Hiroshi Kakiuchida

To enhance the durability of bio‐inspired superwetting coatings for real‐world practical applications, it is necessary to endow them with the methods of reasonable self‐healing abilities found in living organisms. Herein, we have successfully developed hydrogel‐based multifunctional superhydrophilic coatings, which mimic the multifunctionality of the fish body surface, and exhibit remarkably fast and repeatable self‐healing abilities. The aqueous precursor solutions, including poly(vinylpyrrolidone), nano‐clay particles, and a waterborne aminosilane, with and without metal‐ions were prepared by a two‐step integral blend method, followed by spin‐ or spray‐coating onto flat/large‐scale and 3D‐shaped glass substrates. The resulting coatings (approximately 470–720 nm in thickness) were all smooth, highly transparent, superhydrophilic [contact angle ( θ _S ) of water: < 5°] in air and superoleophobic ( θ _S of diiodomethane: > 170°) in water. Despite their submicron thickness, the coatings maintained anti‐fogging performance for over 6 months under high relative humidity exceeding 80%. X‐ray photoelectron spectroscopy and Fourier transform infrared spectroscopy confirmed that the addition of multivalent metal‐ions promoted hydrated ion cluster formation, thereby enhancing water absorption and self‐healing kinetics. The damaged area (approximately 20–56 µm) was completely self‐healed over nine times each within 10 s through hot steam exposure.

Many scientists would have no doubt that water droplets tend to be firmly adhered to hydrophilic surfaces. Contrary to this common belief, we have succeeded in preparing surfaces that are statically very hydrophilic but exhibit surprising water slipping performance. These surfaces were prepared through a chemisorption of ethoxy‐terminated polyethylene glycol (PEG) organosilane (Si‐PEG‐Si) and subsequent alkali‐treatment. We prepared Si‐PEG‐Si‐derived monolayers by varying Si‐PEG‐Si concentrations (10 ⁻¹ –10 ⁻⁸ mol/L), reaction time (5–48 h), and temperature (room‐temperature or 50°C) and monitored changes in static/dynamic wetting behaviors. The optimal condition (10 ⁻⁵ mol/L, 18 h and room‐temperature) achieved the best surfaces with the lowest static water contact angle ( θ _S , ≈35.4°), contact angle hysteresis (Δ θ , ≈2.6°) and sliding angle ( α , ≈8.7°). Subsequent alkali‐treatment further increased hydrophilicity ( θ _S , ≈17.7°) without any marked degradation of dynamic wetting behaviors (Δ θ , ≈2.2° and α , ≈7.3°). Based on sum‐frequency generation spectroscopy, we concluded that the water‐thin layer containing water‐swollen PEG chains worked as a self‐lubricating layer for enhancing the movement of water droplets, while increasing hydrophilicity. Moreover, Si‐PEG‐Si‐derived monolayer‐covered paper filters showed excellent filtration performance of oil/water mixtures with a stable separation‐efficiency because of their water‐driven oleophobicity, resulting in the high purity of the extracted water over multiple separation cycles.

Light‐diffusing polarizers of liquid crystal (LC)/polymer composites are fabricated via a self‐assembly process. The transmittance has a polarization extinction ratio more than ten and is controlled electro‐ or thermo‐responsively. Such stimuli responsiveness results from unidirectionally oriented, highly rotatable LCs existing in LC‐rich domains. The polarizers are photoinduced self‐organizable, electro‐driven controllable, and wavelength adjustable. The structures comprise (sub) micrometer‐long, narrow domains of LC and isotropic polymer phases, and some LCs are oriented perpendicular to the domain boundaries. Such LC orientation becomes self‐assembly ordered through photopolymerization‐induced phase separation (PPIPS), performed by nonuniform photoirradiation with direct or Fourier‐transformed speckle‐pattern projection. The polarization extinction ratio of light diffusion increases, as the compositional ratio of urethane prepolymers increases or the photoirradiation becomes more intense. LCs are found to be oriented more uniaxially in the LC‐richer (polymer‐poorer) phases. The uniaxially oriented LCs contribute to the optical polarization of light diffusion. The electro‐response time of polarization change is the order of 100 ms and microscopically caused by LC reorientation behavior in LC‐rich domains among open polymer networks. Two different light‐source wavelengths can be chosen for PPIPS by adjusting photoinitiators. The light‐diffusing polarizers extend photonic application range from the points of view of users and manufacturers.

With the recent ban on the production and use of long-chain perfluorinated compounds, the development of alternative approaches to prepare liquid-repellent surfaces that avoids the use of such compounds has become an urgent issue. We have succeeded in the development of fish-mimicking hydrophilic transparent hydrogel-based films with long-lasting anti-oil adhesion properties. Such films could be prepared by simply mixing poly(vinylpyrrolidone) (PVP), nanoclay particles (NCPs), and a waterborne aminosilane (AOS) using an integral blend (IB) method. When submerged in water, these films displayed underwater superoleophobicity (advancing and receding contact angles (CAs) of diiodomethane were ~171°/~163°) with low CA hysteresis (less than 8°), because the hydrophilic nature of the films promoted the formation of a thin layer of adsorbed water on the topmost film surfaces, similar to fish scales. Furthermore, when our films were coated onto the inside of poly(ethylene terephthalate) (PET) bottles and pre-wetted using 80 °C hot water vapors, these film surfaces could effectively repel various oils and were able to maintain their oil-repellent properties for more than 5 weeks. These water-driven, non-perfluorinated transparent hydrogel-based films are expected to increase recycling of PET bottles for oils that are generally incinerated or landfilled.

Polarized optical microscopy (POM) images of polymer network liquid crystals (PNLCs) were first analyzed using a pretrained machine learning model for feature extraction and hierarchical clustering. The analyses worked well in predicting and improving the thermoresponsive changes individually in direct luminous and hemispheric solar transmittance, both of which are crucial properties of energy-saving smart windows. The features of a 1280 × 1920–pixel color POM image were extracted by the latest pretrained algorithm, EfficientNet-B7, as a 2560-dimensional vector and then reduced into a two-dimensional space for clustering and visualization using the uniform manifold approximation and projection (UMAP) algorithm while efficiently preserving the global structures of the distance relationship in a high-dimensional space. The feature vectors in the UMAP space were correlated with the thermoresponsive transmittance and classified using hierarchical clustering analysis. The extracted features belonging to some clusters were also correlated with the fabrication parameters. The PNLCs here were produced from various raw materials under different fabrication conditions. These analyses and predictability are extensively applied to different PNLCs for stimuli-responsive optical devices, such as solar- and privacy-control windows.

Molecular interactions between liquid crystals (LCs) and reactive mesogens (RMs) at temperatures across the phase transition regions were comprehensively studied during photopolymerization-induced phase separation (PPIPS) beginning with raw mixtures until the formation of polymer network liquid crystals (PNLCs). Then, the molecules were found to be nonuniformly more and less mobile in response to temperature as PPIPS progressed. Optical birefringence and infrared absorption were carefully measured throughout PPIPS, using 4-cyano-4′-hexylbiphenyl (6CB) and 1,4bis-[4-(3-acryloyloxypropyloxy) benzoyloxy]-2-methylbenzene (RM257) as typical LCs and RMs. Microscopic views of thermoresponsive changes in the molecular orientation order of both LCs and RMs were obtained: LCs and RMs in raw mixtures interacted with one another but uniformly transformed their molecular orientation. Such interactions continuously change to become nonuniform with progress in PPIPS. At the incipient stages of PPIPS, RMs, which are polymerized but not completely networked, inhibit LCs from changing their molecular orientation and vice versa. As PPIPS progresses, some LCs become more mobile and some less mobile owing to RM constraints. The domain configuration of the submicrometer phase separation affects the thermoresponsive mobility of LCs and RMs, that is, LCs become more mobile in LC-richer areas. The quantitative knowledge here provides comprehensive insight that LCs and RMs are mutually constrained and that such interactive behavior varies nonuniformly as PPIPS progresses.

Thermoresponsive switchable window glasses were developed with meso- (submicron- to micron-) scale composites of orientation-co-ordered liquid crystals (LCs) and reactive mesogens (RMs). These structures are called polymer network liquid crystals (PNLCs). These were produced through a simple self-assembly process or photopolymerization induced phase separation (PPIPS), conducted by ultraviolet irradiation. Transparent and hazy states thermoresponsively change, caused by nematic-to-isotropic phase transition. This switchability changes backscattering intensity and consequently controls hemispherical transmittance as well as optical clarity. The PNLCs have practical applications to energy-saving smart windows because of their simple fabrication process, adaptability, and durability.

This study examined the thermal response of polymer-dispersed liquid crystal (PDLC) diffusers, patterned using a two-lens imaging system. Optical modulation was achieved by modifying the PDLC transmittance using temperature-induced changes to liquid crystal (LC) orientation. PDLCs with controllable scattering properties were obtained by irradiating LC-polymer composites with laser speckle patterns. The variation of the scattering characteristics of the PDLCs with temperature, average speckle size, and LC orientation order was analyzed to determine the most suitable parameters for a diffuser for smart window solar-ray control applications. The findings of these experiments demonstrate that using speckle patterns, a one-time laser exposure process, can provide a simple fabrication method of novel optical devices.

Surface modifications for easy removal of liquids and solids from various metal surfaces are much less established than for silicon (Si) or glass substrates. Trimethylsiloxy-terminated polymethylhydrosiloxane (PMHS) is very promising because it can be directly immobilized covalently to a wide variety of metal surfaces by simply heating neat PMHS liquid, resulting in a film showing excellent dynamic omniphobicity. However, such PMHS films are easily degraded by hydrolytic attack in an aqueous environment. In this study, we have successfully improved the hydrolytic stability of the PMHS-covered ultrasmooth metal (Ti, Al, Cr, Ni, and Cu) surfaces by end-capping of the residual Si–H groups of the PMHS films with vinyl-terminated organosilanes, for example, trimethylvinylsilane (TMVS), through a platinum-catalyzed hydrosilylation reaction. The resulting TMVS-capped PMHS film surfaces showed significantly greater stability even after submersion in water for 6 days, with their excellent dynamic dewetting behavior toward water, toluene, n-hexadecane, and ethanol changing little. In addition, they also showed reasonable anti-icing (icephobic) properties with low ice-adhesion strength of less than 50 kPa even after 20 cycles of testing at −15 °C.