Dual Janus foam for directional thermal management

Domain: physics
Published: 2026-02-02
Editorial outcome: major_revision

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.

Introduction
Thermal management is essential for energy conservation and efficient thermal energy harvesting and exploitation1,2. 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 controlling4–7. 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 materials8–12. 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 management13–17. 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 reversed 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 management11,12. 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 generation18–20. 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 circuits22–25. 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 methods27–30.

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
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(divinylbenzene) (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 toward soil (Fig. 1d and Supplementary Fig. 1).

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).

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.

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 (λfd/λrv − 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.

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 onto 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 asymmetry12,36–41. 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-daytime 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).

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 need 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 power21,26. 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.

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.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "A dual Janus foam with dual asymmetries in photothermal and thermal rectification is designed, which exhibit high\nphotothermal property contrast and excellent thermal rectification ratios (122%). The entire material system bears a striking\nresemblance to the author's previously published work (Janus particles stabilized asymmetric porous composites for thermal\nrectification, Nature Communications, 2025, volume 16, Article number: 5650), and also the thermal rectification factor—a\ncrucial metric for thermal rectification—has not been significantly enhanced compared to their previous results. Thus, the\ninnovation of this paper is insufficient for publication in Nature Communications.", "reviewer_2": "In this manuscript, the authors designed dual Janus foams (DFs) with dual asymmetries in photothermal and thermal\nrectification towards thermal management. This work is interesting. However, there are many issues should be addressed.\n1. The directional thermal management performance of DFs should be compared with that of existing asymmetric materials.\n2. The authors claimed that “This method is promising in practical application for its processability in a large area with the\neasy preparation of emulsions and molding.” It seems not easy for the complicated process of both JPs and Janus foam.\nMore experimental evidence is needed.\n3. Outdoor practical application of the DFs should be performed, including performance under different weather conditions\n(sunny, cloudy, rainy) and long-term stability tests, etc.\n4. The authors attribute the thermal rectification performance to the asymmetric porous structure and the introduction of rGO,\nhowever, specific mechanism is not elaborated in detail. The discussion of distribution, orientation, and contribution of rGO\nto thermal rectification is not sufficient.\n5. The mechanical properties of the DFs should be given.\n6. Can the DFs keep stable under wind and rain? This is crucial for practical applications.\n7. In the characterization section, detailed sample preparation and measurement condition should be added for repeatability.", "reviewer_3": "The authors present their design, fabrication, and measurements of the dual Janus foams with a thermal rectification factor of\n122%. TEM, SEM, and EDS are used to characterize the microstructure. Reflectivity is measured to characterize the surface.\nAbsorption power and surface temperature are also reported to demonstrate thermal management. Figures are nicely\nprepared, and the conclusion is well supported. The reviewer recommends publication with the following issues addressed.\nMajor:\n1) Fig. 4 demonstrates how the rectification works. However, one side is always in contact with the ground, and manual\nflipping is requested during thermal management. Without the manual flipping, how do we use this solar-based rectification?\nIn other words, please describe some potential applications without manual flipping.\n2) “The stabilized HIPPE is viscous due to its high volume fraction of the internal phase”. Since this is a water-in-oil\nstructure, is the internal phase water? Are we saying water causes high viscosity?\n3) Absorptivity instead of the absorption power should be reported.\n4) Does the graphene addition impact pore size significantly? What dominates the asymmetric pore sizes in DF-7510G05\njunction? graphene addition, volume fraction, or fabrication process?\n5) Fig. 3j shows that forward and reverse converge to different final temperatures. Since 3i uses a constant heating\ntemperature, I assume the same settings for Fig. 3j. Flipping the sample doesn’t change the thermal resistance. Why do the\nfinal temperatures converge to different values? Is this due to the slow converging slope in the figure (illusion due to short\ntime), different contact resistances, or something else?\n6) Fig. 3a illustrates the transient plane method, but it is not self-explanatory enough. For example, are the back plates\nsamples? Is the heater & sensor device in contact with the samples?\nMinor:\nThe technical writing needs to be emphasized. To list a few:\n1) “construct the stacked structure without the interference between layers” to “construct the stacked structure with a clear\ninterface”\n2) “by labeling the oil phase with fluorescent dye (coumarin 6) (Extended Data Fig. 3)” to “by labeling the oil phase with\nfluorescent dye, coumarin 6. (Extended Data Fig. 3)”\n3) “With the addition of black filler rGO” to “With the addition of black filler Reduced Graphene Oxide (rGO)”\n4) The arrow in Fig. 1k should point from the text to the structure.\n5) “Digital graph of stacked high internal phase Pickering emulsion” to “Photo of stacked high internal phase Pickering\nemulsion”\n6) “(D) The interfacial structure of DJF-7515” to “(D) The wall between pores”\n7) “Since the excellent sunlight absorption ability of rGO” to “Thanks to the excellent sunlight absorption ability of rGO”\n8) “The average reflectivity of the large pore side (rv) is approximately the same (85–90 %) for different Janus foams and\nexperienced a slight decrease, which could probably be influenced by the opposite layer.” to “The average reflectivity of the\nlarge pore side (rv) is approximately the same (85–90 %).” Reflectivity is a surface property for opaque materials.\n9) “The basic rectification performance was achieved through the rational design of the asymmetric porous foam with a sharp\ncontrast in the pore size and porosity and is regarded as the single Janus structure. ” to “The single Janus structure achieved\na basic rectification by different pore sizes and porosities.”\n10) “As the dual Janus foam was applied as the conduction path, the discrepancy of temperature increase is enlarged and\nsuggests an enhanced thermal rectification performance (Fig. 3d, e). ” to “The addition of rGO significantly increases the\nthermal conductivity of the small-pore side, and thus enhances the thermal conductivity asymmetry, improving thermal\nrectification performance. (Fig. 3d, e)”\n11) “The thermal conductivities of different Janus foams in the reverse direction were close (~ 0.6 W/(m·K)), while those in\nthe forward direction increased from ~ 0.7 to ~ 0.13 W/(m·K)” to “The thermal conductivities of different Janus foams in the\nreverse direction were close (0.06 W/(m·K)), while those in the forward direction increased from 0.07 to 0.13 W/(m·K) ”\n12) Fig. 2 h and i use 8002, 4015, 7515, and 7510 instead of xx-rv/df. Why?\n13) Fig. 4 d and h should use 'min' instead of 'second' as the time unit."}, "author_response": "Response\nWe are grateful to receive the reviewer’s comment. We believe the question and challenge\nfrom the reviewer will help us to review our work with deeper and comprehensive\nconsideration and make it more suitable for publication in Nature Communications. There\nare some points we want to make clearer, and we hope this will address the reviewer’s\nconcern.\nThe dual Janus foams prepared and investigated in this work are an extended and upgraded\nmaterial system based on our previous work. In the previous work, we developed a strategy\nto form thermal rectifying materials based on the asymmetric porous structure and\ndemonstrated its basic working mechanism. Here, we developed the basic single asymmetric\nstructure with thermal rectifying conduction behavior to dual Janus structures featuring the\nasymmetric thermal conduction in addition to asymmetric photothermal attributes. The\nmaterial system employed both in this work and the previous work shares a similar basis,\nwhich certifies the generality and expansibility of this strategy. Based on this generalizable\nmethod, we upgrade the material system to realize better performance in directional thermal\nmanagement.\nIn the field of thermal rectifying materials research, rectifying factors are still challenging to\nimprove, especially on the solid macroscopic scale and in mild conditions. Rectifying factors\nof thermal transport are hard to lift compared to electrical counterparts, just like thermal\nconductivities are hard to improve compared to electrical conductivity. According to the\nliterature we have reviewed, the rectifying factors of asymmetric materials on the solid\nmacroscopic scale and in the mild conditions over 100 % are still challenging. And the\nenhancement from 38 % in our previous work to 122 % in this work is actually significant.\nBesides, we also investigate another parameter, the temperature difference ratio, which was\nmore practical and was improved nearly 21 times from 3 % based on a single Janus structure\n(the structure constructed in our previous work) to 65 % based on a dual Janus structure (the\nstructure in this work).\nApart from the above development mentioned, the dual Janus foam prepared in this work\nexhibited more pronounced performance under sunlight on sunny days and maintained the\nbasic thermal rectifying feature without sunlight due to their asymmetric thermal conduction,\nwhich has been verified in our previous work.\nTherefore, the resemblance between these two works is mostly attributed to the preparation\nstrategy’s generalizability and expansibility. But the two works differ not only in the\ndesigned structure, from single asymmetry to dual asymmetries, but also in the improved\nthermal rectifying property and enhanced directional thermal management performance. We\nhope this response will make our work clearer for the reviewer. We thank the reviewer and\nthe helpful challenges with our best sincere.\n\nRemark 1\nThe directional thermal management performance of DFs should be compared with that of\nexisting asymmetric materials.\nResponse\nThank you very much for the suggestions. Additional literature reviews have been conducted,\nand the comparison of directional thermal management performance is displayed below. This\ndisplayed item is now added to the supplementary materials. And more discussion about the\ncomparison is also added in the main text.\nAs far as we reviewed, the previous work, which focused on asymmetric structure and\ndirectional thermal management, could be catalogized into several main types: photothermal\nasymmetry, conduction asymmetry, infrared emissivity asymmetry, and combined ones.\nMost of the work has neglected the contribution of asymmetric conduction to directional\nthermal management, which is actually the integrated result of conduction and radiation\nregulation. Compared with previous work, we have achieved a relatively high surface\ntemperature difference and significant thermal conduction asymmetry. The total thermal\nconductivity of our materials is also relatively low, which has potential for applications\nwhere heat preservation is the main requirement.\n\nRemark 2\n2. The authors claimed that “This method is promising in practical application for its\nprocessability in a large area with the easy preparation of emulsions and molding.” It\nseems not easy for the complicated process of both JPs and Janus foam. More\nexperimental evidence is needed.\nResponse\nThanks for the reviewer’s question. The method demonstrated in this work is easy to\nscale up for the next step in practical applications. The seeded polymerization\ntechnology for preparing Janus particles on an industrial scale has already been\nrealized8. And the high internal phase emulsion for a large area sample was achieved as\ndisplayed in the manuscript (Fig.4a). We also conducted a scale-up preparation of JPs\nin a 5-liter reactor for demonstration (Fig. R1a). Here, as additional experimental\nevidence, the emulsion in large quantities was prepared with a more easy process using\nmechanical stirring, which is frequently used to prepare high internal phase emulsions\nin large quantities (Fig. R1b, c). Meanwhile, all the as-used monomers are industrial\nmaterials. Therefore, this as-synthesized anisotropic porous composites are economical.\nIt is believed that the method proposed here actually has potential for practical\napplications.\n\nRemark 3\nOutdoor practical application of the DFs should be performed, including performance\nunder different weather conditions (sunny, cloudy, rainy) and long-term stability tests,\netc.\nResponse\nThanks for the reviewer’s suggestions. The samples with relatively large areas were\nthen prepared and applied to the small house model, and the outdoor performance was\ninvestigated. The experiment on sunny and cloudy days was performed in Beijing\nduring winter with sunlight exposure during daytime (Fig. R2a). The rainy days,\nhowever, are actually rare in the winter in Beijing. Therefore, we placed a mist\nhumidifier near the sample houses, which were all enclosed in a transparent box to\nmimic the rainy days (Fig. R2b). The temperature regulation results were then collected\nand analyzed.\n\nRemark 4\nThe authors attribute the thermal rectification performance to the asymmetric porous\nstructure and the introduction of rGO, however, specific mechanism is not elaborated\nin detail. The discussion of distribution, orientation, and contribution of rGO to thermal\nrectification is not sufficient.\nResponse\nThanks for the reviewer’s kind suggestion. We feel sorry to miss the detailed\nexplanation about the specific mechanism of thermal rectification performance derived\nfrom the asymmetric porous structure and the help of rGO.\nFirst, the basic thermal rectification property of composites prepared in this work is\nclosely related to the asymmetric porous structure, which is an accumulated\nphenomenon during the heating process with a significant dynamic nature.\nThe asymmetric porous structure resulted in different thermal conductivities of each\nlayer of the Janus foam. Upon heating from one side, the side with higher thermal\nconductivity, the heat will be quickly transported through and accumulated in this side,\nwhich causes strong motivation to transport heat through the second part of the\nconduction process, the side with lower thermal conductivity. On the reverse direction,\nheating against the side with lower thermal conductivity, the heat will be hindered at\nthe very beginning of the composite, and the higher thermal conductivity of the second\npart of heat conduction just cannot work with its full capability, resulting in an overall\nlower thermal conduction speed. Therefore, during the dynamic heating process of the\nasymmetric porous composites, the thermal rectification behavior will occur.\nSecond, based on the discussion above, the thermal conductivity difference between the\ntwo layers will consequently affect the thermal rectification performance. Hence, rGO\nfillers with high thermal conductivity were chosen and introduced into one layer of the\nJanus foam. The introduced rGO fillers formed a smoother heat conduction path inside\none layer of the composites. With more rGO added, the conduction path becomes more\nconnected, and therefore the thermal conductivity of this layer increases, leading to\nimproved thermal rectification performance. Therefore, the distribution of rGO inside\nthe matrix will actually influence the continuity of the thermal conduction path.\nHere, we also observed that a small portion of JPs is close and adhered to rGO particles,\nas in Supplementary Fig. 6 (displayed below in Fig. R5 for convenience). This can be\nattributed to the affinity of the benzene rings of polystyrene in JPs to the hexanol carbon\nrings in the rGO sheets. And this can improve the dispersity of rGO inside the matrix.\nAs to the orientation of rGO, it will influence the efficiency in forming a thermal\nconductive path. However, it is more complicated to modulate and out of the range of\nour current research. Therefore, we didn’t intentionally modulate the orientation of rGO\nin our current system. However, we believe that the orientation of rGO is also an\nimportant factor in our samples' performance, and we hope to take it as a practical\napproach to improve our sample performance in the future.\n\nRemark 5\nThe mechanical properties of the DFs should be given.\nResponse\nThanks for the reviewer’s suggestion. The mechanical properties of DFs were then\ncharacterized and displayed here. Relevant discussions and displaying items have been\nadded in the main text and supplementary information.\nThe Janus foams exhibit similar stress-strain curves in the initial compression stage due\nto the same structure of the large pore layers. Then the curves diverge and show\ndifferent compression modulus. With lower internal volume fraction and higher rGO\nconcentration, DF-4015G15 exhibits the highest modulus during the later stage of\ncompression. All the sample recovered their original height after the compression tests,\nindicating their resilience.\n\nRemark 6\nCan the DFs keep stable under wind and rain? This is crucial for practical applications.\nResponse\nThe performance of DF under windy and rainy conditions was then investigated with\nour outdoor experiments. The results and discussions are given in response to Remark\n3.\n\nRemark 7\nIn the characterization section, detailed sample preparation and measurement condition\nshould be added for repeatability.\nResponse\nThe detailed sample preparation and measurement conditions are now added.\nThe morphology of JPs and the cross-sectional microstructure of dual Janus foam were\ncharacterized using the scanning electron microscope (SEM, JEOL JSM-7900F, Japan).\nJPs were first dispersed in ethanol and then dropped on the clean silicon wafer and dried\nfor characterization. The Janus foams were fractured inside liquid nitrogen to reveal the\nsharp and clear cross-section. All samples for SEM were pretreated with Pt sputtering\ncoating. The accelerating voltage was set at 5 kV.\nThe composition of JPs was analyzed using a transmission electron microscope (TEM,\nJEOL JEM-2100Plus, Japan) equipped with an EDS detector (Oxford Ultim Extreme).\nThe JPs dispersion in ethanol was dropped onto the surface of the carbon film supported\nby a copper grid and then dried for characterization. The accelerating voltage was 200\nkV.\nThe size distribution of JPs in solvents was characterized with the VASCO Kin Particle\nSize Analyzer (Cordouan Technologies, France). JPs were dispersed in desired solvents\nand were characterized with the probing light at 638 nm. The results were analyzed\nusing the Rayleigh scattering model.\nThe microstructure observation of HIPPE was performed using the optical microscope\n(BX53, Olympus, Japan). The as-prepared emulsions were gently placed on the bottom\nof small transparent plastic petri dishes and were observed using transmitted light. The\nfluorescent images were collected by adding a trace of coumarin 6 in the oil phase of\nemulsions and using reflected light under blue light.\nThe 3D microstructure of asymmetric foams was characterized using Micro-CT\n(Diondo d2, Germany). Janus foams were sampled in the size of 2 mm × 2 mm × 4 mm.\nThe voltage of X-ray beam is 90 kV, and the element size is 0.002 mm. Visualization\nwas performed using VGStudio MAX 3.4.\nThe sunlight reflectance of asymmetric foams was characterized with a UV-Vis-NIR\nspectrophotometer equipped with an integrating sphere (Shimadzu 3600-plus, Japan).\nThe samples were tailored to a size of 2.5 cm × 2.5 cm. The test spectrum is 200 nm–\n2500 nm.\nInfrared images were captured using a thermal imager (FOTRIC 322Q, China). The\nsamples' surface temperature upon simulated sunlight was recorded by placing the\ninfrared imager about 40 cm above the samples.\nThermal conductivities were characterized using a thermal constant analyzer (Hot Disk\nTPS2500S, Sweden). The samples for this test are 20 mm in diameter and about 6 mm\nin thickness. The heater and sensor (the sole component) was clamped closely by two\nJanus foams (same composition and structure) in the mirror-symmetric configuration.\nThe heating power is 10 mW. Each test was performed after waiting for 15 min at room\ntemperature to reach temperature equilibrium.\nThe simulated sunlight indoors was produced with a Xenon lamp (CHF-XM500,\nBeijing Perfectlight Technology Co., Ltd, China) equipped with an optical component\n(AM 1.5 G). The Janus foams with a size of 20 mm × 20 mm and a thickness of 6 mm\nwere placed in a designed set-up. Foams were placed on the PTFE substrate and\nsurrounded by polystyrene foam interlayered with aluminum foil to avoid the back\nreflecting and scattering light from the PTFE substrate and reduce the convection from\nthe environment. The temperature of the samples’ surface and bottom was monitored\nusing temperature sensors.\n\nRemark (1)\nFig. 4 demonstrates how the rectification works. However, one side is always in contact\nwith the ground, and manual flipping is requested during thermal management. Without\nthe manual flipping, how do we use this solar-based rectification? In other words, please\ndescribe some potential applications without manual flipping.\nResponse\nThanks for the reviewer’s question. Actually, the thermal rectifying materials could\nfunction with manual flipping to expose different faces and exhibit different properties.\nHowever, they could also work without manual flipping in some certain rationally\ndesigned systems.\nOne scenario is the occasion that the regulated object needs to accumulate as much heat\nas possible from the environment, so the material could be placed with the solar\nabsorption side outward. During the daytime, the Janus foam could accumulate heat\nfrom solar radiation, and with faster heat transport inwards in the forward heat\nconduction direction. During the night, the Janus foam could lag the heat dissipation\noutwards in the reverse heat conduction direction. As in all, the material without manual\nflipping can accumulate heat as much as possible from solar energy with solar\nabsorption side outwards. In other words, the materials could also reject and dissipate\nheat from the object to the environment by being statically placed with the white side\noutward.\nAnother potential and promising application of thermal rectifiers is harvesting energy\nfrom single fluctuating thermal resources like the ambient environment by constructing\na thermal rectifying circuit as afore proposed by other researchers9, 10. Two thermal\nrectifiers with the opposite orientation are placed against two separate thermal\ncapacitors, which then clamp the heat engine (e.g., the thermoelectric generator). In this\nsystem, one thermal rectifier could absorb solar energy during daytime and quickly\ntransport heat from the fluctuating environment and hinder heat dissipation with the\nreverse thermal conduction from inside to outside during nighttime. As a result, the side\nof the heat engine close to the thermal rectifier in this orientation will maintain a high\ntemperature during function. On the other side of the heat engine, with the reverse-\nplaced thermal rectifier, it will maintain a low temperature during function. The well-\nmaintained temperature difference between the two sides of the heat engine is crucial\nfor generating power continuously. Therefore, with the rationally designed system, the\nthermal rectifying materials will harvest energy from fluctuating solar radiation and\nenvironmental temperature without manually flipping the material.\nWe hope our additional discussion will address the reviewer’s concern. The relative\ndiscussion is now added in the main text.\n\nRemark (2)\n“The stabilized HIPPE is viscous due to its high volume fraction of the internal phase”.\nSince this is a water-in-oil structure, is the internal phase water? Are we saying water\ncauses high viscosity?\nResponse\nThanks for the reviewer’s question. Yes, the internal phase of the as-prepared emulsion\nis water. To some extent, we can say the water causes high viscosity since the gradual\naddition of water into the system during the preparation of emulsions is accompanied\nby increased viscosity of the emulsions. But more precisely, it is the increased volume\nfraction of JPs stabilized water droplets (linear to the volume fraction of internal phase\nwater) that causes a higher viscosity of emulsions. The “free space” between JPs\nstabilized water droplets, consisting of low-viscosity oil (the monomer), shrank as the\nvolume fraction of JPs stabilized water droplets increased. Consequently, the mobility\nof the JPs stabilized water droplets decreased, and the viscosity of the whole emulsions\nincreased.\n\nRemark (3)\nAbsorptivity instead of the absorption power should be reported.\nResponse\nThanks for the reviewer’s kind suggestion. The absorptivity of samples is now\ncalculated by assuming the transmissivity is negligible.\nAnd the results are displayed below, which is also provided in the supplementary\nmaterials.\n\nRemark (4)\nDoes the graphene addition impact pore size significantly? What dominates the\nasymmetric pore sizes in DF-7510G05 junction? graphene addition, volume fraction,\nor fabrication process?\nResponse\nThanks for the reviewer’s question. The pore size can be adjusted by varying the\ninternal phase volume fraction and dosage of Janus particles. These two factors\ndominate the asymmetric pore sizes in Janus foams. Larger internal phase volume\nfraction and fewer Janus particles will result in a larger pore size.\nThe addition of rGO introduces one more potential factor to impact the pore size. Here,\nwe conducted additional experiments to investigate the influence caused by rGO.\nTaking the DF-7510G05 as an example, we prepared the Janus foams in the absence of\nrGO in the small pore layers from 7510 emulsions (HIPPE-7510). And the result is\ndisplayed as below. We analyzed the pore size of small pore layers before and after the\nintroduction of rGO based on the Micro-CT characterization and SEM images.\nAs the pore size analysis revealed, the pore size was slightly reduced as 0.5 wt% of\nrGO was added. This is attributed to the additional viscosity of the continuous oil phase\nwith rGO and the enhanced restriction on internal water droplets coalescence, and\nfinally reduced the pore size.\nTherefore, the dominant factors for asymmetric pore sizes of Janus foams are internal\nvolume fraction and Janus particle dosage. Besides, the addition of rGO could also\naffect the pore size of foams.\n\nRemark (5)\nFig. 3j shows that forward and reverse converge to different final temperatures. Since\n3i uses a constant heating temperature, I assume the same settings for Fig. 3j. Flipping\nthe sample doesn’t change the thermal resistance. Why do the final temperatures\nconverge to different values? Is this due to the slow converging slope in the figure\n(illusion due to short time), different contact resistances, or something else?\nResponse\nThanks for the reviewer’s question. Fig. 3j gives the temperature evolution of the upper\nsurface during heating from the bottom in different directions using finite element\nsimulations. Here, the temperature increasing process differed significantly in different\ndirections, indicating the asymmetric heat transport during dynamic heating. The final\ntemperature converges to different values within the investigated time range in the\ntesting experiment. Actually, when approaching an equilibrium state, the temperature\ndifference will dwindle, which requires quite a long time. Therefore, such asymmetric\nthermal conduction behavior bears a dynamic nature and is more suitable for scenarios\nsuch as an environment with fluctuating temperature.\n\nRemark (6)\nFig. 3a illustrates the transient plane method, but it is not self-explanatory enough. For\nexample, are the back plates samples? Is the heater & sensor device in contact with the\nsamples?\nResponse\nThanks for the reviewer’s questions. The details for thermal conductivity\ncharacterization using the transient plane method are not enough here in our manuscript.\nThe single probe serving as both heater and temperature sensor was clamped closely by\ntwo Janus foams, which were placed in a mirror-symmetric configuration. Therefore, it\nis a sandwich-like structure, and the heater & sensor device is in close contact with the\nsamples. For clarity, Fig. 3a was modified with more annotations, and the\ncharacterization details were provided in the experimental methods. We hope this\nrevision will make our method and discussion clearer and understandable.\n\nMinor:\nThe technical writing needs to be emphasized. To list a few：\n1) “construct the stacked structure without the interference between layers” to\n“construct the stacked structure with a clear interface”\n2) “by labeling the oil phase with fluorescent dye (coumarin 6) (Extended Data Fig. 3)”\nto “by labeling the oil phase with fluorescent dye, coumarin 6. (Extended Data Fig. 3)”\n3) “With the addition of black filler rGO” to “With the addition of black filler Reduced\nGraphene Oxide (rGO)”\n4) The arrow in Fig. 1k should point from the text to the structure.\n5) “Digital graph of stacked high internal phase Pickering emulsion” to “Photo of\nstacked high internal phase Pickering emulsion”\n6) “(D) The interfacial structure of DJF-7515” to “(D) The wall between pores”\n7) “Since the excellent sunlight absorption ability of rGO” to “Thanks to the excellent\nsunlight absorption ability of rGO”\n8) “The average reflectivity of the large pore side (rv) is approximately the same (85–\n90 %) for different Janus foams and experienced a slight decrease, which could\nprobably be influenced by the opposite layer.” to “The average reflectivity of the large\npore side (rv) is approximately the same (85–90 %).” Reflectivity is a surface property\nfor opaque materials.\n9) “The basic rectification performance was achieved through the rational design of the\nasymmetric porous foam with a sharp contrast in the pore size and porosity and is\nregarded as the single Janus structure. ” to “The single Janus structure achieved a basic\nrectification by different pore sizes and porosities.”\n10) “As the dual Janus foam was applied as the conduction path, the discrepancy of\ntemperature increase is enlarged and suggests an enhanced thermal rectification\nperformance (Fig. 3d, e). ” to “The addition of rGO significantly increases the thermal\nconductivity of the small-pore side, and thus enhances the thermal conductivity\nasymmetry, improving thermal rectification performance. (Fig. 3d, e)”\n11) “The thermal conductivities of different Janus foams in the reverse direction were\nclose (~ 0.6 W/(m·K)), while those in the forward direction increased from ~ 0.7 to ~\n0.13 W/(m·K)” to “The thermal conductivities of different Janus foams in the reverse\ndirection were close (0.06 W/(m·K)), while those in the forward direction increased\nfrom 0.07 to 0.13 W/(m·K) ”\n12) Fig. 2 h and i use 8002, 4015, 7515, and 7510 instead of xx-rv/df. Why?\n13) Fig. 4d and h should use 'min' instead of 'second' as the time unit.\nResponse\nThanks very much for the reviewer’s careful, meticulous, and thorough revision\nsuggestions. We are really grateful for the advice and have revised the manuscript\naccording to each suggestion.\nAs to points 1, 2, 3, 5, 6, 7, 8, 9, 10, 11, the mentioned phrases and sentences have now\nbeen revised as suggested.\nAs to point 4, the arrow direction has been corrected in the revised version.\nAs to point 12, we feel sorry to mix up the notation of sample names. More clear names\nwith SF-7515-rv, DF-4015G15-fd, SF-7515-rv, and DF-7510G05-fd to replace the\n8002, 4015, 7515, and 7510, respectively, in Fig. 2h. Thanks to the reviewer’s kind\nreminder.\nAs to point 13, the time unit is now ‘min’."}, {"round": 2, "reviewer_comments": {"reviewer_2": "All my comments have been well addressed in the revision.", "reviewer_3": "Thanks for the corresponding revisions. Without manual flipping, the outward side should be designed to have a large absorptivity of solar energy (high frequency) in the daytime and a small emissivity of thermal radiation (low frequency) at night. (spectral selectivity) The reviewer recommends publication."}, "author_response": "Response\nWe are grateful to the reviewer’s the helpful questions and constructive suggestions.\nWe are glad that our former response and revisions addressed the concerns from the\nreviewer well.\n\nResponse\nWe are grateful for the kind and helpful suggestions from the reviewer. It is an inspiring\nidea to rationally design materials with spectral selectivity, especially for the utilization\nof solar energy in buildings. We will further develop new materials with better thermal\nmanagement performance based on the strategy developed in this work. Thanks a lot\nfor the reviewer’s suggestions!"}]

Decision letter