Dual Janus foam for directional thermal management

Abstract

Directional thermal management is critical for energy harvesting and utilization. The common passive radiative cooling and heating strategies free of electricity input mainly focus on regulating the solar and thermal radiation properties of materials while neglecting the thermal conduction process. The directional thermal management for harvesting thermal energy based on the rare thermal rectifying materials is usually in the absence of radiation regulation. Here, the dual Janus foams with dual asymmetries in photothermal and thermal rectification are designed and exhibit high photothermal property contrast and excellent thermal rectification ratios (122 %). The temperature difference ratio of the managed end could be improved nearly 21 times from 3 % based on a single Janus structure to 65 % based on a dual Janus structure. This method provides a comprehensive and inspiring view for better directional thermal management in the future.

Directional thermal management is critical for energy harvesting and the common passive radiative cooling and heating strategies free of electricity input focus on regulating the solar and thermal radiation properties of materials but neglect the thermal conduction process. Here, the authors report dual Janus foams with dual asymmetries in photothermal and thermal rectification showing high photothermal property contrast and thermal rectification ratios.

Introduction

Thermal management is essential for energy conservation and efficient thermal energy harvesting and exploitation^1,2. Energy consumption in heat accounts for about 50% of global final energy consumption, where 96% of the thermal energy is used in industry and buildings³. For energy conservation in thermal regulation, the passive radiative cooling and heating strategy has been proposed to realize the non-electricity-input temperature controlling^4–7. As to thermal energy harvesting, directional thermal management is required.

The current research works on passive radiative cooling and heating are mainly focused on personal and building thermal management by rationally designing the solar and thermal radiation properties of materials^8–12. The thermal energy input through solar radiation is thereafter manipulated, blocked, or absorbed, and the thermal energy output through mid-infrared radiation is regulated to facilitate or hinder thermal dissipation to the environment. These processes are free of electricity and thus avoid energy consumption. The structures with asymmetric radiation properties are also constructed to realize dynamic and directional thermal management^13–17. The typical result of the passive radiative cooling or heating is reducing the fluctuation of objects’ temperature through the day or the year by suppressing the peak of the temperature and elevating the valley. In terms of energy flow, the input and output of thermal energy are widely investigated and manipulated in current research. However, the thermal conduction from the environment to the material and then the object (E-M-O or in the reverse direction O-M-E) is rarely discussed in passive radiative cooling and heating investigations. This is because materials for radiative regulation are typically thin, and the conduction process is negligible in these thermal management situations. As the thickness of materials increases, thermal conduction starts to play a role in thermal management^11,12. Apart from the energy conservation by passive radiative cooling or heating, the harvesting and exploration of thermal energy in a directional way is also an essential part of thermal management. The waste heat and direct temperature gradient have been utilized for heating and thermoelectric power generation^18–20. What is more common than a direct temperature gradient is the single oscillatory heat source, like the day-night temperature fluctuation or periodic temperature variation of the industrial equipment, which is currently difficult to utilize²¹. One of the possible solutions is the directional thermal management based on thermal rectification circuits analogous to electric rectification circuits^22–25. It could transform the oscillatory heat source into a single polarity temperature gradient to drive the thermal engine and generate power²⁶. The potential energy harvested through the directional thermal management will be considerable. However, the key component, thermal rectifying materials, especially those with ideal rectification performance in solid state and macroscopic, are still hard to obtain considering the limited fabrication methods^27–30.

In a comprehensive view, the objective results of passive radiative cooling and heating are also effective for thermal energy harvesting to maintain the temperature gradient by minimizing temperature fluctuation of each end and widening the discrepancy between the two ends. Here, the dual Janus foams (DFs) with dual asymmetric properties, combining the effectiveness of solar radiation regulation and thermal rectification, are prepared with high photothermal property contrast and ideal thermal rectification ratios. The DFs are fabricated with a simple double-layer emulsion template method using amphiphilic Janus particles (JPs) as efficient particulate emulsifiers. The asymmetric photothermal property and thermal rectification functions synergistically and greatly improve the directional thermal management performance. The as-prepared DFs have great potential in all-daytime and all-weather conditions directional thermal management, and will provide a comprehensive and inspiring view of thermal energy harvesting and exploration in the future.

Results and discussion

Preparation of dual Janus foam from Janus particles stabilized Pickering emulsion

The amphiphilic JPs with snowman-like morphology were prepared via the seeded emulsion polymerization method. The hydrophobic polystyrene/poly(divinyl benzene) (PS/PDVB) and hydrophilic silica components were segmented into different parts, thus forming a Janus structure (Fig. 1a). The amphiphilic nature of JPs was verified by composition analysis (Fig. 1b) and distribution characterization in different solvents, butyl acrylate and water (Fig. 1c). JPs have better dispersion ability in oil than in water, implying their stronger affinity towards oil (Fig. 1d and Supplementary Fig. 1).

Fig. 1. Preparation of dual Janus foam from Janus particles (JPs) stabilized Pickering emulsion.

a SEM image of snowman-like JPs. b TEM image of JPs and EDS mappings in terms of carbon, silicon, and oxygen elements. c The size distribution of JPs in different solvents (red line for JPs in butyl acrylate, blue line for JPs in water). d Illustrative graph of JPs dispersion state in different solvents. e Photo of the double-layer high internal phase Pickering emulsion (HIPPE) with HIPPE-8002 and HIPPE-7510G05. The notation “7510G05” stands for 75 vol% of internal phase, 10 wt% of JPs, and 0.5 wt% of reduced graphene oxide. f The optical microscope graph of HIPPE-8002. g The optical microscope graph of HIPPE-7510G05. The inset is the emulsion stained with coumarin 6 in the oil phase. h The emulsion droplet diameter of different HIPPEs. i The photo of the DF-7510G05 sample. Data are presented as mean values ± SD. n > 120. j SEM image of the cross-section microstructure of dual Janus foam-7510G05 (DF-7510G05) from double-layer HIPPE-8002/7510G05. k The SEM image of the pore wall structure of DF-7510G05. l SEM image of the oriented assembly structure of JPs on the pore wall surface. m The 3D structure of DF-7510G05 based on the micro-CT analysis. n The 2D cross-section structure and pore size analysis of DF-7510G05 based on micro-CT characterization. The pore size (o) and porosity (p) analysis of DF-7510G05. Data are presented as mean values ± SD. n > 200.

Based on their amphiphilic property, JPs were employed as the particulate emulsifier to stabilize a high internal phase Pickering emulsion (HIPPE). The amphiphilicity of JPs and their stronger affinity towards the oil phase resulted in the formation of a water-in-oil type emulsion. The stabilized HIPPE is viscous due to its high volume fraction of the internal phase (Supplementary Fig. 2). Therefore, two layers of emulsions with different structures were piled up layer-by-layer to construct the stacked structure with a clear interface (Fig. 1e). By using this method, different HIPPEs and the double-layer ones with varied microstructures and components were achieved (Fig. 1f–h). HIPPE-8002 is used in all double-layer HIPPEs. The water-in-oil type of emulsion was further confirmed by labeling the oil phase with fluorescent dye, coumarin 6 (Supplementary Fig. 3). The continuous phase surrounding the droplets was bright green, suggesting that the continuous phase consists of oil (Fig. 1g inset). By varying the internal phase volume fraction and the dosage of JPs, the size of droplets could be adjusted (Fig. 1h).

The as-prepared double-layer HIPPEs were further used as the templates to fabricate foams with an asymmetric porous structure through emulsion-templated polymerization. These asymmetric foams were regarded as a Janus structure in macroscopic terms and were denoted as Janus foams here. With the addition of black filler Reduced Graphene Oxide (rGO) into one layer of the double-layer HIPPEs, the as-prepared asymmetric foams were in white-gray or white-black appearance bearing the second asymmetry (color) apart from the asymmetric porous structure (Fig. 1i, Supplementary Fig. 4). The foams with dual asymmetric properties were then specified as dual Janus foams (DFs) compared to those with single asymmetry in pores (single Janus foams, SFs). Due to the significant discrepancy of emulsion droplet size between adjacent layers of double-layer HIPPEs, the Janus foams have two layers of porous structure with a sharp contrast between layers (Fig. 1j, Supplementary Figs. 5 and 6). The black filler rGO is distributed only in one layer of the polymer matrix (Fig. 1k). JPs formed a robust layer on the surface of pore walls and provided the composites with stable porous structures (Fig. 1l). The asymmetric porous structure was further characterized using micro-CT analysis (Fig. 1m, Supplementary Figs. 7 and 8). The adjacent two layers have a sharp boundary line, implying the outstanding stabilized effect from JPs (Fig. 1n). The porous structure exhibited significant asymmetry of pore size and porosity between the two layers (Fig. 1o, p, Supplementary Fig. 9). Besides, the addition of rGO mildly affects the pore size of foams, which was slightly reduced as 0.5 wt% of rGO was added (Supplementary Figs. 10–12). This is attributed to the additional viscosity of the continuous oil phase from rGO and the enhanced restriction on coalescence of internal water droplets, which finally reduced the pore size.

Asymmetric photothermal property of dual Janus foam

The addition of black filler rGO in one layer of the double-layer HIPPEs provided the as-prepared Janus foams with asymmetric photothermal properties. According to the dosage of rGO, the foams come out in the appearance of white-white (0 wt%), gray-white (0.5 wt%), and black-white (1.5 wt%) as illustrated in Fig. 2a. When rGO was introduced, one layer of the foam was blackened, and the photothermal conversion capability of this layer was improved, thus giving the foam asymmetric photothermal properties. The solar reflectivity of different Janus foams was measured (Fig. 2b–d). It was found that the difference in pore size and porosity in the absence of rGO (single Janus structure) could only result in a slight variation of solar reflectivity, which is observed in SF-7515 (single Janus foam from double-layer emulsions of HIPPE-8002/7515). Both sides of the SF-7515 have high reflectivity, although the small-pore side (the forward side, fd) was slightly higher than the large pore side (the reverse side, rv) (Fig. 2b).

Fig. 2. Asymmetric photothermal properties of dual Janus foam.

a The illustrative graphs of Janus foams with different photothermal properties in two directions. The solar reflectivity of different Janus foams: SF-7515 (b), DF-7510G05 (c), DF-4015G15 (d). e Illustrative graph of the asymmetric photothermal property. f The average reflectivity of different Janus foams. g The calculated theoretical average absorption power of different Janus foams under sunlight. h Infrared images of the 8002-side (SF-7515-rv) and 4015-side (DF-4015G15-fd) upon heating by sunlight (indoor experiments with a simulated sunlight source). i Temperature of different sides of Janus foams with the upper surfaces under simulated sunlight.

The introduction of rGO into one layer of Janus foams brought about the dual asymmetry in both pore structure and solar reflection property and resulted in the dual Janus foams. With more rGO in DFs, the asymmetry of reflectivity became more pronounced (Fig. 2c, d). Thanks to the excellent sunlight absorption ability of rGO, the DFs exhibited a significant discrepancy in sunlight interaction on the two sides. The black layer with rGO can absorb the incident light and then transfer the energy of light into heat, while the opposite layer will reflect the incident light and reject the input energy of light (Fig. 2e). The average solar reflectivity with the sunlight power as the weight was calculated and analyzed (Fig. 2f). The average reflectivity of the large pore side (rv) is approximately the same (85–90%). The average reflectivity of the small-pore side (fd), however, decreased sharply upon the addition of rGO, from approximately 94 to 18% and even 6%. The widened discrepancy of reflectivity of foams finally results in the enhanced asymmetry of photothermal properties. The theoretical absorption power and absorptivity of different layers of foams were calculated (Fig. 2g, Supplementary Fig. 13). With the average reflectivity of 6%, the small-pore layer (fd) of DF-4015G15 can absorb the energy of sunlight with the power of 940 W/m² (based on the standard sunlight power spectra AM 1.5G). The large pore side (rv) can only absorb the energy in the power of 160 W/m². The significant difference between the two opposite sides results in the asymmetric heating and, thereafter, directional thermal management. Under simulated sunlight, the Janus foams exhibited asymmetric photothermal conversion behavior. With the higher absorption power, the small-pore side (fd) filled with rGO was quickly heated, and the upper surface temperature increased much more rapidly compared with the large pore side (rv) (Fig. 2h, i). DF-7510G05 also has a similar property (Supplementary Fig. 14).

Asymmetric thermal conduction of dual Janus foam

Thermal rectification, the asymmetric heat conduction phenomenon, was achieved with an asymmetric porous structure and then greatly improved with the introduction of thermal conductive filler rGO. The thermal conduction property of foams was characterized with the transient plane source method (Fig. 3a). The single Janus structure achieved a basic rectification by different pore sizes and porosities. The asymmetric porous structure resulted in different thermal conductivities for each layer of the Janus foam. Upon heating from one side, i.e., the side with higher thermal conductivity, the heat will be quickly transported through and accumulated in this side, which causes strong motivation to transport heat through the second part of the conduction process, the side with lower thermal conductivity. On the reverse direction, heating against the side with lower thermal conductivity, the heat will be hindered at the very beginning of the composite, and the higher thermal conductivity of the second part of heat conduction just cannot work with its full capability, resulting in an overall lower thermal conduction speed. Therefore, during the dynamic heating process of the asymmetric porous composites, the thermal rectification behavior will occur. Then, the rectification performance was enhanced by deliberately introducing thermal conductive fillers into one layer of the foams to construct the asymmetric polymer matrix along with the asymmetric pores as the dual Janus structure (Fig. 3b). The as-used rGO formed a smoother heat conduction path inside one layer of the composites. With more rGO introduced, the conduction path becomes more connected, and therefore the thermal conductivity of this layer increases, leading to improved thermal rectification performance. Therefore, the filler rGO in one layer of the Janus foams acts not only to create asymmetric photothermal property but also to enhance the asymmetry of thermal conduction. The asymmetric porous structure was also optimized to obtain the ideal thermal rectification performance.

Fig. 3. Thermal rectification performance of dual Janus foam.

a Illustrative graph of thermal conductivity characterization using the transient plane source method. b The illustrative graph for the basic thermal rectification with a single Janus structure and the enhanced thermal rectification with a dual Janus structure. c–e The temperature increase of the heater in thermal conductivity tests. SF-7515 (c), DF-7510G05 (d), DF-4015G15 (e). f Thermal conductivities of different Janus foams in two directions (fd for forward, rv for reverse). Data are presented as mean values ± SD. n = 3. g Thermal rectification performance comparison among solid state thermal rectifying materials in macroscopic in previous studies. h Thermal conductivities of different uniform foams (UFs). Data are presented as mean values ± SD. n = 3. i Temperature distribution evolution of DF-4015G15 by heating the bottom sides along different directions with a constant temperature of 50 °C in simulations. j The temperature of the upper surface of DF-4015G15 when heating the bottom along different directions in simulations.

Bearing the asymmetric thermal conduction property, Janus foams could dissipate heat at different rates along the two opposite directions, which is directly monitored by the temperature sensor, which also functions as the heater in thermal conductivity characterization. The sensor experienced a varied temperature increase in different directions. When the heat was conducted along the forward direction (fd), it was dissipated at a higher rate and experienced a lower temperature increase than along the reverse direction (rv) (Fig. 3c). The addition of rGO significantly increases the thermal conductivity of the small-pore side, and thus enhances the thermal conductivity asymmetry, improving thermal rectification performance (Fig. 3d, e). The thermal conductivities (λ) of Janus foams in two directions were then calculated and collected (Fig. 3f). The thermal conductivities of different Janus foams in the reverse direction were close (0.6 W/(m K)), while those in the forward direction increased from 0.7 to 0.13 W/(m K) with the addition of rGO and the optimized porous structure. Therefore, the rectification ratio (λ_fd/λ_rv−1) was increased from 18 to 122%, which is relatively high among solid-state thermal rectification materials in macroscopic (Fig. 3g)^31–34.

The thermal conductivity of Janus foams is also relatively low and will be suitable for applications where heat preservation is mainly needed. The thermal conductivities of foams prepared from one layer of emulsion (denoted as uniform foams, UFs) were then characterized (Fig. 3h). Basic parameter of samples including density was also determined (Supplementary Fig. 15). Based on these parameters, the simulations using bi-layer model with constant heating power were performed exhibiting a similar result to the experiments (Supplementary Figs. 16 and 17). This suggests the asymmetric thermal conduction behavior could be attributed to the asymmetric porous structure. Besides the constant heating power situation, another common situation with the constant heating temperature was also simulated (Fig. 3i). In the forward thermal conduction path (heating from the bottom), the upper surface experienced a faster temperature increase, confirming the asymmetric thermal conduction behavior (Fig. 3j).

Enhanced directional thermal management of dual Janus foam

Different from the common directional thermal management based on asymmetric solar and thermal radiation properties or single thermal conduction manipulation, dual Janus foams provide a comprehensive and inspiring idea by combining the dual asymmetric properties: photothermal asymmetry and thermal rectification to enhance directional thermal management performance. This method is promising in practical application for its processability in a large area, with the easy preparation of emulsions and molding. A DF with the size of 30 × 30 cm was then prepared for demonstration (Fig. 4a). The seeded polymerization technology for preparing JPs on an industrial scale is already realized³⁵. We also conducted a scale-up preparation of JPs in a 5-L reactor for demonstration (Supplementary Fig. 18a). The emulsion in large quantities can also be prepared with a more easy process using mechanical stirring, which is frequently used to prepare high internal phase emulsions in large quantities and in industries (Supplementary Fig. 18b, c). The mechanical properties of DFs were then characterized, which is important for their practical applications. The Janus foams exhibit similar stress-strain curves in the initial compression stage due to the same layer of large pore layers. Then the curves diverge and show different compression modulus. With lower internal volume fraction and higher rGO concentration, DF-4015G15 exhibits the highest modulus during the later stage of compression (Supplementary Fig. 19). All the Janus samples recover their original height after the compression tests, indicating their resilience. Therefore, the emulsion template strategy suggests great potential in mass production and in accommodating complicated surface conditions, which is essential for application in the future.

Fig. 4. Directional thermal management performance of dual Janus foam.

a The DF sample in a large area with a size of 30 × 30 cm. b Illustrative graph of the experimental setup for simulated sunlight heating and directional thermal management. c Illustrative graph of dual asymmetric property for enhanced thermal management performance. d–f The upper temperature of different Janus foams when heated by sunlight in different directions. SF-7515 (d), DF-7510G05 (e), DF-4015G15 (f). g, h The bottom temperature of different Janus foams when heated by sunlight in different directions. SF-7515 (g), DF-4015G15 (h). i The summary of the bottom temperature increase of different Janus foams and the difference ratio between the two opposite directions.

A simple model was set up for verification of the directional thermal management of Janus foams, as illustrated in Fig. 4b. The simulated sunlight was produced from an Xeon lamp with the AM 1.5G filter and cast onto the sample’s upper surface through the square hole above the sample. As the sunlight is cast on the sample surfaces, it will either be reflected or absorbed based on their photothermal properties. Then, the absorbed energy of sunlight was converted to heat and warmed the sample up (Fig. 4c). The upper surface temperature of different Janus foams was monitored as displayed in Fig. 4d–f. Due to the asymmetric photothermal property of Janus foams, the upper surface experienced different temperature increases when placed in different directions. As the average reflectivity discrepancy between the two sides of the foams widened, the difference of temperature increase along the two directions enlarged. With the heated upper surface of Janus foams as a heat source and thermal conduction motivation, the energy input from the exterior environment was further directionally managed by the thermal rectification behavior. The bottom temperature could be regarded as the result of directional management, where the bottom represented the managed object (Fig. 4g, h). Notably, the temperature of the upper surface of SF-7515 in the forward direction is lower than in the reverse direction due to the higher reflectivity of the forward side, as shown in Fig. 4d. However, the asymmetric thermal conduction property finally reversed the high-low relationship and resulted in a higher bottom temperature in the forward direction (Fig. 4e). This result suggests that the asymmetric heat conduction property could dominate the directional thermal management performance with similar photothermal properties. The thermal rectification performance was also proved via evaluating the cumulative heating rate (Supplementary Fig. 20). The heating rate is higher in the forward direction for all three samples.

Moreover, the asymmetric photothermal properties could help to magnify the directional management performance with stronger heat conduction motivation in the forward direction. The bottom temperature difference of dual Janus foams in different directions is enlarged compared to single Janus foam (Fig. 4h, Supplementary Fig. 21). The difference ratio between two directions based on the temperature increase at 3600 s increased from 3 to 58% and 65% implying the significant improvement of directional thermal management performance (Fig. 4i). This improvement in temperature difference along opposite directions is a more practical parameter than thermal conductivities and will give guidance for directional thermal management and thermal energy harvesting in the future. The previous work, which focused on asymmetric structure and directional thermal management, could be catalogized into several main types: sole photothermal asymmetry, sole conduction asymmetry, infrared emissivity asymmetry, and combined ones (Supplementary Table 1). Most of the work has neglected the contribution of asymmetric conduction to directional thermal management, which is actually the integrated result of conduction and radiation regulation. Compared with previous work, we have achieved a relatively high surface temperature difference and significant thermal conduction asymmetry^12,36–41. Meanwhile, based on this principle, the asymmetric thermal conduction property could also ensure the directional thermal management in darkness, which implies the potential continuous function of dual Janus foam in all-day time and all-weather conditions⁴².

The outdoor performance was then investigated using the small house models with samples covered. The experiments on sunny and cloudy days were conducted in Beijing during winter with sunlight exposure in daytime. The rainy day, due to the sparsity of winter in Beijing, was simulated by placing a mist humidifier near the sample houses (Supplementary Fig. 22). The inner temperature of small house models was then monitored using thermistor probes (Fig. 5a).

Fig. 5. The regulated inner temperature of the small house model in outdoor experiments.

a The monitored temperature for five days. The daytime temperature during 10:00–16:00 in sunny days (b), cloudy days (d), foggy and rainy days (f), and the night temperature during 20:00 to the next morning 8:00 in sunny nights (c), cloudy nights (e), foggy and rainy nights (g).

During the first three sunny days, the small house model covered with DF in the forward direction experienced a higher temperature than that covered with DF in the reverse direction (Fig. 5b). Such directional management is weakened yet still effective during the night (Fig. 5c). On cloudy days, due to the weakened sunlight intensity, the temperature is lower. But the temperature difference of house models covered with DF in different directions remains (Fig. 5d). The cloudy night is similar to a sunny day’s night (Fig. 5e). In the simulated foggy environment, the sunlight was strongly scattered, and the temperature decreased due to the influence of accumulated water droplets. Therefore, the house models covered with DFs in different directions only showed a small difference. Then, the mist, consisting of micro water droplets, fills the box and tends to accumulate in large water droplets and fall from the upper lid of the box, resulting in simulated rainy days. The sunlight scattering is then mitigated. The temperature also quickly responds to inward sunlight, exhibiting a significant difference between house models (Fig. 5f). During the night, however, the difference is minimized, which could be due to the large humidity and accumulated water droplets on the samples (Fig. 5g). Besides, due to the relatively low thermal conductivity of our DF samples, the house models covered with DFs could better maintain their inner temperature compared to the bare house model, implying the potential in heat-preservation situations. The wind condition is obtained, revealing the relatively stable performance under a windy environment (Supplementary Fig. 23). Overall, according to our outdoor experiments, the DF samples could function well under different weather conditions and have long-term stability, showing the potential for real applications.

Based on this directional heat accumulation/dissipation of dual Janus foams, the foams can be employed in scenarios where the regulated object needs to accumulate as much heat as possible from the environment, or conversely. More importantly, the directional thermal regulation of dual Janus foam is promising in transforming the single oscillatory heat source, like the day-night temperature fluctuation or periodic temperature variation of the industrial equipment, into a single polarity temperature gradient to drive the thermal engine and generate power^21,26. Therefore, the dual Janus structure of foams, combining the asymmetry in photothermal property and thermal conduction, provides a comprehensive and inspiring view for better directional thermal management and promising in practical applications.

Discussion

Dual Janus foams with dual asymmetries, including photothermal and thermal conduction properties, were fabricated through a JP-stabilized emulsions templated method. The asymmetric photothermal property with a discrepancy of about 80% of average solar reflectivity was obtained, and the asymmetric photothermal conversion ability was confirmed. Thermal rectification behavior was achieved by rationally designing the asymmetric porous structure and then optimizing it by introducing thermal conductive fillers, rGO, into one layer and tuning the porous structure. The thermal rectification ratios based on the thermal conductivities characterized using the transient plane source method are increased to 122%, which is outstanding among the solid state thermal rectification materials in macroscopic. With the synergistic contribution from the dual asymmetric properties, the directional thermal management was greatly enhanced. The temperature difference ratios of the managed side could be improved nearly 21 times from 3% based on the single Janus foam to 65% based on dual Janus foam. It is promising to fabricate practical thermal management materials to improve the energy utilization efficiency.

Methods

Materials

Hollow polystyrene (PS) spheres (HP-433, 37.5 wt%) were purchased from Dow Chemical Company. Sodium dodecyl sulfate (SDS) was purchased from Sinopharm Chemical Reagent Co., Ltd. Divinyl benzene (DVB), 2,2′-azobis(2-methylpropionitrile) (AIBN), potassium persulfate (KPS), n-butyl acrylate (BA), and ethylene glycol dimethacrylate (EGDMA) were purchased from J&K Scientific. 3-Methacryloxypropyltrimethoxysilane (MPS) was purchased from Tokyo Chemical Industry Co., Ltd. (TCI). rGO was provided by Nanjing XFNANO Materials Tech Co., Ltd. Absolute ethanol was provided by General-Reagent. Ultrapure water (ρ > 18.2 MΩ cm) was homemade.

Preparation of Janus particles

JPs were prepared with two steps of seeded emulsion polymerization. 26.67 g of HP-433 (37.5 wt%) were dispersed in 320 g of water as the PS seed emulsion. 0.1 g of AIBN was dissolved in 10 g of DVB as the monomer solution, which was then added to 20 mL of SDS aqueous solution (0.24 wt%). The mixture was then emulsified with a high-speed homogenizer for 2 min (13,000 rpm) to obtain the stable DVB monomer emulsion. The monomer emulsion was added to the PS seed emulsion under continuous stirring, and the reaction was carried out at room temperature for 8 h and then at 70 °C for 24 h for polymerization. The resultant was collected and then diluted with water (1:1 wt/wt) as the PS/PDVB seed emulsion for the second step of seeded emulsion polymerization. 3 g of MPS was added to the 3 mL of aqueous solution containing 0.03 g of KPS and 0.08 g of SDS. The mixture was then emulsified using a high-speed homogenizer for 1 min at 13000 rpm as the MPS monomer emulsion. The MPS monomer emulsion was added to 246 g of the diluted PS/PDVB seed emulsions with a peristaltic pump within 20 min. The reaction was then conducted at 70 °C for 24 h. The product particles were collected and washed with repeated centrifugation using ethanol and water, and then freeze-dried for 48 h. The final result particles were denoted as JPs.

Preparation of dual Janus foam

Dual Janus foam was fabricated with layer-by-layer casting JPs-stabilized emulsions. A typical procedure for preparing dual Janus foam is as follows. HIPPE with 80 vol% of internal phase and 2 wt% of JPs as emulsifiers was first prepared. 0.014 g of AIBN was dissolved in the mixture containing 1.5 mL of BA and 0.05 mL of EGDMA as the monomer solution. 0.028 g of JPs was dispersed in the monomer solution with ultrasonication. 6.2 mL of water was added to the JPs dispersion in batches with vortex mixing at intervals. The as-prepared viscous emulsion is denoted as HIPPE-8002. Then, the HIPPE with 75 vol% of internal phase, 10 wt% of JPs, and 0.5 wt% of rGO was prepared. 0.022 g of AIBN was dissolved in the mixture containing 2.4 mL of BA and 0.08 mL of EGDMA as the monomer solution. 0.224 g of JPs was dispersed in the monomer solution with ultrasonication. 0.011 g of rGO was then added to the JPs dispersion, and the mixture was homogenized with further ultrasonication. 7.4 mL of water was added to the particles’ dispersion in batches with vortex mixing at intervals. The as-prepared highly viscous emulsion was denoted as HIPPE-7510G05. The two emulsions were then poured into the glass mold layer-by-layer with HIPPE-7510G05 as the bottom. The stacked emulsion (HIPPE-8002/7510G05) was then sealed and placed in the convection oven at 70 °C for 24 h for polymerization. Thereafter, the product was taken out from the glass mold carefully and dried for 24 h to obtain the porous dual Janus foam, which is denoted as DF-7510G05. The single Janus foam prepared from HIPPE-8002 and HIPPE-7515 without rGO was denoted SF-7515. Janus foams with other compositions were prepared in a similar process.

Characterization

The morphology of JPs and the cross-sectional microstructure of dual Janus foam were characterized using the scanning electron microscope (SEM, JEOL JSM-7900F, Japan). JPs were first dispersed in ethanol and then dropped on the clean silicon wafer and dried for characterization. The Janus foams were fractured in liquid nitrogen to reveal the clear cross-sections. All samples for SEM were pretreated with Pt sputtering coating. The accelerating voltage was set at 5 kV. The composition of JPs was analyzed using a transmission electron microscope (TEM, JEOL JEM-2100Plus, Japan) equipped with an EDS detector (Oxford Ultim Extreme). The JPs dispersion in ethanol was dropped onto the surface of carbon film supported by a copper grid and then dried for characterization. The accelerating voltage was 200 kV. The size distribution of JPs in solvents was characterized with the VASCO Kin Particle Size Analyzer (Cordouan Technologies, France). JPs were dispersed in desired solvents and were characterized with the probing light at 638 nm. The results were analyzed using the Rayleigh scattering model. The microstructure observation of HIPPE was performed using the optical microscope (BX53, Olympus, Japan). The as-prepared emulsions were gently placed on the bottom of small transparent plastic petri dishes and were observed using transmitted light. The fluorescent images were collected by adding a trace of coumarin 6 in the oil phase of emulsions and using reflected light under blue light. The 3D microstructure of asymmetric foams was characterized using Micro-CT (Diondo d2, Germany). Janus foams were sampled in the size of 2 × 2 × 4 mm. The voltage of the X-ray beam is 90 kV, and the element size is 0.002 mm. Visualization was performed using VGStudio MAX 3.4. The sunlight reflectance of asymmetric foams was characterized with a UV-Vis-NIR spectrophotometer equipped with an integrating sphere (Shimadzu 3600-plus, Japan). The samples were tailored with a size of 2.5 × 2.5 cm. The test spectrum is 200–2500 nm. Infrared images were captured using a thermal imager (FOTRIC 322Q, China). The samples’ surface temperature upon simulated sunlight was recorded by placing the infrared imager about 40 cm above the samples. Thermal conductivities were characterized using a thermal constant analyzer (Hot Disk TPS2500S, Sweden). The samples for this test are 20 mm in diameter and approximately 6 mm in thickness. The heater and sensor were clamped closely by two Janus foams (same composition and structure) in the mirror-symmetric configuration. The heating power is 10 mW. Each test was performed after waiting for 15 min at room temperature to reach temperature equilibrium. The simulated sunlight indoors was produced with a Xenon lamp (CHF-XM500, Beijing Perfectlight Technology Co., Ltd, China) equipped with an optical component (AM 1.5G). The Janus foams with a size of 20 × 20 mm and a thickness of about 6 mm were placed in the designed set-up. Foams were placed on the PTFE substrate and surrounded by polystyrene foam interlayered with aluminum foil to avoid the back reflecting and scattering light from the PTFE substrate and reduce the convection from the environment. The temperature of the samples’ surface and bottom was monitored using temperature sensors.

Author contributions

Conceptualization: F.L. Methodology: C.J. Investigation: C.J. and X.H. Visualization: C.J. Funding acquisition: F.L. Writing—original draft: C.J. Writing—review and editing: F.L. and X.Z.

Peer review

Peer review information

Nature Communications thanks Teng Zhang and the other, anonymous, reviewer(s) for their contribution to the peer review of this work. A peer review file is available.

Data availability

The data generated in this study are provided in the Source data file. Additional data are available from the corresponding author upon request. Source data are provided with this paper.

Competing interests

The authors declare no competing interests.

Supplementary information

The online version contains supplementary material available at 10.1038/s41467-026-69140-6.