in improving the superhydrophobicity of the surfaces as well as enhancing the efficiency of the photothermal conversion.^33–35 However, the low radiation and cold temperatures at the poles pose a challenge to conventional photothermal materials and superhydrophobic surface structures,³⁶ and there is a lack of research on efficient materials for the integration of polar ice protection and desalination. The anthill is a widely distributed porous composite structure that provides excellent ventilation, temperature regulation and humidity control through a complex design of channels and chambers.^37,38 The uniform roughness of the surface and the hydrophobic chemical groups effectively reduce the surface energy and enhance the hydrophobicity.

Based on this, in this work, anthill-like nanostructures were successfully prepared by attaching polydopamine and multi-walled carbon nanotubes to the surface of melamine sponge through self-polymerization and thin film deposition techniques. This preparation method generated PMOS with both micro- and nanostructures and high photothermal conversion efficiency. The results show that, thanks to the broad-spectrum absorption properties of the photothermal material and the light-trapping effect of the micro- and nano-porous structure, the PMOS absorbs up to 98% in the full wavelength band, and under single sunlight irradiation, the surface temperature of the PMOS can be rapidly increased to 95 °C within 100 s. In addition, the superhydrophobic surface of the PMOS forms an air cushion layer that aids in active de-icing, while the photothermal conversion causes the ice to rapidly melt and detach from the surface. A 0.5 cm thick piece of Arctic sea ice can also be quickly removed in 540 s under only 0.5 solar irradiation. In addition, PMOS demonstrates excellent interfacial evaporation performance, with Arctic seawater evaporating at an interfacial rate of up to 2.76 kg m⁻² h under one solar irradiation, and up to 2.35 kg m⁻² h even under 0.5 solar irradiation. More importantly, we have successfully explored the integration of evaporation and de-icing at the PMOS interface, which is capable of converting all sea ice with a thickness of 0.5 cm into fresh water within 7200 s. In addition, PMOS exhibits excellent mechanical durability, acid and alkali resistance and self-floating properties. With these features, we believe PMOS has great potential for polar anti-icing/de-icing and desalination.

2. Results and discussion

2.1. Surface characteristics and wettability

As a unique bio-architecture in nature with highly efficient ventilation, thermal insulation and waterproofing properties, anthills are usually covered with rough structures at the micro- and nanometer scale and often contain hydrophobic substances, effectively preventing water infiltration and maintaining a dry environment inside. To analyze their properties, we observed the anthill surface structure. The roughness dimensions of the anthill surfaces are typically tens to hundreds of micrometers (Fig. 1a). The presence of multilevel micro-nanostructures and low surface energy waxes on the surface of the anthill gives it superhydrophobicity and low adhesion properties.^39,40 Based on this, in this study we designed and prepared a photo-thermal superhydrophobic material (PMOS) mimicking an anthill structure, which is shown in Fig. 1a to confer excellent light trapping and superhydrophobic properties to PMOS. As shown in Fig. 1b, the bionic anthill structure was constructed by self-polymerization of polydopamine (PDA) onto the surface of a melamine sponge (MS), followed by fabrication of micro-spine arrays using a thin-film deposition technique (the film thickness is 222 μm). In addition, in order to better explore the properties of PMOS, we analyzed its formation mechanism, in which PDA can undergo a self-polymerization reaction under alkaline conditions to form stable covalent bonds. This process generates an o-benzoquinone structure from the dopamine monomer of PDA, which forms a covalent bond with the amino group on melamine, further enhancing its adhesion and chemical stability. Moreover, the PDA modification layer provides abundant hydroxyl (–OH) groups, which can react with the silanol groups formed by the PFOTES in a dehydration condensation reaction to form a solid Si–O–C covalent bond, which firmly connects the PFOTES to the polydopamine modification layer, thus enhancing its hydrophobicity and reduced surface energy. In order to better verify these mechanisms, we used Fourier transform infrared spectroscopy (FTIR) mapping to determine the formation of chemical bonds by analyzing the positions of the characteristic absorption peaks. The results show that the peaks at 1256.2 cm⁻¹ and 1332.8 cm⁻¹ indicate that C–N bonds have been generated. Meanwhile, the peak at 1153.4 cm⁻¹ represents the formation of Si–O–C covalent bonds. Thus, the spectrogram shows the presence of C–N and Si–O–C covalent bonds, supporting the successful bonding of polydopamine with the melamine sponge, PFOTES (Fig. 1e).

Fig. 1 Design and characteristics of a robust microridge structure inspired by ant nests. (a) Schematic illustration of the PMOS (PDA@MWCNTs@MnO₂@CuO@MS) with interfacial evaporation and anti-icing/de-icing properties. (b) Fabrication process of PMOS. PMOS is prepared via self-polymerization and thin-film deposition techniques. (c) SEM image of the PMOS with an ant-nest-inspired structure. Multi-walled carbon nanotubes (MWCNTs) and polydopamine (PDA) self-polymerize onto a melamine sponge, with MnO₂ and CuO nanoparticles chelated onto the sponge framework. (d) EDS energy spectrum of PMOS. EDS spectrum of the photothermal superhydrophobic sponge, showing the presence of C, O, Si, Cu, and Mn elements. (e) Comparison of Fourier transform infrared (FTIR) spectra of MS and PMOS. The change in PMOS relative to MS indicates a significant change in the surface chemistry. (f) EDS elemental analysis of PMOS. Elemental mapping analysis by EDS, directly displaying the content of C, O, Si, Cu, and Mn elements. (g) Contact angle measurement of MS (melamine sponge). A 5 μL droplet was quickly absorbed on the surface of MS, showing a contact angle of zero. (h) Contact angle measurement of PMS(PDA@MS). The contact angle of a droplet on the surface of PMS was 137.5°. (i) Contact angle measurement of PMOS. The contact angle of a droplet on the surface of PMOS was 154.5°.

In addition, we further investigated the surface morphology changes during PMOS fabrication. Fig. 1c shows that the initial melamine sponge has a smooth surface with clear texture and pores, and has a three-dimensional crosslinked porous network structure, and the low surface free energy nanomaterials can be uniformly attached to its skeleton without destroying the structure. Moreover, it behaves in a superhydrophilic state (Fig. 1g). Due to the inherent adhesion, PDA acts as a connecting layer between the CuO, MnO₂ and MWCNTs particles, and the particles are firmly adhered to the sponge skeleton. As a result, the PMOS was designed and generated with a stable rough structure (Fig. 1c). After contact angle meter testing, the PMOS exhibited superhydrophobic properties with a contact angle of 154.5° (Fig. 1i). We also tested the roll angle of the PMOS, which was 5.2°, demonstrating excellent low adhesion properties. In addition, PMOS exhibits low adhesion, and droplets with different pH values and different types of droplets exhibit superhydrophobicity on the PMOS surface (Fig. S1, ESI†). The surface properties of PMOS with a hydrophobic top surface and hydrophilic bottom surface have significant advantages in interfacial evaporation. The hydrophilic surface effectively adsorbs water molecules and directs water to the interface, while reducing the diffusion resistance of water vapour and ensuring a constant supply of water to the evaporation interface. The hydrophobic surface, on the other hand, prevents the build-up of liquid water on the surface, reduces water coverage at the interface and avoids blocking heat transfer, thus maintaining an efficient evaporation rate. In addition, the elemental composition of PMOS was carried out by EDS, and the SEM-EDS mapping images showed that the five elements of C, O, Si, Mn and Cu were uniformly distributed on the surface, with the highest amount of C and the least amount of Cu (Fig. 1d and f). This also shows the successful branching of Cu and Mn metal elements.

2.2. Photothermal properties of superhydrophobic sponges and delayed icing properties

Good broadband absorption is an important process in the photothermal effect and is further used in anti-icing and de-icing.⁴¹ PMOS demonstrated excellent photothermal conversion capabilities. Fig. 2a shows the absorption spectra of MS and PMOS measured in the full UV-visible-NIR range (200–2500 nm). PMOS has an absorbance of up to 98% (Fig. 2a), showing very high photothermal conversion efficiency. In contrast, untreated melamine sponges had high absorption only in the UV band and less than 50% in the remaining bands.

Fig. 2 Photothermal conversion properties of PMOS and delayed icing properties of the surface. (a) Absorption rate test of the melamine sponge (MS) and photothermal superhydrophobic sponge (PMOS). PMOS has significantly higher light absorption than MS in the full wavelength range. (b) Photothermal conversion properties of MS, PMS, and PMOS under one sun. From the figure, it can be seen that the equilibrium temperature of PMOS under one solar intensity can reach 95 °C, which is much higher than the equilibrium temperature of MS and PMS. (c) Photothermal conversion properties of the photothermal superhydrophobic sponge under three different light intensities. PMOS is able to reach 140 °C at two solar intensities and 112 °C at 1.5 solar intensities. (d) Photothermal stability test of the photothermal superhydrophobic sponge under one sun. PMOS has a stable equilibrium temperature without significant fluctuations after 15 cycles of photothermal stability testing. (e) Comparison of delayed icing times with available research results. We compared the icing delay times of the materials that have been studied, and PMOS has a clear advantage. (f) Mechanism of heat transfer by condensation of liquid droplets. Mechanism of heat transfer by droplet condensation on PMOS surfaces. (g) Delayed icing test. Delayed freezing test of droplets on the surface of an aluminum plate. (h) Delayed icing test. Delayed freezing test of droplets on the PMOS surface.

To evaluate the photothermal conversion properties of PMOS, we compared the photothermal conversion capabilities of three different materials (MS, PMS, and PMOS) at the same light intensity (Fig. 2b). The results showed that the surface temperature of PMOS increased rapidly under single light irradiation, rapidly warming to 90 °C within 150 s and reaching an equilibrium temperature of 95 °C within 280 s. The surface temperature of PMOS was higher than that of PMS (70 °C) and MS (50 °C). In addition, the photothermal performance of PMOS was investigated by direct irradiation under different light intensities (1 sun (1000 W m⁻²), 1.5 sun, and 2 sun), as shown in Fig. 2c: the surface temperature of PMOS increased rapidly within 90 s and reached an equilibrium temperature of 112 °C within 300 s at 1.5 sun, and the temperature of the upper surface was reduced to the room temperature within 10 min after switching off the simulated sunlight. Meanwhile, its equilibrium temperature is close to 140 °C at 2 solar intensities. To further test its photothermal stability, we conducted 15 cycles of switching on/off light experiments under the same light intensity (Fig. 2d), which showed that its equilibrium temperature was stable without obvious fluctuations, demonstrating excellent photothermal stability performance.

Due to the spherical shape of the droplet on the superhydrophobic surface, this reduces the contact area between the droplet and the solid, significantly delaying the freezing time of the droplet.⁴² In order to investigate the delayed icing characteristics of PMOS at low temperatures and to verify its potential in passive anti-icing, we recorded the delayed icing time of liquid droplets on the surface of PMOS and aluminium sheets in a freezing chamber at −20 °C and 35% relative humidity. Fig. 2g and h show that the delayed icing time of the droplets on the PMOS surface reaches 5400 s, which is approximately 216 times longer than that of the aluminium plate. Due to the positively small solid–liquid contact surface on the PMOS surface, the nucleation sites are increased, thus delaying the nucleation and freezing of water droplets. In addition, in order to probe more deeply into the reasons for the ultra-long delayed freezing time of the droplets on the PMOS surface, we also analysed the condensation heat transfer mechanism of the droplets (Fig. 2f). At room temperature, the droplets show a typical Cassie–Baxter state, and the air cushion under the micro- and nano-rough structure slows down the heat transfer. As the temperature decreases, the air cushion layer gradually slows down the heat transfer on the PMOS surface, prolonging the icing process of the droplets. When the temperature drops further, some droplets infiltrate into the tiny pits, air gradually escapes, and the droplets freeze rapidly below the freezing point to form an ice-water mixing state. When the temperature rises, air bubbles accumulate at the top of the droplet until it freezes completely. This process effectively inhibits ice nucleation, slows down ice crystal growth, and significantly extends the freezing time. We also analysed in depth the difference between the delayed icing of droplets on the surface of Al and the surface of PMOS under the same low temperature environment by building a heat transfer model (Fig. S7, ESI†). In addition, to further highlight the delayed icing performance of PMOS, we compared the delayed icing times of the other materials studied (Fig. 2e),^43–50 and PMOS showed extremely excellent delayed icing.

2.3. Interfacial evaporation, deicing integration characteristics of PMOS and deicing characteristics of PMOS

In order to investigate the photothermal de-icing characteristics of PMOS in the low temperature, low irradiation environment of the Arctic, we took out a 2 cm × 2 cm × 0.5 cm ground surface sea ice sample collected from the Arctic (see the ESI† for detailed parameters). Arctic sea ice was placed on an equal-area PMOS surface tilted at 30°, as shown in Fig. 3b, and the temperature of the cryogenic chamber was controlled to be −20 °C with a relative humidity of 50%, and then its de-icing characteristics were measured under low irradiation conditions. As shown in Fig. 3d, at 0.5 low irradiated solar intensity, the Arctic sea ice can be rapidly removed in 540 s. Moreover, the droplets formed by the melting sea ice can be quickly slid off from the PMOS, avoiding secondary icing. In addition, with the gradual increase of irradiation intensity, the de-icing time is also gradually shortened. At 0.8 solar intensity, the removal time of Arctic sea ice is 420 s, while at 1 solar intensity the removal time of Arctic sea ice can reach an amazing 300 s, which shows good photothermal de-icing characteristics.

Fig. 3 De-icing tests of PMOS and integrated evaporation and de-icing tests at its interface. (a) Integration experiment. Diagram of the integrated interface evaporation and de-icing apparatus. (b) Schematic diagram of the photothermal deicing experimental setup. The temperature of the cryogenic chamber within the device is −20 °C and the relative humidity is 50%. (c) Interfacial evaporation testing of PMOS in integrated experiments. The mass of seawater evaporated at the interface during the integration process as a function of time. (d) Photothermal de-icing test. Comparison of the photothermal deicing performance of the photothermal superhydrophobic sponge under 0.5 sun, 0.8 sun, and 1 sun. (e) Arctic sea ice melting processes. Melting processes of Arctic sea ice under single-light conditions during integration.

In addition, in order to investigate how photothermal materials can simultaneously achieve efficient interfacial evaporation and de-icing functions in extreme environments, we designed an integrated experimental setup for photothermal interfacial evaporation and de-icing. The device is used for simultaneous interfacial evaporation and de-icing processes as shown in Fig. 3a. The system uses solar energy to drive the melting removal of Arctic sea ice, while the interfacial evaporator is used to facilitate the collection of fresh water. The device is designed to manage both processes efficiently, with solar energy being focused on the surface to both promote vapour generation and accelerate ice melting. In addition, we selected an ice layer with a radius of 1.6 cm and a thickness of 0.5 cm and placed it on the PMOS surface, and the ice layer melted within 240 s under a single light condition (Fig. 3e), and the resulting seawater evaporated completely within 2 h (Fig. 3c). The interfacial evaporation rate was 2.817 kg m⁻² h⁻¹. The experiment integrated interfacial evaporation and de-icing processes into the same system, successfully utilising solar energy to achieve dual functionality. The results of the experiments demonstrate a viable approach to effectively dealing with cold temperatures and freshwater scarcity in the Arctic or similar environments.

2.4. Interfacial evaporation tests of Arctic seawater

With ample vapour escape channels and excellent light-to-heat conversion characteristics, PMOS is an excellent choice for solar interface evaporators. In addition, PMOS also has excellent self-floating properties that can be useful for floating on the ocean surface (Fig. S2, ESI†). Fig. 4a and b show test photographs of the experimental setup. The photothermal layer captures more sunlight by lengthening the light path and reducing reflections, increasing the efficiency of the photothermal conversion and thus effectively promoting vapour generation.⁵¹ Meanwhile, the hydrophilic layer of the melamine sponge (MS) ensures an adequate water supply and accelerates salt dissolution. The thermal insulation layer locks the heat from the surface of the photothermal layer and helps the evaporator to achieve self-evaporation on the water surface. The above combined properties enable PMOS to exhibit a highly efficient interfacial evaporation rate and excellent desalination capability.

Fig. 4 Interfacial evaporation tests of Arctic seawater. (a) Photograph of the interfacial evaporation device. It illustrates the actual test environment of the interfacial evaporation unit. (b) Schematic diagram of the interfacial evaporation device. This PMOS interfacial evaporator is mainly composed of two parts: the absorption layer and the thermal insulation layer. (c) Infrared spectrum of the surface temperature of the interfacial evaporation material under one sun. After exposure to a single solar intensity the PMOS surface temperature increases rapidly and reaches the equilibrium temperature within 60 s. Turning off the sunlight simulator, the PMOS surface temperature returned to room temperature within 300 s. (d) Temperature changes of the absorption layer and hydrophobic layer of the interfacial evaporation material under different light intensities. After analysing the bottom surface temperature of the PMOS interface evaporator with the change of light intensity is almost not much different, while the temperature of the upper surface with the increase in light intensity of its surface temperature is also gradually increased. (e) Evaporated mass versus time for different salt concentrations. With increasing salt concentration, the quality of freshwater evaporated at the interface of the PMOS gradually decreases over the same period of time. (f) Changes in PMOS evaporation rates at different salt concentrations. With increasing salt concentration, the interfacial evaporation rate of PMOS decreases over the same period of time. (g) The actual evaporation effect of the PMOS interface evaporator. Arctic seawater mass versus time during evaporation using an interfacial evaporator, compared to dark evaporation as well as direct evaporation of pure water, at a single sunlight intensity. (h) Evaporating mass versus time of Arctic seawater at different low irradiation light intensities. With the gradual increase in light intensity, the mass of seawater evaporated by PMOS gradually increased over the same period of time. (i) Evaporation rates of PMOS at different low irradiance solar intensities. With the gradual increase of light intensity, the rate of PMOS evaporation gradually increased at the same time. (j) Evaporated mass of Arctic seawater as a function of time in a ten-hour evaporation test. The mass of evaporating Arctic seawater gradually increases during the first three hours and stabilises after three hours. (k) Variation in evaporation rate of Arctic seawater during a 10-hour evaporation test. The rate of evaporation of Arctic seawater increases gradually during the first three hours and stabilises after three hours. (l) Changes in the PMOS evaporator surface over 10 hours. During ten hours of evaporation, no salt crystals precipitated from the PMOS surface, showing good salt resistance.

To further investigate the photothermal conversion properties of PMOS under interfacial evaporation conditions, we placed it in Arctic seawater and tested it under 1, 1.5 and 2 solar intensity conditions. Fig. 4d shows that at 1 solar intensity, the surface temperature of the photothermal layer stabilises at 60 °C, while the hydrophilic layer is maintained at 26 °C; at 1.5 light intensities, the temperature of the photothermal layer reaches 75 °C; while at 2 light intensities, the temperature reaches a maximum of 85 °C. Despite the increase in light intensity, the temperature of the hydrophilic layer remained almost constant. In addition, an infrared (IR) thermographic camera was applied to monitor the temperature distribution of the evaporator under a device that simulated solar radiation. When the xenon lamp was switched on, the temperature of the photothermal layer increased rapidly and reached an equilibrium of 60 °C within 60 s; when the lamp was switched off, the temperature decreased rapidly and returned to the initial state within 5 minutes. The infrared image (Fig. 4c) shows that the heating and cooling processes are mainly concentrated on the upper surface of the sponge. Such a significant localised heating effect not only provides a higher surface temperature, but also significantly improves the interfacial evaporation efficiency. Furthermore, in order to quantitatively assess the practical effectiveness of the evaporator, we compared its performance under dark evaporation and pure water evaporation conditions, and the results show that the evaporation rate of the contained evaporator is significantly higher at 2.76 kg m⁻² h⁻¹ compared to water alone. In addition, the interfacial evaporation rate of PMOS in seawater with different salt concentrations was also explored. PMOS was placed in solutions with different NaCl concentrations, as shown in Fig. 4e and f, and the evaporation rate of PMOS reached 2.43 kg m⁻² h⁻¹ in a 3.5 wt% NaCl solution irradiated by one solar intensity for one hour. The evaporation rate gradually decreased with increasing NaCl concentration at 1 solar. However, even in a 30 wt% NaCl solution, the PMOS evaporator achieved a high evaporation rate of 2.04 kg m⁻² h⁻¹ without salt crystallisation on its surface. This is mainly due to the porous structure of PMOS, which allows water to migrate quickly from the inside of the sponge to the surface and evaporate rapidly through the photothermal effect. The efficient mass transfer process significantly increases the evaporation rate and ensures excellent evaporation performance at high salt concentrations.

In addition, in response to the low irradiation intensity environment in the Arctic, the experiments also explored the interfacial evaporation rate of PMOS at 0.5 and 0.8 solar irradiation intensities. As shown in Fig. 4(h) and (i), the results show that the evaporation rate of PMOS reaches 2.35 kg m⁻² h⁻¹ at 0.5 solar intensity and increases to 2.47 kg m⁻² h⁻¹ at 0.8 solar intensity, which indicates that PMOS possesses a good interfacial evaporation performance even under low-temperature and low-irradiance conditions. In addition, to address the need for fresh water in the extreme Arctic environment, we used Arctic seawater from melting Arctic sea ice, placed a photothermal interface evaporation material in it, and tested the change in evaporation rate of the material over a 10 h period at 1 solar intensity. As can be seen from Fig. 4(j) and (k), the evaporation rate gradually increases in the first three hours, mainly because the temperature on the surface of the material gradually increases and reaches stability. In the initial stage, the material needs some time to absorb light energy and convert it into heat energy, so that the surface temperature gradually increases, thus accelerating the evaporation process. In addition, as the material surface reaches thermal equilibrium, the moisture migration channel is gradually opened, and the continuous replenishment of surface moisture further promotes the increase of evaporation rate. Eventually, the evaporation rate stabilises when the light-heat conversion and water supply reach equilibrium. In addition, it was observed that no salt crystals appeared on the PMOS surface. This is mainly due to the low adhesion property of the PMOS surface, which prevents the attachment and deposition of salt crystals, thus effectively avoiding the formation of salt crystals. This property enables PMOS to maintain stable superhydrophobicity in high salt solutions and prevents the effect of salt crystallisation on the material.

2.5. Stability test

The application of anti-icing surfaces is critical in polar environments, especially in the Arctic, where excellent durability and versatility of materials are required to cope with the harsh natural conditions. For this purpose, we tested the mechanical strength and chemical stability of PMOS. The good adhesion of the PMOS surface to the substrate was verified by tape and sandpaper rubbing tests. Fig. 5a shows the specific experimental design of the tape test: we attached the tape to the material surface, placed a 500 g circular weight on it, rolled it over at 20 mm s⁻¹, and repeated the process 200 times. The results show that the PMOS surface remains hydrophobic even after 200 peeling cycles.

Fig. 5 PMOS mechanical durability and corrosion resistance testing. (a) 200 cycles of tape peel test of PMOS. The contact angle of the PMOS surface can still reach close to 150° after 200 tests of tape peeling experiments. (b) 200 cycles of friction test of PMOS. After 200 cycles, the contact angle of the PMOS surface can still reach nearly 150°. (c) PMOS 12d acid and alkali resistance test. After immersing PMOS in strong acid and alkali solutions for 12 days, the contact angle of its surface can still reach 150°. (d) 50 cycles of icing-de-icing cycle test of PMOS. After 50 icing/de-icing cycles, the contact angle on the PMOS surface can still reach 150°. (e) Absorbance test of PMOS after 12 d acid–base corrosion. After 12 d of acid and alkali corrosion, the absorbance of PMOS changed little over the full band. (f) SEM images of PMOS after 50, 100, and 200 friction cycle tests respectively. After observing after friction, the micro-spine structure on its surface was worn to some extent, but the main micro-nano rough structure was not damaged. (g) SEM images of PMOS after 12 d of corrosion in strong acid and alkali solutions with pH = 1 and pH = 13, respectively. After acid and alkali corrosion, part of the fibre skeleton of the sponge breaks, but the microstructure of its surface is still preserved.

In the second mechanical strength test, we placed a 500 g weight on top of 1000 grit sandpaper and rubbed it over the surface of the material at a rate of 30 mm s⁻¹ for 6 cm. This experiment is based on 50 frictions as a cycle, and five points in each cycle are selected for hydrophobic angle measurement, and the average value is taken as the result. As shown in Fig. 5b, the PMOS still exhibits excellent hydrophobicity after 200 friction cycles. In addition, comparative analysis of the surface morphology of PMOS after abrasion (Fig. 5f) shows that the surface micro-ridge structure remains largely intact after 200 rubbing cycles. The micro and nano raised structure on the PMOS surface effectively disperses the stress generated by mechanical friction and prevents local damage from spreading over a large area. Each micron or nano-structured unit bears the external force independently during the friction process, and the stress dispersion mechanism significantly reduces the overall wear level of the material and enhances the wear resistance. In addition to this, stress tests on PMOS have once again demonstrated its excellent mechanical durability (Fig. S4, ESI†).

In addition, PMOS remained superhydrophobic after 12 d of immersion in acidic and alkaline solutions (Fig. 5c), and its full-band optical absorption remained almost unchanged (Fig. 5e), demonstrating excellent chemical stability. This stability is mainly due to the protective effect of the surface air film. Further analysis of the surface morphology of the corroded PMOS (Fig. 5g) shows that the overall morphology of the three-scale structure (consisting of irregular micro-ridges, non-uniform clusters and nanosheets) remains essentially unchanged despite the fracture of some of the fibres,⁴⁹ ensuring the durability of the material.

To evaluate the de-icing effectiveness of PMOS and its ability to be reused, 50 icing-de-icing cycle tests were conducted. The results of this test are illustrated in Fig. 5d, where the data shows that after 50 cycles, the contact angle of the PMOS surface decreases slightly but remains around 150°, which ensures that the material has excellent photo-thermal de-icing performance during the day. In addition, we also investigated the stability of PMOS in low-temperature and high-humidity environments. The contact angle of the PMOS surface was maintained above 150° during a month-long contact angle test, demonstrating its excellent stability (Fig. S8, ESI†). As you can see, PMOS not only offers excellent mechanical and chemical stability, but also long-term anti-icing/de-icing capabilities. These properties indicate that durable and versatile photothermal superhydrophobic sponges show great potential for applications in polar anti-icing/de-icing and interfacial evaporation.

3. Conclusion

Inspired by the structure of an anthill, we have prepared a biomimetic photothermal anti-icing/de-icing material, PMOS, by self-polymerisation and thin film deposition. Thanks to the synergistic effect of its micro- and nanoscale rough surface structure and low surface energy, PMOS exhibits superhydrophobic properties. In addition, the design effectively reduces the heat transfer efficiency of the droplets and significantly improves the free energy barrier required for ice nucleation. Experimental results show that PMOS has a delayed icing time of 5400 s for a 20 μL droplet in a −20 °C environment. In terms of photothermal performance, the equilibrium temperature of PMOS can reach 95 °C at one solar intensity. This highly efficient performance stems primarily from the multilevel micro- and nanostructures on its surface, which provide more than 98% absorption of sunlight, effectively capturing and converting light energy. In addition, PMOS demonstrated fast de-icing on a 0.5 cm thick sample of Arctic sea ice, with a de-icing time of only 300 s at one solar intensity, and 540 s even at 0.5 solar illuminations. PMOS also excels in interfacial evaporation, evaporating Arctic seawater at a rate of 2.76 kg m⁻² h⁻¹ at one sunlight intensity. In addition, PMOS has excellent acid and alkali resistance and mechanical stability to better adapt to the harsh polar environment. More importantly, we have successfully explored the integrated function of interfacial evaporation and de-icing of this composite structure, which provides a feasible solution in the field of polar photothermal anti-icing/de-icing and seawater desalination.

4. Experimental

4.1. Materials

Melamine sponges were purchased from JD.com; polydimethylsiloxane (PDMS), specifically Sylgard 184 base and curing agent, was obtained from Dow Corning; multi-walled carbon nanotubes (MWCNTs) with a diameter of 3–15 nm, a length of 15–30 μm, and a purity of >98% were supplied by Shenzhen Suiheng Technology Co., Ltd; manganese dioxide (MnO₂, 100 nm, 99.9%), copper oxide (CuO, 50 nm, 99.9% metal-based powder), 1H,1H,2H,2H-perfluorooctyltriethoxysilane (98%), and anhydrous ethanol (≥99.8%) were purchased from Adamas-beta (Shanghai) Chemical Reagent Co., Ltd.

4.2. Sample fabrication

The preparation process of micro- and nanostructured PDA@MWCNTs@MnO₂@CuO@MS photothermal superhydrophobic sponges (PMOS) was divided into three main steps. First, a 3 cm × 3 cm × 0.2 cm melamine sponge was immersed in 100 mL of anhydrous ethanol, removed after three repeated immersions, and placed in a vacuum oven at 80 °C for drying. Next, 0.5 g of dopamine hydrochloride was added to 50 mL of tris buffer solution with pH 8 and the sample was placed on a magnetic stirrer for 12 hours in a vacuum environment. Subsequently, the dried tris sponge was immersed in the mixed solution, removed and dried in a vacuum drying oven at 100 °C for 6 h to finally obtain PDA@MS.

The second spraying method was used to spray the molten co-mingled solution onto the surface of the PDA@MS by firstly adding 0.5 g of PDMS and 0.05 g of curing agent to 100 mL of cyclohexane solution and stirring for 1 h. Subsequently, 0.09 g of MWCNTs, 0.2 g of MnO₂, and 0.2 g of CuO were added to the above mixing solution and stirred for 6 h to form the spraying solution. The spraying solution was added into the spraying pot and the pressure of the spray gun was adjusted to 0.3 MPa, and the nozzle was at about 15 cm away from the PDA@MS for spraying. The sprayed PDA@MS was placed into the vacuum drying oven at 60 °C for 1 h and taken out to obtain PDA@MWCNTs@MnO₂@CuO@MS.

The third step was to reduce the surface energy by immersing the PDA@MWCNTs@MnO₂@CuO@MS surface into the PFOTES mixing solution. Firstly, a mixed solution of deionised water and ethanol with a volume ratio of 1 : 9 was configured, with a total volume of 100 mL; 0.5 g of PFOTES solution was added, and the solution was stirred for 3 h. Subsequently, the PDA@MWCNTs@MnO₂@CuO@MS surface was immersed into the above mixed solution. Remove it and put it into a vacuum drying oven at 80 °C for 3 h and then remove it.

4.3. Material characterization

Scanning electron microscopy (SEM, Zeiss) was used to characterise the morphology and microstructure at an accelerating voltage of 5.0 kV. Elemental content was determined using energy dispersive spectrometer (EDS) measurements. A photothermal superhydrophobic sponge surface absorption rate test was carried out with a Shimadzu UV-3600i Plus, Japan. Fourier transform infrared spectroscopy (FTIR) testing of PMOS surfaces was performed using a Nicolet Nexus 470 tester. The contact angle (CA) was analysed using a contact angle meter JC2000D (Shanghai Zhongchen Digital Technology Equipment Co., Ltd), and the CA test was carried out at five different positions to obtain the average value, with a test droplet of 5 μL. PMOS photothermal conversion characterisation experiments were carried out using a solar simulator (CEL-PE300L-3A, Beijing, China). The delayed icing characteristics of droplets on the PMOS surface as well as de-icing and defrosting performances were investigated using a customised cryogenic freezing chamber.

Author contributions

T. L., X. L., W. L., H. B., and M. L. prepared the draft. S. L. and Z. Q. reviewed and edited the manuscript. All authors have read and agreed to the published version of the manuscript.

Data availability

Data cannot be shared publicly due to phase research protection. Requests for access should be directed to the corresponding author.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This study was sponsored by the National Natural Science Foundation of China (NSFC 52106268) and the Nature Science Foundation of Hubei Province (2022CFB316).

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Footnotes

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4mh01887g

‡ Lead contact.

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