Fast Dual-Stimuli-Responsive Dynamic Surface Wrinkles with High Bistability for Smart Windows and Rewritable Optical Displays

Smart Wrinkled Interfaces: Patterning, Morphing, and Coding of Polymer Surfaces by Dynamic Anisotropic Wrinkling

In contrast to traditional static surfaces, smart patterned surfaces with periodical and reversible morphologies offer limitless opportunities for encoding surface functions and properties on demand, facilitating their widespread application as functional building blocks in various devices. Advances in intelligently controlling the macroscopic properties of these smart surfaces have been accomplished through various techniques (such as three-dimensional printing, imprint lithography and femtosecond laser) and responsive materials. In contrast to the sophisticated techniques above, dynamic anisotropic wrinkling, taking advantage of dynamic programmable manipulation of surface wrinkling and its orientation, offers a powerful alternative for fabricating dynamic periodical patterns due to its spontaneous formation, versatility, convenient scale-up fabrication, and sensitivity to various stimuli. This review comprehensively summarizes recent advances in smart patterned surfaces with dynamic oriented wrinkles, covering design principles, fabrication techniques, representative types of physical and chemical stimuli, as well as fine-tuning of wrinkle dimensions and orientation. Finally, advanced applications of these smart patterned surfaces are presented, along with a discussion of current challenges and future prospects in this rapidly evolving field. This review would offer some insights and guidelines for designing and engineering novel stimuli-responsive smart wrinkled surfaces, thereby facilitating their sustainable development and progressing toward commercialization.

Synthesis and Characterization of New Chiral Smectic Four-Ring Esters

Orthoconic antiferroelectric liquid crystals (OAFLCs) represent unique self-organized materials with significant potential for applications in photonic devices due to their sub-microsecond switching times and high optical contrast in electro-optical effects. However, almost all known OALFCs suffer from low chemical stability and short helical pitch values. This paper presents the synthesis and study results of two chiral AFLCs, featuring a four-ring structure in the rigid core and high chemical stability. The mesomorphic properties of these compounds were investigated using polarizing optical microscopy and differential scanning calorimetry. Spectrometry and electro-optical studies were employed to estimate the helical pitch, tilt angle, and spontaneous polarization of the synthesized compounds and the prepared mixtures. All studied compounds exhibit enantiotropic chiral smectic mesophases including the SmA*, the SmC*, and a very broad temperature range of the SmCA* phase. Doping top-modern antiferroelectric mixture with synthesized compounds offers benefits such as increased helical pitch and tilt angle values without significantly influencing spontaneous polarization. This allows the prepared mixture to be regarded as an OAFLC with high optical contrast, characterized by an almost perfect dark state. These valuable physicochemical and optical properties suggest significant potential of studied materials for practical applications.

Transfer-Printing Hydrogel-Based Platform for Moisture-Driven Dynamic Display and Optical Anti-Counterfeiting

Humidity-responsive materials offer a promising approach to achieving tunable metasurface systems due to their fast and reversible swelling responses to moisture, which enables many important applications, such as real-time humidity sensing, optical switches, dynamic displays, and optical information encryption. However, the humidity-responsive structural coloration generally cannot provide a high spatial resolution and requires a complex patterning process. Here, we present a scalable moisture-driven color-changing Fabry-Pérot (FP)-like cavity composed of a polyvinyl alcohol layer sandwiched between an upper gold nanoparticles assembly and a bottom gold mirror. Through nanoparticle contact printing, we pixelated these cavities with sub-micrometer sizes without crosstalk and achieved an ultrahigh display resolution of ∼400 nm. Meanwhile, these nanoparticle-based FP (NBFP) cavities exhibit more vibrant colors than those of conventional film-based ones due to broadband absorption of the disordered nanoparticle assembly. Moreover, the NBFP cavities exhibit a rapid response (<300 ms), benefiting from the membrane pores formed in the gaps between the adjacent nanoparticles. Finally, we demonstrated the applications of the NBFP cavities in optical anti-counterfeiting and dynamic multi-color printing. These results suggest that our approach will help to realize a colorful, fast, and ultrahigh-resolution dynamic display device in optical security and colorimetric sensing.

Wrinkled Interfaces: Taking Advantage of Anisotropic Wrinkling to Periodically Pattern Polymer Surfaces

Periodically patterned surfaces can cause special surface properties and are employed as functional building blocks in many devices, yet remaining challenges in fabrication. Advancements in fabricating structured polymer surfaces for obtaining periodic patterns are accomplished by adopting "top-down" strategies based on self-assembly or physico-chemical growth of atoms, molecules, or particles or "bottom-up" strategies ranging from traditional micromolding (embossing) or micro/nanoimprinting to novel laser-induced periodic surface structure, soft lithography, or direct laser interference patterning among others. Thus, technological advances directly promote higher resolution capabilities. Contrasted with the above techniques requiring highly sophisticated tools, surface instabilities taking advantage of the intrinsic properties of polymers induce surface wrinkling in order to fabricate periodically oriented wrinkled patterns. Such abundant and elaborate patterns are obtained as a result of self-organizing processes that are rather difficult if not impossible to fabricate through conventional patterning techniques. Focusing on oriented wrinkles, this review thoroughly describes the formation mechanisms and fabrication approaches for oriented wrinkles, as well as their fine-tuning in the wavelength, amplitude, and orientation control. Finally, the major applications in which oriented wrinkled interfaces are already in use or may be prospective in the near future are overviewed.

Novel Green Reversible Humidity-Responsive Hemiaminal Dynamic Covalent Network for Smart Window

Recently, smart windows have attracted widespread attention on account of their unique features, yet traditional smart windows still rely on external energy support to accomplish dynamic reversible switching, which not only confines usage but also causes waste of energy. For this purpose, we have prepared hemiaminal dynamic covalent network (HDCN) film with outstanding flexibility and strength by a simple and low-cost method, in which the modulus is 206.28 MPa and the elongation at break is 39.02%. Additionally, the transition from a transparent to an opaque state is achieved when the film is stimulated by humidity, and the dynamic transformation of the film to different phases of transparency is obtained when the film is exposed to different relative humidities (60-99%). Most importantly, HDCN film fulfills the modern green requirements and enables complete dissolution in a certain mildly acidic solution, avoiding environmental pollution when the material is discarded due to loss of function. The dynamic tunability of HDCN film demonstrates great advantages and potential in smart windows and anticounterfeiting.

Smart Polymer Surfaces with Complex Wrinkled Patterns: Reversible, Non-Planar, Gradient, and Hierarchical Structures

This review summarizes the relevant developments in preparing wrinkled structures with variable characteristics. These include the formation of smart interfaces with reversible wrinkle formation, the construction of wrinkles in non-planar supports, or, more interestingly, the development of complex hierarchically structured wrinkled patterns. Smart wrinkled surfaces obtained using light-responsive, pH-responsive, temperature-responsive, and electromagnetic-responsive polymers are thoroughly described. These systems control the formation of wrinkles in particular surface positions and the reversible construction of planar-wrinkled surfaces. This know-how of non-planar substrates has been recently extended to other structures, thus forming wrinkled patterns on solid, hollow spheres, cylinders, and cylindrical tubes. Finally, this bibliographic analysis also presents some illustrative examples of the potential of wrinkle formation to create more complex patterns, including gradient structures and hierarchically multiscale-ordered wrinkles. The orientation and the wrinkle characteristics (amplitude and period) can also be modulated according to the requested application.

Review on Mechanoresponsive Smart Windows: Structures and Driving Modes

The growing awareness about the global energy crisis and extreme weather from global warming drives the development of smart windows market. Compared to conventional electrochromic, photochromic, or thermochromic smart windows, mechanoresponsive smart windows present advantages of simple construction, low cost, and excellent stability. In this review, we summarize recent developments in mechanoresponsive smart windows with a focus on the structures and properties. We outline the categories and discuss the advantages and disadvantages. Especially, we also summarize six unconventional driving modes to generate mechanical strain, including pneumatic, optical, thermal, electric, magnetic, and humidity modes. Lastly, we provide practical recommendations in prospects for future development. This review aims to provide a useful reference for the design of novel mechanoresponsive smart windows and accelerate their practical applications.

Surface Wrinkling for Flexible and Stretchable Sensors

Recent advances in nanolithography, miniaturization, and material science, along with developments in wearable electronics, are pushing the frontiers of sensor technology into the large-scale fabrication of highly sensitive, flexible, stretchable, and multimodal detection systems. Various strategies, including surface engineering, have been developed to control the electrical and mechanical characteristics of sensors. In particular, surface wrinkling provides an effective alternative for improving both the sensing performance and mechanical deformability of flexible and stretchable sensors by releasing interfacial stress, preventing electrical failure, and enlarging surface areas. In this study, recent developments in the fabrication strategies of wrinkling structures for sensor applications are discussed. The fundamental mechanics, geometry control strategies, and various fabricating methods for wrinkling patterns are summarized. Furthermore, the current state of wrinkling approaches and their impacts on the development of various types of sensors, including strain, pressure, temperature, chemical, photodetectors, and multimodal sensors, are reviewed. Finally, existing wrinkling approaches, designs, and sensing strategies are extrapolated into future applications.

Photothermal Dual Passively Driven Liquid Crystal Smart Window

Photochromic or thermochromic liquid crystal (LC) smart windows have attracted wide attention due to their spontaneous transmittance modulation under different environments. There remains a challenge for the LC smart windows that can be modulated with light and temperature simultaneously owing to the difficulty in selecting photothermal molecules. Herein, we selected a photothermal molecule, isobutyl-substituted diimmonium borate (IDI), which shows excellent characteristics of a photothermal material used in smart windows, such as transparency in the visible light range with a slight brown color, good compatibility with the LC system, and excellent photothermal effect compared with common photothermal materials. Thus, a photothermal dual-driven smart window is developed by doping IDI into chiral LC mixtures, which can efficiently modulate the transmittance at different temperatures (or light intensities) by varying the phase state from the homeotropically oriented smectic phase (transparent) to the focal conic cholesteric phase (opaque). The transmittance is high (70%) when the ambient temperature is low and the light intensity is weak, allowing more sunlight to enter the room. The transmittance is low (20%) when the ambient temperature is high and the light intensity is strong, which prevents sunlight from entering the room. The proposed smart window will have a promising application in terms of energy saving and personalized privacy protection.

Dynamic multifunctional devices enabled by ultrathin metal nanocoatings with optical/photothermal and morphological versatility

Smart devices characterized by micro-/nanotopographies, such as cracks, wrinkles, folds, etc., have been fabricated for widespread application. Here, with the combination of multiscale hierarchical architecture, ultrathin metal nanocoatings with high optical/photothermal tunability and morphological versatility, and surface/interface engineering, a set of multifunctional devices with multistimuli responsiveness was fabricated. These devices can adapt to external stimuli with reversible and instantaneous responses in optical signals, which include strain-regulated light-scattering properties, photothermal-responsive wrinkled surface coupled with moisture-responsive structural color, and mechanically controllable light-shielding properties. The structural designs that rationally overlay micro-/nanostructured ultrathin nanocoatings with other elements are the key to realize this advanced system, which provides avenues for designing versatile, tunable, and adaptable multifunctional devices.

Inspired by the intriguing adaptivity of natural life, such as squids and flowers, we propose a series of dynamic and responsive multifunctional devices based on multiscale structural design, which contain metal nanocoating layers overlaid with other micro-/nanoscale soft or rigid layers. Since the optical/photothermal properties of a metal nanocoating are thickness dependent, metal nanocoatings with different thicknesses were chosen to integrate with other structural design elements to achieve dynamic multistimuli responses. The resultant devices demonstrate 1) strain-regulated cracked and/or wrinkled topography with tunable light-scattering properties, 2) moisture/photothermal-responsive structural color coupled with wrinkled surface, and 3) mechanically controllable light-shielding properties attributed to the strain-dependent crack width of the nanocoating. These devices can adapt external stimuli, such as mechanical strain, moisture, light, and/or heat, into corresponding changes of optical signals, such as transparency, reflectance, and/or coloration. Therefore, these devices can be applied as multistimuli-responsive encryption devices, smart windows, moisture/photothermal-responsive dynamic optics, and smartphone app–assisted pressure-mapping sensors. All the devices exhibit high reversibility and rapid responsiveness. Thus, this hybrid system containing ultrathin metal nanocoatings holds a unique design flexibility and adaptivity and is promising for developing next-generation multifunctional devices with widespread application.

Deformable Thermo-Responsive Smart Windows Based on a Shape Memory Polymer for Adaptive Solar Modulations

Thermo-responsive smart windows that control solar transmission are expected to be the promising solution to excessive building energy consumption and overheating of solar cell devices. The two performance indices, namely, the luminous transmission (T_lum) and the solar modulation (ΔT_sol), are often intrinsically limited by conventional thermo-responsive materials, which restrict their applications in smart windows. Alternatively, constructing a deformable surface morphology of smart windows can be an effective strategy to modulate the solar transmission. Here, we report a new category of thermo-responsive smart windows with a deformable surface morphology, which can be custom designed to achieve both desirable ΔT_sol and T_lum according to the sunlight incident angles of actual applications. This design is based on a thermo-responsive shape memory polymer and an optical coating, which is termed the butterfly-wing-like smart window (BSW). The BSW reversibly transforms from a temporary shape of flat topography to a predefined original shape of tilted configuration upon heating. It is demonstrated that the BSW has a high ΔT_sol of 32.6% and an excellent T_lum(average) of 64.5%. This work provides a new design strategy and mechanism for thermo-responsive smart windows.

Photo-Oxidation-Controlled Surface Pattern with Responsive Wrinkled Topography and Fluorescence

Wrinkles and photo-oxidation reactions are widely found in soft materials, which are intimately associated with the failure of materials and structures. It is expected that the photo-oxidation process could also have a positive effect on the material and its surface. Here, we report the photo-oxidation of 2-(4-dietheylaminophenyl)-4,5-bis(4-methoxyphenyl) imidazole (DEA-TAI) into a wrinkled bilayer system to control surface wrinkle and fluorescent patterns, in which a supramolecular polymer network composed of carboxylic acid-containing copolymer (PS-BA-AA; PS=poly(styrene), BA=butyl acrylate; AA=acrylic acid) and DEA-TAI were used as the skin layer. Ultraviolet (UV) irradiation can induce photo-oxidation of the imidazole ring of DEA-TAI to weaken the intermolecular hydrogen bonding between PS-BA-AA and DEA-TAI, resulting in the release of stress in the bilayer system. The wrinkled morphology and fluorescence of the surface can be simultaneously regulated by photo-oxidation of DEA-TAI under UV light, and the resulting wrinkles are extremely sensitive to the pH value, which can be quickly and reversibly erased by NH₃ gas. Smart surfaces with specific hierarchical wrinkles and fluorescence can be achieved by selective irradiation with photomasks, which may find potential applications in smart displays and multi-code information storage.

Multicolor and Multistage Response Electrochromic Color-Memory Wearable Smart Textile and Flexible Display

Electrochromic materials have great application in soft displays and devices, but the application of ideal electrochromic textiles still faces challenges owing to the inconvenience of a continuous power supply. Here, electrochromic color-memory microcapsules (ECM-Ms-red, -yellow, and -blue) with a low drive voltage (2.0 V), coloration efficiency (921.6 cm² C^-1), a practical response rate (34.4 s^-1), multistage response discoloration, and good color-memory performance (>72 h) and reversibility (≥1000 cycles) are developed. The color-memory performance is controlled by the energy difference of oxidation-reduction reactions. A multicolor and multistage response electrochromic color-memory wearable smart textile and flexible display are developed that are convenient and energy-efficient for application. The design philosophy of color-memory based on controllable energy difference of reactions has great potential application in sensors and smart textiles.

Soft skin layers for reconfigurable and programmable nanowrinkles

Wrinkling skin layers on pre-strained polymer sheets has drawn significant interest as a method to create reconfigurable surface patterns. Compared to widely studied metal or silica films, softer polymer skins are more tolerant to crack formation when the surface topography is tuned under applied strain. This Mini-review discusses recent progress in mechano-responsive wrinkles based on polymer skin materials. Control over the skin thickness with nanometer accuracy allows for tuning of the wrinkle wavelength and orientation over length scales from nanometer to micrometer regimes. Furthermore, soft skin layers enable texturing of two-dimensional electronic materials with programmable feature sizes and structural hierarchy because of the conformal adhesion to the substrates. Soft skin systems open prospects to tailor a range of surface properties via external stimuli important for applications such as smart windows, microfluidics, and nanoelectronics.

Transparency and AIE tunable supramolecular polymer hydrogel acts as TEA-HCl vapor controlled smart optical material

Stimuli-responsive optical materials attract lots of attention due to their broad applications. Herein, a novel smart stimuli-responsive supramolecular polymer was successfully constructed using a simple tripodal quaternary ammonium-based gelator (TH). The TH self-assembles into a supramolecular polymer hydrogel (TH-G) and shows aggregation-induced emission (AIE) properties. Interestingly, the transparency and fluorescence of the TH-G xerogel film (TH-GF) could be reversibly regulated by use of triethylamine (TEA) and hydrochloric acid (HCl) vapor. When alternately fumed with TEA and HCl vapor, the optical transmittance of the TH-GF was changed from 8.9% to 92.7%. Meanwhile, the fluorescence of the TH-G shows an "ON/OFF" switch. The reversible switching of the transparency and the fluorescence of the TH-GF is attributed to the assembly and disassembly of the supramolecular polymer TH-G. Based on these stimuli-response properties, the TH-GF could act as an optical material and shows potential applications as smart windows or fluorescent display material controlled by TEA and HCl vapor.

Dynamic wrinkling pattern exhibiting tunable fluorescence for anticounterfeiting applications

A dynamic surface pattern with a topography and fluorescence in response to environmental stimulus can enable information recording, hiding, and reading. Such patterns are therefore widely used in information security and anticounterfeiting. Here, we demonstrate a dynamic dual pattern using a supramolecular network comprising a copolymer containing pyridine (P4VP-nBA-S) and hydroxyl distyrylpyridine (DSP-OH) as the skin layer for bilayer wrinkling systems, in which both the wrinkle morphology and fluorescence color can be simultaneously regulated by visible light-triggered isomerization of DSP-OH, or acids. Acid-induced protonation of pyridines can dynamically regulate the cross-linking of the skin layer through hydrogen bonding, and the fluorescence of DSP-OH. On selective irradiation with 450 nm visible light or acid treatment, the resulting hierarchical patterned surface becomes smooth and wrinkled reversibly, and simultaneously its fluorescence changes dynamically from blue to orange-red. The smart surfaces with dynamic hierarchical wrinkles and fluorescence can find potential application in anticounterfeiting.