Bio‐Inspired, Scalable, and Tri‐Mode Stimuli‐Chromic Composite for Smart Window Multifunctionality

Smart window is promising to save building energy and reduce carbon emissions. The fast development leads to a high demand for multifunctionality not limited to energy saving, while the material design and fabrication are challenging. Herein, a scalable method is developed for tri‐mode light regulations: thermo‐, mechano‐, and hydro‐/solvato‐chromisms. The film is constructed of a bio‐inspired hierarchical‐structured surface and a functional elastomer base. Through combined experiments and simulations, the triple‐stimuli‐chromic mechanisms of strain‐induced surface structure deformations, wettability‐controlled reflective index matches, and thermal‐responsive nanostructural resonances, respectively are revealed. Besides a good energy‐saving performance, the robust method shows several advantages: 1) independent energy‐saving and privacy functionalities, 2) an additional hydro‐/solvato‐chromic mode to control privacy in extreme circumstances, and 3) designable patterns and colors to meet high aesthetic demand. The work may inspire the future development of multifunctional smart windows and spatio‐temporal light control methods.


Introduction
3][4] DOI: 10.1002/adfm.10][11] Recent development of smart window technology can be mainly classified as electro-, [12][13][14][15][16] thermo-, [17][18][19] mechano-, [20][21][22] photo-, [23] and hydro-/solvato-chromism, [24,25] based on the different stimuli-responsiveness mechanisms. [8]owever, individual stimulus-chromism provides limited functionalities for smart windows: most previous works focus on energy-saving functionality only, [8,26] while giving less attention to other functionalities such as on-demand privacy, aesthetic appearance, and so on.For example, the most-investigated thermochromic vanadium dioxide (VO 2 ) can only modulate the near-infrared (NIR) solar transmittance, while overlooking the unfavoured yellow-brown appearance and aesthetic demands. [3,26,27]Recent works regarding dual-stimuli responsive material for smart windows have made certain progress, such as enhanced solar modulation (ΔT sol ), [28] ultra-broad band-selective regulation, [29] or integrated energy-saving and privacy functionalities. [30]Further development is expected for multi-stimuli responsive materials for light modulation to extend multifunctionalities. [8,31] However, the materials for smart windows are rarely reported to be responsive to three or more stimuli.This may be due to the challenges in the complex material/structure design, the sophisticated fabrication process, and the multiple coupling effects of materials under different stimuli.The fast development of the technology leads to an increasing demand for multi-stimuli-responsive material design with scalable fabrication methods.
Creatures with optical camouflage/modulation capability represent exciting inspiration sources for the design of light modulation materials. [32,33]The chameleon is known as a camouflage expert in nature by dynamically changing their visible appearance to blend in with different surroundings.They change skin colors between the relaxed and excited states by adjusting the guanine nanocrystal lattice, [34] which performs as a dynamic Brag diffrac-tion mirror to selectively reflect light with a certain wavelength range (Figure 1a-i).Another example is the diphylleia grayi, a typical skeleton flower that shows bright white petals under dry conditions while changing to glassy transparent when being wetted on rainy days. [24,35]The diphylleia grayi petals have a skeleton network structure with significant lacunae and intercellular spaces.The white-to-transparency change happens when replacing air with water in these spaces, a reflective index re-matching phenomenon (Figure 1a-ii).The two light modulation mechanisms based on structural change and reflective index matching are widely observed in nature, [36,37] while are rarely integrated and investigated for smart windows.
Inspired by these light modulation experts in nature, we develop a robust method to achieve tri-mode light management in a thin film to meet smart window multifunctionalities.The film is constructed of a functional elastomer with a bio-inspired hierarchical-structured surface.It shows the bandselective thermo-, mechano-, and hydro-/solvato-chromism.The method exhibits three advantages for smart windows: 1) independent energy-saving and privacy functionalities, 2) an additional hydro-/solvatochromic mode to control the privacy functionality, and 3) designable patterns and colors to meet high aesthetic demand.We reveal that the mechano-and hydro-/solvato-chromic modes can modulate luminous transmittance by a strong scattering effect, while the thermochromic mode regulates solar energy by tunable NIR absorbance.The films were prepared through a scalable two-step deposition process, and the colors and patterns are designable to meet aesthetic purposes.We further compare the produced sample with widely used single-layer and doubleglazed glasses in three cities and prove the sample surpasses the other two regarding their energy-saving performance.

Design Principles and Fabrication
The film is designed to exhibit a triple-mode functionality for light modulation and is classified to be thermo-, mechano-, and hydro-/solvato-chromics depending on the different applied stimuli (Figure 1b).The original state is defined as the film at room temperature under a dry and fully releasing condition, and it is designed to exhibit a relatively high transmittance in the ultraviolet-visible-near infrared (UV-vis-NIR) regions (Figure 1b-i).The similar reflective index between polydimethylsiloxane (PDMS) and silica (SiO 2 ) nanospheres combined with a low concentration of VO 2 nanoparticles (NPs) (nominal 1.5 wt%) gives rise to a relatively high transmittance in the UV-vis-NIR region.The size of SiO 2 and VO 2 is important.Suitable VO 2 NPs are expected to be small enough to reduce scattering and to support the nanostructural resonance in NIR region, [28,30,38] which should be much less than the visible wavelength (400-800 nm).The SiO 2 nanospheres are expected to have their diameters closed to visible wavelength, and a standard size with a very narrow size distribution is required to produce structural colors, of which the size-color effect is discussed in Section 2.5.When being stretched (mechanochromic mode), the stretched film can reduce the transmittance across the broadband UV-vis-NIR region due to the changes from a flat to a rough surface with a biomimetic structure, accompanied by a strong scattering effect (Figure 1b-ii).When the rough surface is covered by the liquid (water for hydrochromic mode and organic solvent for solvatochromic mode), the opaque film turns to be transparent with a relatively high transmittance in the UV-vis-NIR regions (Figure 1b-iii).Under elevated temperature (thermochromic mode), the film selectively blocks NIR transmittance (Figure 1b-iv).
In the experiment, the produced film is observed with a transparent yellow-brown appearance in its original state (the inserted photograph of Figure 1b-i), while changing to be translucent when being stretched (the inserted photograph of Figure 1b-ii), and to be transparent again when both strain and liquid stimuli are applied (the inserted photograph of Figure 1b-iii).This straininduced transparent-to-translucent change is also observed in the control sample without VO 2 (Figure S1, Supporting Information), but not observed in the control sample without SiO 2 (Figure S2, Supporting Information), suggesting the SiO 2 -VO 2 -PDMS layer is essential for the mechano-and hydro-/solvatochromic modes.Under heating-cooling cycles, the film maintains the same appearance as that in the original state (the in-serted photograph of Figure 1b-iv), while the NIR transmittance is significantly reduced.All these three modes are reversible and show high switching speeds.A switching speed within several seconds is preferred for practical smart window applications. [8]e find that the mechano-and hydro-/solvato-chromic modes change their visible transmittance within seconds (Movies S1 and S2, Supporting Information), and the thermochromic VO 2 was known for its ultrafast phase transition in/below the microsecond level. [39]Moreover, the film can be further designed and modified for aesthetic purposes.The film's color under stretched states is adjustable by using SiO 2 nanospheres with different sizes to selectively reflect incident light with a certain wavelength (Figure 1b-v).Also, optical patterns are achievable by depositing SiO 2 nanospheres on selective patterned areas (Figure 1bvi).These two parameters-color and pattern, are critical for aesthetic purposes.Detailed material/structure change principles, light-matter interaction mechanisms, and resultant advantages are discussed in the following sections.
The preparation method is facile and scalable, including a deposition of closed-packed SiO 2 nanospheres (Figure 1c-i) and a subsequent deposition of the PDMS-VO 2 composite (Figure 1cii).[42] The VO 2 -PDMS composite is made of welldispersed VO 2 NPs in PDMS matrix, where the preparation method was systematically investigated in the previous report. [28]fter curing the PDMS, the film can be easily peeled off from the substrate (Figure 1c-iii).The produced film is characterized as a bi-layer structure that is composed of VO 2 -PDMS and SiO 2 -VO 2 -PDMS composite layers (Figure 1c-iii).The deposited SiO 2 nanospheres are hexagonally close-packed (Figure S3a, Supporting Information), and the configuration is proved to be wellmaintained in the PDMS-VO 2 matrix after the second deposition and curing processes (Figure S1b, Supporting Information).The size of applied VO 2 NPs has an average diameter of ≈50 nm (Figure S3c,d, Supporting Information) and is well-dispersed in a PDMS matrix in a low concentration (Figure S4, Supporting Information).The abovementioned preparation parameters are applied and VO 2 concentration is fixed to be nominal 1.5 wt% for the produced film in the following discussion unless specified.

Mechanochromics Based on Surface Structure Deformation
To understand the light modulation performance, the ballistic and total transmittance is measured using a point and a spherical detector, respectively (Figure 2a-i,b-i).In the mechanochromic mode, the ballistic transmittance decreases significantly under an applied strain from 0% to 100% for the whole UV-vis-NIR spectrum (Figure 2a-ii).For example, the ballistic transmittance at 500 nm decreases from ≈44.5% to ≈21.5%.In the total transmittance measurement, the spectra show only slight decreases for the stretched film (Figure 2b-ii).The haze factor significantly increases from 1.3% in the releasing state to 47.7% in the 100%-stretched states.The surface structure is crucial and its formation mechanism and effect on the incident light are investigated.Under optical microscopy, the produced sample is observed to show a relatively uniform surface in its releasing state The total (ballistic and diffusive) transmittance is recorded using an integrated sphere.The applied stains are 0% and 100% for releasing and stretched states, respectively.c) Optical microscopy images of the rough surface under a global strain of c-i) 0% and c-ii) 100%.Insets are the respective diffraction patterns using a 650-nm laser.d) SEM images of the composite surface with three hierarchical structures of d-i) wrinkles, d-ii) cracks, and d-iii) humps.e) Illustration of three structural generations of e-i) wrinkles, e-ii) cracks, and e-iii) humps.f) FEM simulation of the maximum principal strain contours of f-i) wrinkles, f-ii) cracks, and f-iii) humps.
(Figure 2c-i), and changes to a highly rough surface in the stretched states (Figure 2c-ii and Figure S5, Supporting Information), which is consistent with the SEM result (Figure S6, Supporting Information).The insets of Figure 2c are the corresponding diffraction patterns in the released and stretched state, respectively.The rough surface is characterized by a hierarchical structure consisting of three structural features in different scales: 1) the parallel wrinkle structures with a pitch size of ≈3-4 um (Figure 2d-i), 2) randomly distributed cracks with size of several to tens micrometers (Figure 2d-ii), and 3) hexagonally closed packed (HCP) humps with a pitch size of ≈450 nm (Figure 2d-iii).
The formation of a wrinkling structure is due to the strain mismatch between the PDMS-VO 2 and the SiO 2 -VO 2 -PDMS layer under stretching.The SiO 2 -VO 2 -PDMS containing a high volume ratio of SiO 2 (the composite Young's modulus is calculated to be ≈71.9MPa according to Mori-Tanaka method [43] ) is much stiffer than the PDMS-VO 2 with its Young's modulus close to pure PDMS (1.8 MPa) given the negligible concentration of VO 2 NPs (nominal 1.5 wt%). [28]This difference will generate a sig-nificant strain mismatch on their interface under the same applied strain, resulting in the formation of wrinkles under stretching.We conducted a two-layer finite element model (FEM) to reveal the mechanism of wrinkling generation.During the stretching (y-axis), the Poisson deformation leads to a squeezing effect along the x-axis (Figure 2e-i), resulting in periodic wrinkling structures formed along the y-direction.High strains are observed near the surface area caused by the strain mismatch effect (Figure 2f-i).The dimension of wrinkles can be precisely predicted at various stretching strains , according to the finitedeformation buckling theory. [30]In addition, film cracking is observed in the direction aligned perpendicularly to the wrinkles (Figure 2c-ii).When the maximum tensile stress at the midpoint between film strips increases larger than the critical fracture strength of films (Figure 2e-ii), cracks started to initiate and propagate at the interface between SiO 2 nanospheres and PDMS matrix (Figure 2f-ii and Movie S3, Supporting Information).This is consistent with the experimental result: the crack area is observed with lots of exposed SiO 2 nanospheres (Figure 2d-ii).In the experiment, it is observed that increasing the thickness of the SiO 2 -VO 2 -PDMS layer can induce more cracks in the stretched samples (Figure S7, Supporting Information).
The generation of nanoscale humps is due to the different stiffness between SiO 2 (73.1 GPa) and PDMS (1.8 MPa).During the stretching process, this difference leads to the gap's areas filled up by PDMS deforming more than the areas of SiO 2 nanospheres where the humps form.More specifically, the surface PDMS in gap areas shrinks along the z-direction due to the Poisson effect, leading to the formation of humps (Figure 2e-iii).The structure evolution of humps is consistent with the FEM simulation result, where the stretching force leads to the appearance of humps due to the large deformation of PDMS surrounding SiO 2 nanospheres, and the strain is observed much higher on PDMS than it on SiO 2 nanospheres (Figure 2f-iii).Besides, slight delamination is observed on the SiO 2 -VO 2 -PDMS interfaces (Figure 2f-iii).The humps tend to become increasingly apparent and the microcracks generate at the nanosphere-matrix interfaces with larger tensile strain (Movie S4, Supporting Information).
To understand how the hierarchical structure affects the incident light, we conducted the diffraction test using visible laser sources and recorded the diffraction patterns for these lasers passing through the film.Under a 650-nm laser irradiation, a focused beam point pattern is recorded when passing through the film on its releasing state (inset of Figure 2c-i), while it gradually changes to a laterally dispersive pattern through the stretched samples (insets of Figure 2c-ii and Figure S8, Supporting Information).Similar phenomena are observed using 532-and 450nm lasers (Figure S8, Supporting Information).The result indicates that the hierarchical rough surface plays a significant role in inducing diffuse transmittance, thus leading to a high haze factor of 47.7% on 100%-stretched state.These cracks, wrin-kles, and humps with the nano-microscale sizes lead to strong scattering for the incident light in broadband, and they can recover to a flat surface when releasing the strain due to intrinsic elastomeric characteristics of PDMS.The sample with an increased SiO 2 -VO 2 -PDMS layer thickness produces relatively more biaxial-elongated patterns (Figure S9, Supporting Information), indicating that the crack-induced scattering effect can be promoted by adjusting the top layer thickness.

Hydro-/Solvato-Chromics Based on Reflective Index Match
The hydro-and solvato-chromics are different regarding the liquid (water/organic liquid) used to cover the surface.The hydro-/solvato-chromic mode is investigated on the 100%-stretched film using water (hydrochromic mode) as an example The light modulation performance is observed to be similar to the mechanochromic: the ballistic transmittance exhibits a decreasing trend from dry to wet states (Figure 3a), while the total transmittance spectra are close for these two states (Figure 3b).The transmittance valley at ≈1900 nm (Figure 3b) is attributed to the absorbance of the O-H bond in water and varies according to the difference of covering liquids.On 100%-stretching, the haze factor changes from 47.7% in the dry state to 0.5% in the wet state.Also, the addition of water to wet the released film shows a negligible effect: similar ballistic spectra are observed in the dry (Figure 2b-ii) and wet (Figure S10, Supporting Information) states of 0%-stretched film, except the transmittance valley at ≈1900 nm.Although this mode has similar spectral performance to the mechanochromic mode (Figure 2a,b), their mechanisms are different: the mechanochromic mode regulates the transmittance by tuning the surface structures, while the hydro-/solvatochormic mode modulates the transmittance by changing the interfacial property.The surface layer made of PDMS-SiO 2 has a composite reflective index ≈1.44 (the reflective index of SiO 2 and PDMS are 1.44 and 1.43, respectively) that is significantly different from the air of ≈1.0.This difference gives rise to a reflective index gap of ≈0.44 on the interface, making the rough surface exhibit strong scattering.Taking hydrochromic mode as an example, the introduction of water (reflective index of ≈1.33) is capable of significantly reducing the gap of the interfacial reflective index to ≈0.11, thus damping the scattering effect of the rough surface and promoting the ballistic transmittance.
To understand the effect of the reflective index gap on the transmittance light, we calculated the e-field distributions across the flat surface and the rough surface covered by air or water (Figure 3c) using the finite-difference time-domain (FDTD) method.A simplified wrinkle structure is applied in the simulation for the rough surface.It is observed that a relatively strong efield is accumulated on the rough PDMS-air interface (Figure 3ci), while is much weaker on the rough PDMS-water (Figure 3cii) and flat PDMS-air (Figure 3c-iii).More importantly, the efield re-distributed significantly in the air with a rough surface (Figure 3c-i), while is relatively negligible in water with a rough surface (Figure 3c-ii) or in air with a flat surface (Figure 3c-iii).This result suggests that the large reflective index gap and the surface roughness are the important factors to modulate the nearfield e-field distribution, thus shaping the wavefront significantly and inducing a strong scattering in the far-field light transmittance.The strong scattering leads to a strong diffuse transmittance and a high haze factor (47.7%) in the dry stretched state.The solvatochromic mode is achievable based on a similar mechanism by using organic liquids that are colorless, highly transparent in the UV-vis-NIR region, and have a reflective index close to PDMS (≈1.44).It is demonstrated that the translucent rough surface (the inserted photo in Figure 1b-ii) changes to be transparent when it is covered by solvents such as ethanol, 2butanol, di(propylene glycol) methyl ether (DPM), and dimethylformamide (DFM) (Figure 3d).
[46] With similar optical performance to the mechanochromic mode, the hydro-/solvato-chromic mode can serve as an alternative method for on-demand privacy control in daily life.More importantly, this mode shows an advantage to tuning films into clear (high ballistic transmittance) under both stretched and released states, which can serve as a predominated control in extreme circumstances.For example, in a skyscraper fire, where a good sight view is essential for rescue while some windows could be stuck in the privacy state (low ballistic transmittance) due to sudden evacuation and/or emergency power shutdown.The integration of these modes may complicate the fabrication and increase the cost, which needs to be improved in future development before its practical application.

Thermochromics Based on Nanostructural Resonance
The thermochromic mode shows a similar transmittance contrast and spectrum in both ballistic (Figure 4a) and diffusive transmittance (Figure 4b) spectra.Besides, a negligible change is found in haze factor, which is 1.3% on 20 °C and 1.1% on 90 °C samples.These results suggest a fundamental difference in the light modulation mechanism than the aforementioned two modes.As temperature increases from 25 to 90 °C, the film shows a significant decrease in the NIR region and a slight change in the UV-vis region (Figure 4a,b).At 2000 nm, the ballistic transmittance decreases from ≈77.1% to ≈52.7%.This is due to the switchable NIR absorbance of phase-changed VO 2 .Under elevated temperatures, the film reduces NIR transmittance due to the thermal-responsive crystal phase transition of VO 2 from the low-temperature monoclinic (M) to the high-temperature rutile (R) phases. [27,47]This crystal transition can promote the carrier density and change the reflective index of VO 2 (R/M), generating the phenomenon of nanostructural resonances in metallic VO 2 (R) only and causing a strong absorbance in NIR region with a peak at ≈1200 nm. [27,47]It is noted the same sample is used in the measurement to illustrate the spectral changes in all three stimuli-chromic modes (Figures 2a,b, 3a,b, 4a,b), and the same condition (in dry, released, and 25 °C state) is applied to present the original state for reference in all three modes (Figure 1b), giving similar total transmittance spectra.
To better understand the effect, we conduct the FDTD simulation on a VO 2 (R/M) NPs in PDMS matrix on 1200-nm wavelength.It is observed that there is a strong e-field localized on the VO 2 (R) surface (Figure 4c-i), but not on VO 2 (M) (Figure 4c-ii).Besides, VO 2 (R) exhibits a much higher absorbance than VO 2 (M) (Figure 4c-iii and Figure S11, Supporting Information).In comparison, the SiO 2 embedded in PDMS is observed with little efield disturbance (Figure S12, Supporting Information).The crystal transition loop is measured in terms of the transmittance in ≈1200 nm.A heating-cooling hysteresis loop is observed, and its critical transition temperature is found to be ≈65 °C (Figure 4d).The thermochromic mode based on tunable absorbance is different from the mechano-and hydro-/solvato-chromic modes of scattering effect.VO 2 is the only material that exhibits high absorbance in the composite film, making it also possible to tune the transmittance by adjusting the VO 2 concentration, and the themochromics is observed on both the stretched (Figure 4e) and released films (Figure S13, Supporting Information).
The result also suggests that the thermochromic mode is independent of the mechano-and hydro-/solvato-chromic modes, which is promising to achieve independent control of privacy and energy-saving functionalities for a smart window.This is extremely challenging for most previous studies that are solely based on tunable scattering or absorbance. [48,49]More importantly, this work provides two methods for on-demand privacy control, and the hydro-/solvato-chromic mode is suitable to integrate with those double-glazed glasses with a limited gap and interlayer volume, especially considering the emerging investigations of smart window integrated with facile fluid systems. [45,46]

Demonstrations of Mass-Production, Aesthetics, and Energy Saving
The preparation method is also robust and promising for largescale production, as demonstrated by the production of up to 10 pieces of films with a size ≈40 cm 2 for each (Figure 5a).Besides, the proposed film design can be further modified to meet diverse aesthetic purposes.Dynamic mechanochormic patterns are achieved by introducing the pre-designed masks during the SiO 2 deposition process, which is to selectively deposit the SiO 2 layer on target areas.For example, we prepared a positive "fish" (Figure 5b-i) and a negative "butterfly" pattern (Figure 5bii) by selectively depositing SiO 2 on the patterned area and the rest of pattern area, respectively.An opaque "fish" and transparent "butterfly" patterns appear upon mechanical stretching and disappear when releasing (Figure 5b).Moreover, the SiO 2 -VO 2 -PDMS layer can be produced with distinguishing colors by using mono-dispersed SiO 2 nanospheres with different sizes of 200, 250, and 300 nm (Figure S14, Supporting Information), the films display distinguished colors at the stretched states, covering blue (200 nm), green (250 nm) to red (300 nm) (Figure 5c).The different appearances due to structure colors are induced by the assembly of SiO 2 NPs, which is consistent with previous reports. [22,50]eanwhile all the films return to transparent, yellow-brown color upon strain release (Figure S15, Supporting Information).In practical applications, color and pattern are critical components to meet a higher aesthetic purpose. [51,52]Previous works on SiO 2 -PDMS composites demonstrated mechanochromics and color modulation (Table S1, Supporting Information), while relatively less research effort was given to their potential in smart windows and was reported to achieve on-demand privacy control only. [20,22]In comparison, this work based on SiO 2 -PDMS-VO 2 composites integrates tri-mode stimuli-chromics with tunable color and patterns, which are not reported in previous works (Table S1, Supporting Information).More importantly, these properties are beneficial to smart window multifunctionalities.In comparison with state-of-the-art smart window categories (Table 1), to the best of our knowledge, this work presents the first design that can simultaneously meet several important functionalities of smart windows, including on-demand privacy, energy saving, additional control in extreme circumstances, and independent control for these functionalities, as well as aesthetic purpose.
We further demonstrate the energy-saving capability by conducting a building energy simulation using the Energy Plus method.In the simulation, an apartment model was applied with the feature size illustrated in Figure 5c.Different windows are compared: the smart window is constructed by the composite film sandwiched by two glasses and is compared with singlelayered or double-glaze glasses.The indoor temperature control and window performance was set at >26 °C for cooling and <18 °C for heating.[55][56] The apartment heating, ventilation, and air conditioning (HVAC) energy is analyzed in three cities-Barcelona, Melbourne, and Aukland (Figure 5d).The result shows that the smart window outperforms the single-layered and double-glazed glasses regarding the annual energy-saving performance in all three cities (Figure 5e).Taking Melbourne as an example, the annual HVAC energy consumption of the apartment is calculated to be 188.5, 204.7, and 182.6 MJ m −2 when equipped with single-layered, double-glazed, and smart windows, respectively.It is also observed that the smart window saves more energy than single-glazed windows in every month of the year, and is more energy-efficient than double-glazed windows from March to December (Figure 5f).Similar resulting tendencies are also found for the apartment in Barcelona and Aukland (Figure S16, Supporting Information).This result suggests the film in this work is promising to be applied to smart window applications for energy-saving purposes.

Conclusion
In summary, we present a facile and scalable method to produce films with triple-mode light regulation functionalities, including thermo-, mechano-, and hydro-/solvato-chromisms.The film is constructed of a bio-inspired hierarchical surface and a functional elastomer base.By using both experiment and simulation methods, we demonstrate the three modes are achieved by three different principles: tunable surface roughness for mechanochromics, dynamic reflective index matching for hydro-/solvato-chromics, and reversible crystal phase transition for thermochromics.The film is scalable and exhibits several advantages in smart window applications: 1) independent on-demand energy-saving and privacy functionalities, 2) an extra hydro-/solvatochromic mode for privacy control in extreme circumstances, and 3) designable patterns and colors for aesthetic purposes.To our knowledge, this work presents the first design to simultaneously meet these functionalities of smart windows.Moreover, the produced film is proven to have a better energy-saving performance than the widely used single-layered and double-glazed glasses in Barcelona, Melbourne, and Aukland.The work may inspire the future development of multifunctional smart windows, stimuli-responsive materials, and solar-tothermal regulations.

Experimental Section
Sample Preparation: The closed-packed SiO 2 layer was prepared by a convective assembly/self-assembly method.In general, monodispersed SiO 2 nanospheres (Nanorainbow Biotechnology, diameters of 200, 250, 300, and 450 nm) were well-dispersed into ethanol (Sigma Aldrich, 99.8%) in 2 wt%.Then, a 0.9-mL solution was poured into a plastic petri dish with a diameter of 5.6-cm, followed by being partially sealed to slow down the ethanol evaporation speed.It took ≈3 days to fully dry the solvent.[42] A 3.0-mL solution was used four orientations to avoid the impact of orientation.Five people and the electrical devices with a total power of 10 W m −2 were applied as internal loads in the building.The hourly weather data for a typical meteorological year was employed as the external boundary conditions.The weather data of Melbourne, Barcelona, and Auckland were used.Three samples, single-layered glass, double-glazed glass (air gap of 8 mm), and the smart window were used, and their optical data were summarized in Table S2, Supporting Information.

Figure 1 .
Figure 1.Design principles and fabrication.a) Illustrations of the light modulation capability and mechanism of a-i) chameleon and a-ii) Diphylleia grayi.b) Schematic of the stimuli-chromic and aesthetic mechanisms.The tri-mode stimuli-chromics is illustrated by the b-i) original, b-ii) mechano-, b-iii) hydro-/solvato-, and b-iv) thermo-chromic states.The corresponding smart window functionalities are indicated below the dash rectangles.Insets are photographs to show the visible appearances of the film in different states.Schematic of the b-v) color tuning and b-vi) patterning mechanisms for aesthetic purposes.c) Schematic of the two-step deposition method and the configuration of the produced film, including a c-i) self-assembly of SiO 2 and c-ii) a subsequent deposition of PDMS-VO 2 composite.c-iii) The film is easily peeled off from the substrate, and the structure is illustrated.

Figure 2 .
Figure 2. Mechanochromics based on surface structure deformation.a,b) Schematics of ballistic and a-i,b-i) total transmittance measurement setups, and a-ii,b-ii) the transmittance UV-vis-NIR spectra of the film of its mechanochromic modes.The total (ballistic and diffusive) transmittance is recorded using an integrated sphere.The applied stains are 0% and 100% for releasing and stretched states, respectively.c) Optical microscopy images of the rough surface under a global strain of c-i) 0% and c-ii) 100%.Insets are the respective diffraction patterns using a 650-nm laser.d) SEM images of the composite surface with three hierarchical structures of d-i) wrinkles, d-ii) cracks, and d-iii) humps.e) Illustration of three structural generations of e-i) wrinkles, e-ii) cracks, and e-iii) humps.f) FEM simulation of the maximum principal strain contours of f-i) wrinkles, f-ii) cracks, and f-iii) humps.

Figure 3 .
Figure 3. Hydro-/solvato-chromics based on reflective index match.a) Ballistic and b) total transmittance UV-vis-NIR spectra of the hydrochromic modes.c) Calculated e-field on the c-i) rough PDMS-air, c-ii) rough-PDMS-water, and c-iii) flat PDMS-air interfaces.The interfaces are indicated by the white dashed lines.d) Photographs of the rough surfaces when being covered by water (hydrochromics) and several solvents (solvatochromics).The two abbreviations are di(propylene glycol) methyl ether (DPM) and dimethylformamide (DMF).

Figure 4 .
Figure 4. Thermochromics based on responsive nanostructural resonances.a) Ballistic and b) total transmittance UV-vis-NIR spectra of the thermochromic modes.c) Calculated e-field on the c-i) VO 2 (M)-PDMS and c-ii) VO 2 (R)-PDMS interfaces, as well as the absorbance field c-iii) of VO 2 (R) in PDMS matrix.d) Thermal hysteresis loop of the VO 2 NPs in the composite film.e) Transmittance spectra of the composite film produced using different concentrations of VO 2 NPs.The transmittance spectra are measured on 100%-stretched films.

Figure 5 .
Figure 5. Demonstrations of mass-production, aesthetics, and energy saving.a) Photographs of 10 pieces of 40-cm 2 films.b) Photographs of the produced film with a hiding "fish" pattern or a "butterfly" in the inverse pattern.c) Photographs of the produced films by using SiO 2 nanospheres with different sizes: 200 nm for blue, 250 nm for green, and 300 nm for pink.d) Illustration of the apartment used in the simulation.e) The calculated annual energy use of the apartment in Barcelona, Melbourne, and Aukland.The apartment equipped with different glasses is compared, including single-layered glasses, double-glazed glasses, and the smart window in this work.f) Calculated monthly energy usage in Melbourne for the apartment with different glasses.