Research ArticleMATERIALS SCIENCE

Iridescence-controlled and flexibly tunable retroreflective structural color film for smart displays

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Science Advances  09 Aug 2019:
Vol. 5, no. 8, eaaw8755
DOI: 10.1126/sciadv.aaw8755

Abstract

Structural color materials, which use nano- or microstructures to reflect specific wavelengths of ambient white light, have drawn much attention owing to their wide applications ranging from optoelectronics, coatings, to energy-efficient reflective displays. Although various structural color materials based on specular or diffuse reflection have been demonstrated, neither efficient retroreflective structural colors nor iridescent and non-iridescent colors to different observers simultaneously were reported by existing artificial or natural structural color materials. Here, we show that by partially embedding a monolayer of polymer microspheres on the sticky side of a transparent tape, the spontaneously formed interferometric structure on the surface of air-cushioned microspheres can lead to unique structural colors that remain non-iridescent under coaxial illumination and viewing conditions, but appear iridescent under noncoaxial illumination and viewing conditions. Our findings demonstrate a smart, energy-efficient, and tunable retroreflective structural color material that is especially suitable for nighttime traffic safety and advertisement display applications.

INTRODUCTION

Structural color materials rely on optimized structural designs instead of pigments or dyes to generate colors that stand out from the surrounding visual environment. They have attracted considerable scientific and industrial interest because of their unique ability to manipulate the flow of light, a number of striking optical effects, as well as a wide range of prospective applications in sensors, displays, optical or optoelectronic devices, coatings, and security labels (18). For example, structural color materials are very promising candidates for energy-efficient reflective displays that can provide color images by reflecting the selected wavelengths of ambient illumination, instead of using active light-emitting elements (913). A variety of natural and artificial structural color materials have been reported in either specular or diffuse reflection (1417). For example, periodic dielectric structures, such as single- or multilayer thin films, photonic crystals, and diffraction gratings, usually give rise to brilliant (high saturation and brightness) and iridescent (angle dependent) structural colors that are concentrated around the specular direction of the incident light (812, 14). However, this specular reflection also poses special challenges for display applications because the structural colors can only be observed within a limited range of angles due to the anisotropy of these periodic structures (8, 9, 14). In contrast, diffuse structural colors can have wide viewing angles and are always non-iridescent (angle independent) due to the coherent scattering from isotropic structures with only short-range order, such as quasi-periodic colloidal structures (1820). Nevertheless, the lack of long-range periodicity also leads to relatively broad and weak reflection peaks, making these diffuse structural colors appear less saturated and bright for display purposes (21). In recent years, although brilliantly diffuse structural colors have received increasing attention and can be achieved by mimicking the hierarchical periodic structures in Morpho butterflies and tarantulas (2224), the fabrication of such complex multilayer structures is generally time-consuming and costly to scale up for practical applications (25). Retroreflection is a specific type of reflection in which the incident light is reflected back toward the light source. In current retroreflective materials, glass bead-type or corner cube-type retroreflectors are widely used to provide a retroreflective function, and their coloration is achieved through an undercoating layer containing pigments or dyes (2631). Incorporation of iridescent or non-iridescent structural coloration into retroreflector elements is an exciting prospect. It will expand the scope of retroreflective applications and avoid using pigments or dyes, which may gradually fade with time and cause huge pollution during production. Unfortunately, neither natural nor artificial materials that can efficiently produce retroreflective structural colors were reported. Moreover, although planar retroreflective metasurfaces were reported using carefully aligned gradient subwavelength patterns (32, 33), spectral selective retroreflection has not been realized.

Here, we report a retroreflective structural color film (RSCF) that can simultaneously display iridescent and non-iridescent vivid structural colors to observers at different viewing directions relative to the illumination direction, making it especially useful for smart traffic safety and advertisement display applications at night. In our approach, by partially embedding a monolayer array of polymer microspheres in the sticky polyacrylate layer of a transparent tape, a thin-film interference structure can be spontaneously formed at the microsphere/polyacrylate interface due to interfacial debonding. The interferometric effect together with total internal reflection (TIR) that occurs respectively on the embedded and unembedded sections of the microsphere surface can lead to the structural coloration of retroreflected light. Moreover, because of the spherical symmetry of the microsphere, iridescent or non-iridescent structural colors with wide viewing angles can be observed under coaxial or noncoaxial illumination and viewing conditions, respectively. These unprecedented optical properties allow traffic safety devices based on the proposed RSCF to serve as color-stable traffic signals for drivers and, simultaneously, as color-changing indicators for pedestrians to warn them of approaching vehicles. Furthermore, an RSCF-based display under fixed white-light illumination can present a flickering color signal that can be observed by moving observers, with no need for active displays. This type of structural color films, with unique and flexibly tuned optical properties that cannot be achieved by the existing artificial or natural materials, provides a smart and highly efficient approach for the improvement of nighttime traffic safety and may also find applications in smart advertisements, windows/coatings used in buildings, entertainment, anti-counterfeiting, and artistic decoration.

RESULTS

Fabrication of RSCF

Figure 1 illustrates the fabrication process and the properties of an RSCF assembled from 15-μm monodisperse polystyrene (PS) microspheres. In brief, a monolayer of closely packed PS microspheres is first deposited on a substrate using the colloidal assembly method (34, 35), as shown in Fig. 1A. Then, the microsphere array is transferred onto the sticky side of a transparent tape by pressing the tape onto the microsphere array (Fig. 1B) and subsequently peeling the microspheres off the substrate with the tape (Fig. 1C), resulting in a flexible and robust RSCF that can exhibit a retroreflective structural color from the tape side of the RSCF (Fig. 1D). Figure 1E presents a cross-sectional scanning electron microscopy (SEM) image of the resulting RSCF, showing 15-μm PS microspheres partially embedded in a 13-μm-thick polyacrylate adhesive layer on a 25-μm-thick polypropylene backing. The embedded depth of the microspheres is approximately 7.2 μm. Figure 1 (F and G) compares the photographs of the tape side and the microsphere side (i.e., the incident and reflected lights follow optical path I and path II, respectively, in Fig. 1E) of the RSCF under normal viewing but using different illumination conditions. Under indirect diffuse illumination (e.g., scattered light from an incandescent lamp or daylight), both the tape side and the microsphere side of the RSCF appear translucent and colorless (Fig. 1F). However, when a directional white light source [e.g., a white light-emitting diode (LED) flashlight] illuminates the RSCF at normal incidence, the tape side remarkably and surprisingly reflects a uniform and brilliant bluish green color, while the microsphere side does not reflect any color, as shown in Fig. 1G. To reveal the microscopic origin of the reflection color, a reflection optical micrograph of the RSCF from the tape side was acquired under normal illumination and viewing conditions at ×700 magnification (Fig. 1H), revealing that each PS microsphere produces a luminous bluish green circle. The color of this reflection circle is independent of the arrangement of the microspheres. As indicated by the arrow in Fig. 1H, the color of an individual microsphere is identical to that of a group of closely packed microspheres. This finding suggests that the observed bluish green reflection color in Fig. 1G is the summed result of the reflection from all microspheres. As shown by the bluish green curve in Fig. 1I, the normal reflectance spectrum of the RSCF as measured from the tape side contains two interference resonances, at 520 and 788 nm, with reflectivities of up to 37 and 42%, respectively. In contrast, the reflectivity from the microsphere side is lower than 1% throughout the visible regime (see the blue curve).

Fig. 1 Fabrication and optical properties of the proposed RSCF.

(A to D) Schematic illustration of the fabrication of an RSCF by transferring a monolayer array of PS microspheres from a substrate onto the sticky side of a transparent tape. (E) Typical cross-sectional SEM image of an RSCF assembled from 15-μm PS microspheres, showing that the microspheres are partially embedded in the polyacrylate adhesive layer of the tape. (F) Photographs of the tape side (path I) and microsphere side (path II) of the RSCF under diffuse daylight. (G) Photographs of the tape side (path I) and microsphere side (path II) of the RSCF under a white LED light source, showing a bright bluish green reflection color from the tape side. (H) Reflection optical micrograph of the RSCF observed from the tape side, showing a bluish green reflection circle from each microsphere. (I) Reflectance spectra measured from the tape and microsphere sides of the RSCF at normal incidence and reflection. Scale bars, 20 μm (E and H) and 1 cm (F and G).

Unique optical reflection properties of RSCF

The different reflection properties on the two sides of the RSCF can be attributed to the asymmetric structure of the partially embedded microsphere-tape system. At the surface of the top hemisphere of a microsphere (the hemisphere that is exposed to air), TIR can be achieved at the microsphere/air interface when the incident light follows path I in Fig. 1E (i.e., from the optically denser microsphere medium to the optically thinner air medium; see figs. S1 and S2 for further details). In this case, the light is reflected back in its direction of incidence, resulting in the retroreflection phenomenon. For incident light traveling along path II, however, the TIR condition cannot be met due to the similar refractive index distribution across the microsphere/polyacrylate interface (see section S1 for a theoretical analysis and fig. S3 for an experimental validation).

Although the TIR phenomenon can explain the single-side retroreflection behavior, the distinct resonances in the reflectance spectrum of the RSCF (Fig. 1I) suggest that the thin-film interference is the major mechanism giving rise to the observed color. We propose a previously unidentified interferometric retroreflection mechanism based on the unique structure of air-cushioned microsphere/polymer bilayer, in which microspheres are suspending on the polymer films by fibrils, as illustrated in Fig. 2A, by considering an air gap of uniform thickness formed between each PS microsphere and the sticky polyacrylate layer of the tape (fig. S4). According to the theory of particulate-filled polymer composites (3639), the formation of this air gap can be mainly attributed to the partially embedded microspheres acting as stress concentrators and thus promoting debonding at the microsphere/polyacrylate interface because of the different elastic properties of the rigid PS microspheres compared with those of the soft polyacrylate layer. The required debonding stress is expected to decrease with increasing thermal stress, increasing microsphere diameter, decreasing Young’s modulus of the polyacrylate layer, or decreasing adhesion strength between the PS and polyacrylate phases. For example, when the RSCF is cooled from the sample preparation temperature (e.g., from 25° to 20°C), the mismatch between the thermal expansion coefficients of the PS and polyacrylate phases will lead to compressive thermal stress around the microspheres, which will accelerate the debonding process. This interfacial debonding will also result in many stretched polyacrylate fibrils linking the microspheres to the polyacrylate layer, which will thus be able to support the suspension of the microspheres over the air gap. Comparison of the reflectance spectra between the freshly prepared and cooled RSCF further confirms this process (fig. S5). As illustrated in Fig. 2A, the TIR light line propagates through the air gap twice, resulting in the generation of interference-induced color. If the embedded cross-sectional plane of the microsphere is smaller than the TIR-induced circle, then the TIR light line cannot interact with the air gap (fig. S6), and consequently, the reflection color disappears. All these results validate the proposed interferometric retroreflection mechanism of an air-cushioned microsphere. Remarkably, the RSCF quickly recovers from water immersion and exhibits good spectral stability at temperatures ranging from −196° to 100°C (fig. S7), demonstrating its structural stability for practical applications.

Fig. 2 Color tunability of RSCFs.

(A) Schematic illustration of the generation of retroreflective structural color in a single air-cushioned microsphere/polymer bilayer model. (B) Photographs of RSCFs assembled with 5- to 38-μm PS microspheres under normal illumination and viewing conditions. (C to E) Schematic illustrations (C), photographs (D), and angle-resolved reflectance spectra (E) of the non-iridescent colors reflected by an RSCF assembled from 15-μm PS microspheres under coaxial illumination and viewing conditions at different viewing angles α. (F to H) Schematic illustrations (F), photographs (G), and angle-resolved reflectance spectra (H) of the iridescent colors reflected by an RSCF assembled from 15-μm PS microspheres under oblique illumination at an angle of β and at a fixed viewing angle of 0°. (I) CIE chromaticity coordinates of the RSCF samples represented in (B), (D), and (G). Scale bars, 1 cm (B, D, and G).

The color of interferometric retroreflection can be tuned by changing the diameters of the PS microspheres. As shown by the photographs in Fig. 2B, RSCFs assembled with 6- to 18-μm PS microspheres exhibit different colors under normal illumination and viewing conditions. Their corresponding Commission Internationale de l’Eclairage (CIE) chromaticity coordinates, plotted as black open circles in Fig. 2I, show that the wide color tunability can be achieved by using microspheres with different diameters. As shown in fig. S8, the effective thicknesses of air gaps for the 6- to 18-μm samples can be extracted by fitting the measured spectra. The results reveal that larger microspheres tend to have thicker air gaps as a result of the lower required debonding stresses. Because of the multiple reflection peaks at visible wavelengths, RSCFs assembled from 5-, 22-, and 38-μm PS microspheres have a white appearance. When PS microspheres with diameters below 4 μm are used as the building blocks, no obvious color can be observed due to the complete embedding of these microspheres in the sticky layer. Furthermore, the reflection colors of RSCFs can be varied by changing the microsphere material. For instance, by using poly(methyl methacrylate) (PMMA) microspheres during our RSCF fabrication, more colors can be realized, enabling more colorful displays, as will be shown later.

Figure 2 (C to H) demonstrates the unique capability of our RSCF (taking 15-μm PS microspheres as an example) to exhibit non-iridescent colors under coaxial illumination and viewing conditions (as illustrated in Fig. 2C) and iridescent colors under noncoaxial illumination and viewing conditions (as illustrated in Fig. 2F). Specifically, Fig. 2D shows photographs of the RSCF under coaxial illumination and viewing angles (with α values from 0° to 30°). The reflection color remains constant, as confirmed by the angle-resolved reflectance spectra shown in Fig. 2E. As α increases from 0° to 30°, a stable reflection peak at 520 nm can be observed. This is attributed to the spherical symmetry of the microsphere that leads to identical interferometric conditions when the illumination and viewing directions are coaxial. The corresponding CIE coordinates are located within a small bluish green region, as shown by the red dots in Fig. 2I. In contrast, Fig. 2G shows photographs acquired in the noncoaxial case, i.e., the normal reflection images of the RSCF were recorded under different incidence angles β of 1°, 5°, 10°, 13°, 15°, 20°, and 30°. One can see a striking color change, corresponding to the strongly angle-dependent reflection spectra, as shown in Fig. 2H. The corresponding CIE coordinates indicated by blue open circles in Fig. 2I range from green to purple and then back to pink as β increases from 1° to 15°. The RSCF exhibits a similar color change effect when the viewing angle is varied under a fixed illumination angle, i.e., an iridescent effect when the illumination angle and viewing angle are not fixed with each other, corresponding to the angle dependence of the thin-film interference induced by the spherical air gap. These non-iridescent and iridescent effects can also be observed in other RSCFs assembled from microspheres with different diameters and compositions (movies S1and S2), showing a wide range of color tunability, as is required for colorful retroreflective display applications.

We further compare the measured reflectance intensity (Fig. 3, A to F) and photographs (Fig. 3, G to O) of an RSCF assembled from 15-μm PS microspheres, a commercial pigment-based glass bead-type retroreflective sheeting, and the wing of a Papilio blumei butterfly. For three samples under coaxial illumination and viewing conditions, their reflection peaks remain unchanged at different viewing angles of α from 0° to 30°, and present 37, 30.5, and 2.5% of reflectivity at 0°, as indicated by the white dotted lines in Fig. 3 (A to C). Although very few biophotonic structures were also reported to show weak color retroreflection [e.g., the P. blumei butterfly (40, 41) as shown in Fig. 3G under diffuse daylight], our results reveal that the retroreflective intensity of the P. blumei butterfly is significantly lower than the other two samples. This is also confirmed by the poor visibility of the P. blumei butterfly (middle) compared to the RSCF (left) and the commercial retroreflective material (right) under coaxial illumination and viewing conditions at different angles α of 0°, 10°, 20°, and 30° (Fig. 3, H to K). Moreover, under noncoaxial condition of a normal viewing angle and different illumination angles of β from 0° to 30°, the main and secondary reflection peaks (as indicated by the white and black dotted lines in Fig. 3D, respectively) of the RSCF exhibit a red shift with increasing β, while the reflection peaks of the other two samples do not change (Fig. 3, E and F). Furthermore, the RSCF has two relatively high reflectance zones as β is in the range from 5° to 20° (as indicated by the red dotted circles in Fig. 3D); thus, the RSCF can produce iridescent colors over larger illumination-to-viewing angles than the other two samples can (Fig. 3, L to O, and movie S3). All these results demonstrate that our RSCFs we present here have more and wider practical applications than the current commercial retroreflective materials and the natural structural color films.

Fig. 3 Comparison of the reflective properties.

(A to C) Measured reflectance intensity of an RSCF (A), a commercial pigment-based glass bead-type retroreflective sheeting (B), and the wing of a P. blumei butterfly (C) under coaxial illumination and viewing conditions at different angles of α. (D to F) Measured reflectance intensity of the RSCF (D), the commercial retroreflective sheeting (E), and the P. blumei butterfly (F) under oblique illumination angle β and at a fixed viewing angle of 0°. (G) Photographs taken normal to the RSCF (left), the P. blumei butterfly (middle), and the commercial retroreflective sheeting (right) under diffuse daylight. (H to K) Photographs of the three samples taken under coaxial conditions at different viewing angles of 0° (H), 10° (I), 20° (J), and 30° (K). (L to O) Photographs taken normal to the three sample surfaces and at an oblique illumination angle of 1° (L), 5° (M), 10° (N), and 20° (O). Scale bars, 2 cm (G to O).

Large-scale and versatile fabrication of RSCF for practical applications

The RSCFs we present here are flexible and feasible for large-scale fabrication. Figure 4 (A and B) shows a 1-m-long and a 6-cm-wide RSCF, which was fabricated using 15-μm PS microspheres on commercially available transparent tape via roll-to-roll processing, as viewed under diffuse daylight and under illumination from a vehicle’s headlights at night, respectively.

Fig. 4 Nighttime traffic safety and advertisement applications of RSCFs.

(A and B) Photographs of a 1-m-long and 6-cm-wide RSCF fabricated from 15-μm PS microspheres, showing no color under diffuse daylight (A) but a bright green color under illumination by vehicle headlights at night (B). (C) A schematic model of a nighttime traffic scene in which a 60-cm triangular traffic sign fabricated from RSCFs is located on the roadside and illuminated by the headlights of a moving vehicle and a pedestrian is walking toward the traffic sign. (D to F) Photographs of the pedestrian’s view of the traffic sign when the distance L between the vehicle and the sign is 80 m (D), 50 m (E), and 30 m (F), demonstrating a smart color-changing visual indication that warns the pedestrian of the approaching vehicle. (G to I) Corresponding photographs of the driver’s view of the traffic sign at distances L of 80 m (G), 50 m (H), and 30 m (I), demonstrating that a saturated and stable color signal is presented to the driver. (J0 to J15) Wide variety of colors formed by depositing different sizes of PS and PMMA microspheres on polyacrylate-coated plexiglass plates. The photographs were taken normal to the sample surfaces and at an illumination angle from 0° (J0) to 15° (J15) at about every 1° interval. The words and symbols of the triangle sign and the circular sign in (D) to (I) were made with different RSCFs assembled from 13- and 15-μm PS microspheres and 15-μm PMMA microspheres. The nine-triangle plates in J0 to J15 were made with 7- to 18-μm PS microspheres and 15-μm PMMA microspheres. Scale bars, 10 cm (A and B), 30 cm (D to I), and 4 cm (J0 to J15). (Photo credit: Wen Fan, Fudan University)

To demonstrate the practical applications of our RSCF, we further fabricated a traffic sign using our RSCF for nighttime traffic safety application as an example and placed it on the roadside at a height of 1.7 m. In a typical nighttime traffic scene, as illustrated in Fig. 4C, a vehicle is moving forward with its headlights on, while a pedestrian is walking toward the traffic sign. For the driver of the vehicle, his or her eyes and the headlights are oriented at approximately the same angle to the traffic sign (i.e., coaxial illumination and viewing conditions). Thus, as the vehicle approaches the sign, the driver can see a luminous and stable colorful traffic signal at distances of 80, 50, and 30 m, as evidenced by the photographs shown in Fig. 4 (G to I), respectively. In contrast, for the slowly moving pedestrian, the angle between the observer’s line of sight and the axis of the headlight illumination of the traffic sign varies with the position of the moving vehicle. This angle increases slowly when the vehicle is far from the sign and more rapidly as the vehicle comes closer to the sign. Thus, the pedestrian can see a nearly constant color signal when the vehicle is far away but a dynamically changing color signal when the vehicle is close by (see Fig. 4, D to F, for the colors of the traffic sign when the vehicle is 80, 50, and 30 m away, respectively). Road traffic accidents are one of the top 10 causes of death worldwide, and half of all road traffic deaths occur among pedestrians, cyclists, motorcyclists, and other vulnerable road users (42). Thus, this color change can function as a smart visual indicator or warning signal for pedestrians to actively avoid collision with the approaching vehicles, especially for a pedestrian who is hearing impaired or wearing earphones or a headset. These unique simultaneous non-iridescent (to drivers) and iridescent (to pedestrians) signaling effects can also be exploited in the development of cylindrical reflective delineators, which are widely used as safety warning devices (fig. S9 and movie S4). This intelligent RSCF can also be used as an electricity-free roadside billboard to provide attractive dynamic color advertising information to pedestrians by using the headlights of passing vehicles, but to provide stable color advertising information to drivers without unnecessarily distracting drivers’ attention from the road (fig. S10 and movie S5).

This RSCF can also be fabricated by precoating a sticky layer on one side of a transparent substrate. Figure 4 (J0 to J15) shows the photographs of an RSCF-based puzzle fabricated from nine polyacrylate-coated triangle plexiglass plates (4 cm in length and 5 mm in thickness) and different sizes of PS and PMMA microspheres (7- to 18-μm PS microspheres and 15-μm PMMA microspheres). These nine plates exhibit a variety of colors including violet, orange, blue, sky blue, yellow, bluish green, magenta, green, and orange-red under normal illumination and viewing conditions (Fig. 4J0), and change their colors in response to different angles of incident light from 1° to 15° at about every 1° interval (Fig. 4, J1 to J15). Such color-changing features can be implemented in current nighttime traffic signs/signals/advertisements to enable unique smart display functionalities.

In accident-prone areas, active lighting and traffic signals are required to deliver warnings or alert information to drivers and pedestrians. Flickering and color-changing signals, which usually require advanced control circuits and systems, are widely used for this purpose. Here, we demonstrate a smart traffic signal prototype using an RSCF display with no need for active control circuits or systems. As illustrated in Fig. 5A, a white light LED lamp is fixed next to the prototype speed limit sign as the light source. As fast-moving drivers pass by, their viewing angles change rapidly, resulting in a flickering colorful speed limit display, as shown in Fig. 5 (B to F). This effect is attributed to the angle-dependent oscillations of the reflection peaks with a changing viewing angle, similar to that shown in Fig. 2H. The flickering frequency is proportional to the observer’s speed (movie S6). Therefore, this passive flickering colored image can serve as an interactive warning signal to road users to enhance their awareness of speed limits or other road hazards and accidents.

Fig. 5 Flickering RSCF display at night.

(A) Schematic model of an interactive speed limit sign fabricated from RSCFs. (B to F) Photographs showing the flickering effect of a normally illuminated speed limit sign viewed from the side window of a moving vehicle that is passing by the sign from right to left. The words and symbols of the sign were made with different RSCFs assembled from 6-, 7-, 8-, 9-, 10-, 13-, 15-, 15.5-, and 18-μm PS microspheres and 15-μm PMMA microspheres. Scale bars, 10 cm (B to F). Photo credit: Wen Fan, Fudan University.

CONCLUSION

In summary, we have demonstrated a conceptually new type of RSCF that exploits a unique interferometric retroreflection mechanism of air-cushioned microsphere/polymer bilayer model. Neither synthetic nor natural structural color materials nor commercial retroreflective products reported to date have the unusual characteristics of our RSCF, i.e., a vivid color retroreflectivity, iridescent/non-iridescent color effects with wide viewing angles under different illumination and viewing conditions, and wide color tenability, even compared with the latest work based on total internal reflection and interference in microdroplet systems (43). These unprecedented optical properties and the versatile scalable fabrication method provide a straightforward solution for various large-area applications of robust and intelligent structural color films, such as smart display screens, anti-counterfeiting labels, indicators, smart coatings, and artistic decoration. This solution is particularly useful for enabling novel smart traffic signs that can deliver color-coded signals to drivers and pedestrians at night, and nighttime advertisement displays. We can even envision manufacturing RSCFs of this type and other extraordinary materials with binary or multiple colloidal microspheres to achieve more elaborate composite architectures, which may find further applications if the colloidal microspheres or polymer layers are responsive to various stimuli.

MATERIALS AND METHODS

Materials

The monodisperse PS microspheres with diameters of 500 nm and 1, 2, 4, 5, 6, 7, 8, 9, 10, 13, 15, 15.5, 18, 22, and 38 μm were purchased from Shanghai Huge Biotechnology Co. Ltd. The monodisperse PMMA microspheres with a diameter of 15 μm were purchased from Wuxi Knowledge & Benefit Sphere Technology Co. Ltd. The transparent adhesive tape that consists of a polyacrylate adhesive layer and a polypropylene backing layer was purchased from Ningbo Deli Group Co. Ltd.

Fabrication of RSCF

To fabricate an RSCF, a monolayer array of closely arranged PS or PMMA microspheres was first deposited on a poly(dimethylsiloxane) (PDMS) substrate. Then, the sticky side of a transparent adhesive tape was gently pressed onto the monolayer array of microspheres. The resulting microsphere-contained tape was peeled off from the PDMS substrate and allowed to stand overnight at ambient temperature. In Fig. 4 (J0 to J15), the RSCFs were fabricated by spin-coating a thin layer of polyacrylate adhesive on a triangle plexiglass plate and then adhering a monolayer array of PS or PMMA microspheres on the polyacrylate layer.

SEM observation

The cross-sectional morphologies of RSCFs were observed on a Zeiss Ultra 55 SEM after gold coating.

Optical observation

The photographs and movies of RSCFs were taken with a digital camera (X-T20, Fujifilm, Japan). The micrograph in Fig. 1H was taken using an optical microscope (KH-7700, Hirox Co. Ltd., Japan) equipped with an MX-10C coaxial vertical lighting zoom lens and an OL-70II objective lens at a total magnification of ×700 in reflection mode. In Fig. 2 (B and D), a white LED ring flash was used for coaxial illumination. In Fig. 2G, a white LED flashlight was used for noncoaxial illumination. In Fig. 4 (B and D to I), a vehicle (BMW 530Li) equipped with xenon headlights was used to test the performance of RSCFs at night.

Reflectance spectra

The surface reflectance spectra in Fig. 1I were measured at normal incidence and reflection from the tape side or the microsphere side of the RSCF using a fiber optic spectrometer (PG2000-Pro-EX, Ideaoptics Technology Ltd., China) equipped with a Y-shaped coaxial optical fiber bundle. The angle-resolved reflectance spectra in Fig. 2 (E and H) and the reflectance intensity in Fig. 3 (A to F) were measured using an angle-resolved spectrometer (R1, Ideaoptics Technology Ltd., China).

SUPPLEMENTARY MATERIALS

Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/5/8/eaaw8755/DC1

Section S1. Analysis of TIR condition

Section S2. Microscopic characterization and analysis

Section S3. Analysis of air-cushioned microsphere/polymer bilayer model

Fig. S1. TIR condition.

Fig. S2. Influence of the embedded depth/diameter ratios of PS microsphere on the reflectance spectra of RSCFs assembled from 15-μm PS microspheres.

Fig. S3. Control experiments with 15-μm PS microspheres partially or completely embedded in the polyacrylate layer.

Fig. S4. Analysis of PS microsphere/polyacrylate interface.

Fig. S5. Comparison of the reflectance spectra between the freshly prepared (ambient temperature: 25°) and the overnight stored RSCF (ambient temperature: 20°) assembled from 15-μm PS microspheres.

Fig. S6. Control experiments with 15-μm PS microspheres shallowly embedded in a thinner polyacrylate layer.

Fig. S7. Spectral stability of the RSCF assembled from 15-μm PS microspheres.

Fig. S8. Measured and simulated reflectance spectra of RSCFs.

Fig. S9. RSCF-based cylindrical reflective delineators on a curved road.

Fig. S10. RSCF-based billboard for drivers and pedestrians.

Movie S1. The non-iridescent colors of RSCFs.

Movie S2. The iridescent colors of RSCFs.

Movie S3. Demonstration of the large illumination-to-viewing angle of RSCF in the noncoaxial case.

Movie S4. RSCF-based cylindrical reflective delineators seen by a driver or a pedestrian.

Movie S5. RSC-based billboard seen by a driver or a pedestrian.

Movie S6. RSC-based interactive warning indicator.

This is an open-access article distributed under the terms of the Creative Commons Attribution-NonCommercial license, which permits use, distribution, and reproduction in any medium, so long as the resultant use is not for commercial advantage and provided the original work is properly cited.

References and Notes

Acknowledgments: Funding: We appreciate the financial support provided for this research by the National Key Research and Development Program of China (2017YFA0204600) and the National Natural Science Foundation of China (51721002 and 51673045). Q.G., D.J., and H.S. appreciate the financial support provided by the U.S. NSF (ECCS1507312 and CMMI1562057). Author contributions: L.W., Q.G., J.Z., and W.F. conceived the concept and designed the research. J.Z. and W.F. conducted the experiments. L.W. proposed the air-cushioned microsphere/polymer bilayer model. Q.G., D.J., H.S., L.S., and W.L. conducted the simulations. L.W., Q.G., J.Z., and W.F. designed and conducted the optical applications. L.W., Q.G., W.F., J.Z., and D.J. wrote the manuscript. All authors discussed the results and commented on the manuscript. Competing interests: L.W., J.Z., and W.F. are authors on a patent application related to this work, filed with the State Intellectual Property Office of the P. R. China (application no. 201711202613.3; filed on 20 November 2017). L.W., J.Z., W.F., Q.G., D.J., and H.S. are authors on another patent application related to this work (U.S. application no. 16/175787; filed on 30 October 2018). The authors declare no other competing interests. Data and materials availability: All data needed to evaluate the conclusions in the paper are present in the paper and/or the Supplementary Materials. Additional data related to this paper may be requested from the authors.
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