Elsevier BV Acta Medica Okayama 0017-9310 264 2026 Improving thermal stability of a microcavity emitter for utilization under atmospheric environment 128798 EN Kazuma Isobe Faculty of Environmental, Life, Natural Science and Technology, Okayama University Shota Morishige Graduate School of Environmental, Life, Natural Science and Technology, Okayama University Taiyo Sato Graduate School of Environmental, Life, Natural Science and Technology, Okayama University Yutaka Yamada Faculty of Environmental, Life, Natural Science and Technology, Okayama University Akihiko Horibe Faculty of Environmental, Life, Natural Science and Technology, Okayama University With the development of micro-fabrication technology, various metamaterials with controlled emission spectra have been proposed as thermal emitters. However, general metamaterials have a risk of deformations and degradation at high temperatures in atmospheric conditions, which is inconvenient for use as a thermal emitter. In this study, we propose a concept to enhance the thermal durability of microcavity-type metamaterials. Although typical microcavities are entirely composed of metal to excite the resonance of electromagnetic waves, we assessed the feasibility of a microcavity consisting of silicon with minimal metal coatings. While usual metals are oxidized at high temperatures, gold is rarely oxidized due to its chemical stability. However, the gold layer deposited on the Si substrate has the potential to melt below 400 �‹C due to the formation of an Au-Si eutectic alloy, which has a much lower melting point than pure gold. Therefore, we focused on the gold-tungsten bilayer as a suitable metal coating for the silicon microcavity, thereby preventing oxidation and melting that would otherwise influence the emission spectra of the thermal emitter. The numerical analysis ensured that the proposed microcavity exhibited electromagnetic resonance, similar to that of a microcavity entirely composed of metal, unless the metal coating was too thin. The fabricated microcavity with the gold-tungsten coating also exhibited a thermal emission within a limited wavelength range, due to the microcavity resonance. Moreover, the heating experiment revealed that the microcavity with a gold-tungsten coating maintained its emissivity even when heated to 400 �‹C, which is higher than the oxidation point of tungsten and the melting point of the Au-Si eutectic alloy. Consequently, the gold-tungsten coating would be a reasonable approach to improve the stability of the microcavity-type metamaterial at high temperatures under oxidative conditions. No potential conflict of interest relevant to this article was reported.

Wiley Acta Medica Okayama 1613-6810 2025 Droplet Transportation on Janus Harp Wires for Enhanced Fog Harvesting e06765 EN Yutaka Yamada Faculty of Environmental, Life, Natural Science and Technology, Okayama University Taku Ishikawa Graduate School of Environmental, Life, Natural Science and Technology, Okayama University Kazuma Isobe Faculty of Environmental, Life, Natural Science and Technology, Okayama University Akihiko Horibe Faculty of Environmental, Life, Natural Science and Technology, Okayama University Ensuring freshwater resources is a vital issue for human beings worldwide. Fog harvesting is one promising way to provide water from unconventional sources. However, clogging by the captured liquid depresses the fog harvesting performance. Here, a harp-shaped Janus harvesting system, which has thin wires with a superhydrophobic side facing the fog stream and a superhydrophilic back side to transport the droplets, is used to yield simultaneous fog capturing and water transport abilities. Attached droplets on the Janus wire transported along the periphery avoided clogging and enhanced the performance. The Janus system thus suppressed the increase and fluctuations of actual shade coefficients, which indicated blockage of the fog stream. This optimized the design of the harvester. Experiments using a multilayered Janus harvester demonstrated a significant enhancement compared with that constructed with mono-wettability wires. Overall, the results indicated the promise of droplet transportation on single wires for improving fog harvesting, as well as for other applications such as oil mist recovery and demulsification. No potential conflict of interest relevant to this article was reported.

American Chemical Society (ACS) Acta Medica Okayama 2574-0962 8 13 2025 Microagglomerate of VO2 Particles Packing Paraffin Wax Using Capillary Force as a Latent Thermal Energy Storage Medium 9595 9603 EN Kazuma Isobe Faculty of Environmental, Life, Natural Science and Technology, Okayama University Kaketo Yamauchi Graduate School of Environmental, Life, Natural Science and Technology, Okayama University Yutaka Yamada Faculty of Environmental, Life, Natural Science and Technology, Okayama University Akihiko Horibe Faculty of Environmental, Life, Natural Science and Technology, Okayama University This study proposed a material to retain paraffin wax with vanadium dioxide (VO2) particles as a latent thermal energy storage medium, an alternative to core–shell microcapsules containing phase change materials. VO2 microparticles, which were synthesized through a sol–gel method and annealing process, were dispersed in the oil-in-water microemulsion to obtain microagglomerates of VO2 microparticles. The average diameter of microagglomerates was 5 ƒÊm, and they retained paraffin wax at the vacancies among VO2 particles. Although the microagglomerates had no complete shells similar to core–shell microcapsules, the microagglomerates successfully trapped paraffin wax droplets without any leakage even in a high-temperature environment. It was because capillary forces acting among VO2 particles strictly prevented any leakage of paraffin waxes. The differential scanning calorimetry revealed that the microagglomerates contained only 16.5 wt % of n-octadecane, used as a paraffin wax. However, since VO2 particles can release or absorb latent heat due to their metal–insulator phase transition, the proposed microagglomerates exhibited higher thermal energy storage densities than phase change microcapsules whose shells do not show phase transitions. Moreover, the microagglomerates exhibited higher thermal conductivity than microcapsules with amorphous inorganic shells because the VO2 particles were crystallized through annealing. The proposed microagglomerate is a promising form for further improving the thermal energy storage density and thermal performance of the latent thermal energy storage medium, especially in the temperature range of 30 to 70 �‹C. No potential conflict of interest relevant to this article was reported.

American Chemical Society (ACS) Acta Medica Okayama 0743-7463 41 11 2025 Droplet Impact Behavior on Convex Surfaces with a Circumferential Wettability Difference 7640 7647 EN Taku Ishikawa Graduate School of Environmental, Life, Natural Science and Technology, Okayama University Yutaka Yamada Faculty of Environmental, Life, Natural Science and Technology, Okayama University Kazuma Isobe Faculty of Environmental, Life, Natural Science and Technology, Okayama University Akihiko Horibe Faculty of Environmental, Life, Natural Science and Technology, Okayama University Controlling the bouncing behavior of the impacting droplets is an important issue for splay cooling, icing prevention, and other applications. The bouncing behavior of impacting droplets on superhydrophobic curved surfaces and flat substrates with a wettability difference has been widely investigated, and droplets impacting these surfaces show shorter contact times than those on superhydrophobic flat surfaces and droplet transport. However, there have been few studies on the droplet impact behavior on curved surfaces with a wettability difference, where efficient droplet control could be achieved by combining the features. In the present study, droplet impact experiments were conducted using copper cylinders with different circumferential wettabilities from hydrophilic to superhydrophobic, varying the impact velocity, cylinder diameter, and rotation angle. Droplets that impacted the wettability boundary showed asymmetric deformation and moved to the hydrophilic side, owing to the driving force of the wettability difference. Moreover, the droplet behavior was classified into four types: the droplet bounced off the surface, the droplet bounced off the surface and split, the droplet attached to the surface, and the droplet attached to the surface and split. The droplet behavior was estimated by using the maximum spreading width of the droplet impacted on the flat substrate. We evaluated whether the droplets attached to the surface or bounced off the surface after impact using the Weber number and rotation angle, and the estimations were in agreement with the experimental results for cylinder diameters of 4 and 6 mm. No potential conflict of interest relevant to this article was reported.

American Chemical Society (ACS) Acta Medica Okayama 0743-7463 40 32 2024 Effect of Droplet-Removal Processes on Fog-Harvesting Performance on Wettability-Controlled Wire Array with Staggered Arrangement 16994 17000 EN Yutaka Yamada Faculty of Environmental, Life, Natural Science and Technology, Okayama University Junya Oka Graduate School of Natural Science and Technology, Okayama University Kazuma Isobe Faculty of Environmental, Life, Natural Science and Technology, Okayama University Akihiko Horibe Faculty of Environmental, Life, Natural Science and Technology, Okayama University Development of freshwater resources is vital to overcoming severe worldwide water scarcity. Fog harvesting has attracted attention as a candidate technology that can be used to obtain fresh water from a stream of foggy air without energy input. Drainage of captured droplets from fog harvesters is necessary to maintain the permeability of harp-shaped harvesters. In the present study, we investigated the effect of the droplet-removal process on the amount of water harvested using a harvester constructed by wettability-controlled wires with an alternating and staggered arrangement. Droplet transfer from hydrophobic to hydrophilic wires, located upstream and downstream of the fog flow, respectively, was observed with a fog velocity greater than 1.5 m/s. The proportion of harvesting resulting from droplet transfer exceeded 30% of the total, and it reflected more than 20% increase of the harvesting performance compared with that of a harvester with wires of the same wettability: this value varied with the adhesive property of the wires and fog velocity. Scaled-up and multilayered harvesters were developed to enhance harvesting performance. We demonstrated certain enhancements under multilayered conditions and obtained 15.99 g/30 min as the maximum harvested amount, which corresponds to 13.3% of the liquid contained in the fog stream and is enhanced by 10% compared with that without droplet transfer. No potential conflict of interest relevant to this article was reported.

The Japan Society of Mechanical Engineers and The Heat Transfer Society of Japan Acta Medica Okayama 1880-5566 19 1 2024 Radiative energy transfer via surface plasmon polaritons around metal–insulator grating: For better understanding of magnetic polariton 23-00531 EN Kazuma ISOBE Department of Advanced Mechanics, Graduate School of Natural Science and Technology, Okayama University Yutaka YAMADA Department of Advanced Mechanics, Graduate School of Natural Science and Technology, Okayama University Akihiko HORIBE Department of Advanced Mechanics, Graduate School of Natural Science and Technology, Okayama University Katsunori HANAMURA School of Engineering, Department of Mechanical Engineering, Tokyo Institute of Technology A conventional metal–insulator nanograting has the potential to transmit near-infrared thermal radiation because an electromagnetic wave is resonated in the grating structure. Surface plasmon polaritons (SPPs) take place at the interface between the metal and the insulator with boundaries at both ends. Physicists formulated the resonance frequency of the grating from the Fabry–Pérot interference between the grating thickness and the wavelength of SPPs in a short-range coupled mode. On the other hand, engineering researchers often use a lumped-element model assuming a resonant circuit consisting of an inductance of metal and a capacitance of metal-insulator-metal grating structure. Furthermore, they have considered that the resonant circuit excites a strong magnetic field independent of SPPs. This study compares each physical model and numerical simulation results, then clearly shows that all resonance frequencies and features of the circuit resonance can be described by the Fabry–Pérot interference of the SPPs in short-range coupled mode. Moreover, the estimated resonance frequencies obviously correspond to the local maxima of the transmittance of the nanograting with the various thicknesses and pitches. In this case, a strong magnetic field can be observed in the insulator layer as if it might be an isolated magnetic quantum. However, since materials show no magnetism at near-infrared frequencies, the magnetic response appears due to the contribution of SPPs. No potential conflict of interest relevant to this article was reported.

Elsevier BV Acta Medica Okayama 0017-9310 221 2024 Bayesian optimization of periodic multilayered slabs for passive absorptivity control 125047 EN Kazuma Isobe Department of Advanced Mechanics, Graduate School of Environmental, Life, Natural Science and Technology, Okayama University Tsuyoshi Yamamoto Department of Advanced Mechanics, Graduate School of Environmental, Life, Natural Science and Technology, Okayama University Yutaka Yamada Department of Advanced Mechanics, Graduate School of Environmental, Life, Natural Science and Technology, Okayama University Akihiko Horibe Department of Advanced Mechanics, Graduate School of Environmental, Life, Natural Science and Technology, Okayama University A vanadium dioxide (VO2) film grown on a titanium oxide crystal shows a metal–insulator transition at room temperature with drastically changed optical properties. A multilayered slab with a sub-micron scale VO2 film was proposed to utilize its unique properties for passive intensity control of sunlight absorption and radiative cooling. Its optimal geometries were numerically explored using the Bayesian optimization (BO) method. BO was applied for three types of multilayered slabs, those having one, two, or three isolated slabs of different widths. For each type of multilayered slab, BO could optimize geometric variables with practical calculation times considering the total number of possible combinations of variables, which is subsequently referred to as the total number of candidates. Optimization results revealed that two isolated slabs had the most suitable spectral absorptivity in both hot and cold environments. The infrared absorptivity of the double slab was kept low in cold conditions to suppress radiative cooling. However, the double slab exhibited good radiative cooling performance under hot conditions. Electromagnetic energy density surrounding the slab illustrated that metallic VO2 and gold placed in a parallel manner excited the coupled mode of surface plasmon polaritons to enhance absorptivity. Radiative cooling faded for the triple slab because each slab could couple with radiation propagating only across a portion of the cross-sectional area. Through three BO trials, improvement of the VO2 visible reflectivity was recognized as a future issue for further development of passive sunlight absorption control. No potential conflict of interest relevant to this article was reported.

AIP Publishing Acta Medica Okayama 0003-6951 123 23 2023 Water/ice mixture- and freezing-front motion in a non-isothermal liquid bridge 231601 EN Yutaka Yamada Faculty of Environmental, Life, Natural Science and Technology, Okayama University Kodai Okano School of Engineering, Okayama University Kazuma Isobe Faculty of Environmental, Life, Natural Science and Technology, Okayama University Akihiko Horibe Faculty of Environmental, Life, Natural Science and Technology, Okayama University We experimentally investigate the water/ice mixture- and freezing-front behavior in a water liquid bridge under isothermal and non-isothermal conditions. We find rapid propagation, temporary suspension, and regression of the water/ice mixture front, and finally, it merges with the freezing front when part of the liquid bridge is higher than the freezing temperature. However, freezing-front propagation follows dendritic ice formation, and a protrusion forms at the middle of the liquid bridge as long as the whole liquid bridge is lower than the freezing temperature. We explain those phenomena by quasi-stationary heat-transfer considerations. No potential conflict of interest relevant to this article was reported.

American Chemical Society (ACS) Acta Medica Okayama 0743-7463 39 44 2023 Analysis of Evaporation of Droplet Pairs by a Quasi-Steady-State Diffusion Model Coupled with the Evaporative Cooling Effect 15587 15596 EN Yutaka Yamada Faculty of Environmental, Life, Natural Science and Technology, Okayama University Kazuma Isobe Faculty of Environmental, Life, Natural Science and Technology, Okayama University Akihiko Horibe Faculty of Environmental, Life, Natural Science and Technology, Okayama University Multidroplet evaporation is a common phase-change phenomenon not only in nature but also in many industrial applications, including inkjet printing and spray cooling. The evaporation behavior of these droplets is strongly affected by the distance between neighboring droplets, and in particular, evaporation suppression occurs as the distance decreases. However, further quantitative information, such as the temperature and local evaporation flux, is limited because the analytical models of multidroplet evaporation only treat vapor diffusion, and the effect of the latent heat transfer through the liquid–vapor phase change is ignored. Here, we perform a numerical analysis of evaporating droplet pairs that linked vapor diffusion from the droplet surface and evaporative cooling. Heat transfer through the liquid and gas phases is also considered because the saturation pressure depends on the temperature. The results show an increase in the vapor concentration in the region between the two droplets. Consequently, the local evaporation flux in the proximate region significantly decreases with decreasing separation distance. This means that the latent heat transfer through the phase change is diminished, and an asymmetrical temperature distribution occurs in the liquid and gas phases. These numerical results provide quantitative information about the temperature and local evaporation flux of evaporating droplet pairs, and they will guide further investigation of multiple droplet evaporation. No potential conflict of interest relevant to this article was reported.

Springer Science and Business Media LLC Acta Medica Okayama 0195-928X 44 5 2023 Synthesis and Characterization of Silica-Encapsulated n-Tetracosane and the Effect of Surface Modification by Silane Coupling Agents 69 EN Kyosuke Okuno Graduate School of Natural Science and Technology, Okayama University Kazuma Isobe Graduate School of Natural Science and Technology, Okayama University Akihiko Horibe Graduate School of Natural Science and Technology, Okayama University Yutaka Yamada Graduate School of Natural Science and Technology, Okayama University Microencapsulation of n-tetracosane, whose melting point is approximately 50 degrees C, in a silica shell has been performed through the sol-gel method using tetraethyl orthosilicate (TEOS) as the precursor for silica-shell formation. Additionally, two types of silane coupling agents were used to modify the surface of the microcapsules to change the wettability. The morphology of the microcapsules was observed by scanning electron microscopy. The chemical composition was characterized by Fourier transform infrared spectroscopy. The results confirmed the presence of n-tetracosane and silica in the synthesized microcapsules. Wettability analysis showed hydrophobic and hydrophilic features because of the added silane coupling agents. From the results of differential scanning calorimetry measurements, the encapsulation ratio of the microcapsules increased with decreasing TEOS/n-tetracosane ratio, and the highest encapsulation ratio was 87.1 % at a TEOS/n-tetracosane ratio of 0.25. The pH in the microcapsule solution was affected by addition of a silane coupling agent, and shifting the pH to the basic side lowered the encapsulation ratio owing to enhancement of silica condensation. After 100 differential scanning calorimetry cycles, there was no significant degradation in the phase-change temperatures and enthalpies, which confirmed the good phase-change stability and repeatability. Therefore, the microcapsules are a potential material for thermal-energy-storage systems to effectively utilize energy. No potential conflict of interest relevant to this article was reported.

Royal Society of Chemistry (RSC) Acta Medica Okayama 2046-2069 12 22 2022 Droplet motion on a wrinkled PDMS surface with a gradient structural length scale shorter than the droplet diameter 13917 EN Yutaka Yamada Graduate School of Natural Science and Technology, Okayama University Kazuma Isobe Graduate School of Natural Science and Technology, Okayama University Akihiko Horibe Graduate School of Natural Science and Technology, Okayama University Droplet transportation using a wettability gradient surface has attracted much attention owing to applications such as in microfluidic devices. A surface with a spatial structural gradient was prepared through a simple and cost-effective process even though understanding of droplet behavior on the structure was still limited. Here, we report impinging droplet motion on a gradient wrinkled surface. Surfaces were prepared through hard film deposition on soft pre-strained polydimethylsiloxane (PDMS) with a mask installed with a slit to control the amount of deposition, which is related to the wavelength of the wrinkles. Droplets were impinged with varying position with respect to the structure, and the droplet motion was observed in the direction away from the region under the slit. We found an asymmetric contact angle and alternate motion on both sides of the three-phase contact line during the motion according to the gradient of the wrinkle wavelength. These results may help not only to understand the behavior of droplet impingement on a gradient structural surface but also to further develop applications using directional droplet transfer. No potential conflict of interest relevant to this article was reported.

Springer Science and Business Media LLC Acta Medica Okayama 0195-928X 43 3 2022 Absorptivity Control Over the Visible to Mid-Infrared Range Using a Multilayered Film Consisting of Thermochromic Vanadium Dioxide 44 EN Kazuma Isobe Graduate School of Natural Science and Technology, Department of Mechanical Engineering, Okayama University Minoru Tomioka Faculty of Engineering, Department of Mechanical Engineering, Okayama University Yutaka Yamada Graduate School of Natural Science and Technology, Department of Mechanical Engineering, Okayama University Akihiko Horibe Graduate School of Natural Science and Technology, Department of Mechanical Engineering, Okayama University Vanadium dioxide (VO₂) is a phase transition material that exhibits metallic or insulating characteristics depending upon its temperature. In this study, a multilayered film consisting of ₂, silicon dioxide (SiO₂) and gold was proposed as a metamaterial that switches its absorptivity over a broad wavelength range depending on the ambient temperature as a fundamental element of a building pigment. At high temperatures, the multilayer showed a high absorptivity at mid-infrared wavelengths, promoting radiative cooling. Simultaneously, the multilayer presented a low absorptivity in the visible and near-infrared wavelengths, enhancing sunlight absorption. The daily average heat flux can possibly be suppressed in summer in comparison with a gray body whose emissivity was 0.8. Conversely, at a lower temperatures, the multilayer showed opposite absorptivity in both the mid-infrared and visible ranges, and its daily average heat flux increased in winter. The metal–insulator phase transition of VO₂ caused a drastic shift of the resonant wavelength related to surface phonons and surface plasmons at an infrared wavelength, and optical interference at a visible wavelength, originating at the interface of the SiO₂ layer. Thus, the radiative heat flux for both sunlight absorption and radiative cooling was simultaneously controlled depending on the temperature of VO₂. No potential conflict of interest relevant to this article was reported.

American Chemical Society (ACS) Acta Medica Okayama 1944-8244 13 29 2021 Wettability Difference Induced Out-of-Plane Unidirectional Droplet Transport for Efficient Fog Harvesting 35079 35085 EN Yutaka Yamada Graduate School of Natural Science and Technology, Okayama University Eiji Sakata Graduate School of Natural Science and Technology, Okayama University Kazuma Isobe Graduate School of Natural Science and Technology, Okayama University Akihiko Horibe Graduate School of Natural Science and Technology, Okayama University Securing freshwater resources is a global issue for ensuring sustainable development. Fog harvesting is attracting great attention as a method to collect water without any energy input. Previous reports that were inspired by insects and plants have given insights such as the effectiveness of in-plane wettability and structural differences for droplet transport, which might enhance artificial water harvesting efficiency. However, further efforts to transfer droplets while maintaining performance are needed because droplet motion owing to these effects is limited to the in-plane direction. In this study, we report droplet transport between three-dimensional copper wire structures with nanostructured hydrophobic and superhydrophilic features. This mechanism enhanced the fog harvesting capability by more than 20% compared with the cumulative value of individual wires. In addition, the relationship between the droplet height and spacing of wires affected the performance. Our results show the importance of out-of-plane directional droplet transport from the wire surface assisted by differences in wire wettability, which minimizes limiting factors of fog harvesting including clogging and droplet shedding. Furthermore, the proposed arrangement reduces the overall system width compared with that of a two-dimensional arrangement while maintaining the amount of harvested water. These results provide a promising approach to designing large-scale and highly efficient fog harvesters. No potential conflict of interest relevant to this article was reported.