Sustainable and Inexpensive Polydimethylsiloxane Sponges for Daytime Radiative Cooling

Abstract Radiative cooling is an emerging cooling technology that can passively release heat to the environment. To obtain a subambient cooling effect during the daytime, chemically engineered structural materials are widely explored to simultaneously reject sunlight and preserve strong thermal emission. However, many previously reported fabrication processes involve hazardous chemicals, which can hinder a material's ability to be mass produced. In order to eliminate the hazardous chemicals used in the fabrication of previous works, this article reports a white polydimethylsiloxane (PDMS) sponge fabricated by a sustainable process using microsugar templates. By substituting the chemicals for sugar, the manufacturing procedure produces zero toxic waste and can also be endlessly recycled via methods widely used in the sugar industry. The obtained porous PDMS exhibits strong visible scattering and thermal emission, resulting in an efficient temperature reduction of 4.6 °C and cooling power of 43 W m−2 under direct solar irradiation. In addition, due to the air‐filled voids within the PDMS sponge, its thermal conductivity remains low at 0.06 W (m K)−1. This unique combination of radiative cooling and thermal insulation properties can efficiently suppress the heat exchange with the solar‐heated rooftop or the environment, representing a promising future for new energy‐efficient building envelope material.


Table of contents
Note S1. Outdoor radiative cooling tests. Note S2. CMOSOL modelling of PDMS in a daytime radiative cooling test.
Note S3. Estimation of cooling powers for porous and pristine PDMS emitters Note S4. Estimation of heat fluxes through different roofing materials Supporting Video S1: Observation of the flexibility and durability of the proposed porous PDMS sponge Figure S1. The refractive index (n) of PDMS from ref. [R1-R2].

Note S1. Outdoor radiative cooling tests
In our outdoor radiative cooling tests, a proof-of-concept experimental setup was built (Figure. S1) to simultaneously measure the surface temperature of a porous PDMS sponge and a pristine PDMS film. Both emitters were trimmed into a circular shape with the diameter of 3 cm and the thickness of 5 mm. To minimize the parasitic heat loss, we used polystyrene foam to build the setup and wrapped it with the aluminum tape. During the test, both emitters were placed under direct solar illumination.

Note S2. COMSOL modelling of PDMS in a daytime radiative cooling test.
A theoretical model was developed by coupling the Heat Transfer in Solids interface with the Surface-to-Surface Radiation interface in the software (see an example [R3]). We studied the model in the spectrum range from 0.4-20 µm. In the modelling, the solar irradiance was assumed to 1000 W/m 2 with the diffuse solar irradiance, I solar-diffuse ( )=0, and the ambient temperature (T amb ) of 35 ℃. The emissivity of atmosphere was assumed to 1 in the range of 2.5-8 µm and 13-20 µm, where the rest of the emissivity was assumed to be 0. The absorptivity of the porous and pristine PDMS films were defined by the spectra measured in Figure 2d and 2e, respectively.
In this simulation, we studied two different convection heat transfer coefficients of q= 4 and 10 W/(m 2 ·K) to numerically validate the maximum and minimum steady-state temperature distribution obtained in Figure 4b. The obtained results are plotted in Figure 4d and Figure S4.

Note S3. Estimation of cooling powers for porous and pristine PDMS emitters
The cooling power of PDMS emitter can be estimated using equation 1 in the main text, as described below: In equation 1, the power emitted from the PDMS emitter is Here ∫  is the angular integral of the emitting surface over the hemisphere, is the spectral radiance of a blackbody at a temperature T, h is Planck's constant, is the Boltzmann constant, c is the speed of light, is the wavelength, and A is the area of the emitter.
is the spectral emissivity of the emitter. Using this equation, the emissivity of the porous and pristine PDMS emitters can be converted from measured absorption in Figure 2d and 2e, following Kirchhoff's radiation law.
The absorbed atmospheric radiation from the emitter is Here the angle-dependent emissivity of atmospheric ( , ) is given by 1 cos , where the atmospheric transmittance in zenith direction (0) is estimated by (0) = 1 − (0) . In this estimation, we modeled the atmospheric emittance in MODTRAN (using the atmosphere model: mid-latitude summer) [R4] .
The absorbed solar irradiation is Here 1.5 ( ) is the AM1.5 Global tilt solar illumination with an irradiance of 1000 W/m 2 . The solar irradiance (i.e. the orange spectrum in Fig. 2a) was obtained from national renewable energy laboratory [R5] .
The non-radiative heat loss due to heat conduction and convection is Using these equations, we then estimated the net cooling powers of the porous and pristine PDMS emitters, respectively, as shown in Figure 4e.

Note S4. Estimation of heat fluxes through different roofing materials
The heat flux through different roofing materials was estimated using Fourier's Law ( Q = −kA ), where Q is the heat flux through the plane, k is the thermal conductivity of the material, L is the thickness of the plane, and A is the plane area. In this estimation, we assumed a linear temperature profile within the single-layered roofing material. The thermal conductivity of the asphalt shingle used in this estimation is 0.5 W/(m·K) [R6] . To estimate the heat flux through the roof in outdoor environment, we employed a hot plate to heat one side of the roofing material to the temperature observed in Figure 5d of the main text, and measured the temperature on the other side. As a result, the temperature difference between the two sides is 7.6 °C for the commercial shingle, 4.6 °C for the white-painted shingle and 6.4 °C for the porous PDMS sponge, respectively. Therefore, the corresponding heat fluxes through the commercial single, white-painted shingle and porous PDMS sponge are 760 W/m 2 , 460 W/m 2 and 76 W/m 2 , respectively.