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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.

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.

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.

Recently, there has been considerable interest in heat storage materials. Among them, sodium acetate trihydrate (melting point: 58 °C; latent heat: 264 kg/kg) is a suitable latent heat storage material for recovering and using exhaust heat in a temperature range lower than 100 °C, which is discharged in homes and factories. The purpose of this study was to investigate the heat storage and release behavior of sodium acetate trihydrate in a thin-vessel heat storage unit (160 mm × 130 mm × 4.8 mm) made of glass and polycarbonate. The sodium acetate trihydrate in the unit was heated or cooled by flowing water in water channels (width: 1 mm) disposed on both sides of the heat storage unit. From the experimental results, performance deterioration was evident in the heat storage and release process of sodium acetate trihydrate due to phase separation during heat storage and supercooling during heat release. Therefore, a thickener and a nucleating agent were added to sodium acetate trihydrate to suppress phase separation and supercooling effects. The results revealed that the thickener and nucleating agent were unevenly distributed in the case of repeated heat storage and release. The heat transfer numerical simulation adopting the lumped capacitance model was confirmed to be useful for investigating the melting behavior of sodium acetate trihydrate.

This paper deals with the characteristics of a straight pipe inner flow of the nanosuspension that has non-Newtonian viscosity. The parameters are set in the mass composition of 10–20 mass% of dispersoid and in the temperature range of 20–70 ℃ that includes the melting point of tetracosane. The experimental study is carried out by using the flow pressure loss measurement apparatus. The test section that made of a stainless straight pipe has an inner diameter 8 mm, and it has the length 1000 mm. The inner flow of a straight pipe is occurred by using a pump that is controlled by an inverter. The pressure drop is measured by a differential pressure gauge. The measured pressure loss is used for the calculation of the flow friction loss coefficient and the pump power. The experimental data are evaluated by the variation of the pressure loss coefficient with Reynolds number that defined by the non-Newtonian behavior. Viscosity data by previous study data that correlated by the power law method are used for Reynolds number calculation. The viscosity including non-Newtonian characteristics had been estimated by using a rotary viscosity meter. The measuring ranges are shear rate

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.

Microencapsulated phase change material (MPCM) slurries are energy-carrier fluids. Their latent heat utilization has been investigated by a new method based on the temperature properties of the phase-change fluid, in which match of the working and phase-change temperature ranges is considered. Based on the working-temperature range (3–15 °C) in heat-exchange circulation, the dynamic heat-transfer characteristics and utilization of the MPCM slurry with the core material n-docosane (C22H46) are discussed. The experimental results indicate that the effects of the stirring rate and flow rate on the dynamic heat-transfer characteristics are similar to those of water, and they have little influence on utilization of the 10% MPCM slurry, whereas the corresponding effect of the concentration is relatively large. When investigating the effect of the concentration, it was found that a low phase-change ratio suppressed movement of the phase-change-temperature range owing to the residual solid core material acting as a crystal nucleus, while the heat-transfer performance and latent heat utilization of the MPCM slurry suppressed each other. The 10% MPCM slurry is the most suitable energy carrier fluid, but supercooling still needs to be resolved. The experimental results provide a reference for practical efficient application of MPCM slurries as energy carrier fluids.

We carry out a comparative analysis of experimental results and numerical simulations on the melting of the Phase Change Material tetracosane contained within a cube and heated from below. Simulations employ a Stefan number Ste = 0.65, Prandtl number Pr = 54, and Rayleigh number covers a range up to 108. The simulations show distinct regimes of the melting process: (i) conductive regime, (ii) stable growth regime, (iii) coarsening regime, (iv) turbulent regime. We show how the solid/liquid interface easily observed in the experiment is enough to identify these regimes and so the internal state of the velocity and temperature fields.

The hollow inside of a carbon nanotube (CNT) has great potential not only for flow rate enhancement of nanocapillary, but also for a material container which can be applied for drug delivery and nanoparticle infusion. However, these applications focus on after liquid infusion into CNTs, whereas the understanding of the filling process is still limited. We conducted capillary filling experiments using individual open-ended CNTs, which were stuck into an ionic liquid and visualized by scanning transmission electron microscopy. The results showed that the meniscus stopped inside the CNT, which is not predicted by the Lucas–Washburn equation. To explain this discrepancy, the intermolecular force between the liquid and CNT inner wall was proposed to provide an additional friction force. In addition, voids were observed in the liquid inside the CNT. The generation mechanism of voids was proposed to be induced by the instability of the thin liquid layer along the CNT inner surface caused by the advance of the th...

Natural convection heat transfer from an upwards-facing horizontal heated surface above which a heated cylindrical pipe was placed was estimated experimentally. The purpose of this study was to clarify the effects of the basic parameters on the flow characteristics with a cylindrical heater, and to determine available factors to enhance the heat transfer from the horizontal heater. The parameters estimated were the distance between the horizontal heater and the cylindrical heater, and the size and heat flux of each heater. Flow characteristics have three distinct patterns: no reversed flow, reversed flow that reaches the horizontal heater, and reversed flow that does not reach the heater. Heat transfer was enhanced using the cylindrical heater. In the cases of no reversed flow and reversed flow not reaching the horizontal heater, the heat transfer enhancement increased with the modified Rayleigh number of the cylindrical heater. In the range of low numbers, the reversed flow reaching the heater provided heat transfer enhancement.

Today, latent heat storage technology has advanced to allow reuse of waste heat in the middle-temperature range. This paper describes an approach to develop a latent heat storage system using middle-temperature waste heat (~100oC - 200oC) from factories. Direct contact melting and solidification behavior between a heat-transfer fluid (oil) and a latent heat storage material mixture were observed. The mixture consisted of mannitol and erythritol (Cm = 70 mass %, Ce = 30 mass %) as a phase-change material (PCM). The weight of the PCM was 3.0 kg and the flow rate of the oil, foil, was 1.0, 1.5, or 2.0 kg/min. To decrease the solidified height of the PCM mixture during the solidification process, a perforated partition plate was installed in the PCM region in the heat storage vessel. PCM coated oil droplets were broken by the perforated partition plate, preventing the solidified height of the PCM from increasing. The solidification and melting processes were repeated using metal fiber. It was found that installing the metal fiber was more effective than installing the perforated partition plate to prevent the flow out problem of the PCM.

There have been conflicting reports as to whether surface wettability is effective in the freezing delay enhancement of attached water droplets. It is an important problem in the development of anti-icing surfaces needed for applications, such as aircraft wings and infrastructures. Here, we prepared precooled ambient conditions and surfaces which included smooth, microstructured, and two nanostructured surfaces with hydrophobic coatings to create an environment closer to the actual environment and to avoid frost formation, which enhances wetting transition and nucleation. Static and dynamic wetting characteristics of each surface were investigated as the fundamental properties and the freezing behavior of precooled water droplets were observed. A distinct elongation of the freezing delay time was observed for droplets on nanostructured surfaces which have static contact angles >150°, in contrast to those on smooth and microstructured surfaces. However, the difference in droplet adhesion induced by nanostructures showed a negligible impact on freezing delay. These results indicated that the reduction of the actual contact area between the solid and liquid phases restricted ice nucleus formation.

This paper deals with the output improvement of heating and cooling cycle by using the work-fluid including phase change material. The experimental study is carried out by heat exchange between work-fluid and heat transfer surface. The work-fluid is flown to a high temperature or a low temperature heat transfer surface from the narrow path. In order to increase the amount of the heat transmission, a trace of Diethylether (boiling point 34.8 °C), as a phase change material (PCM), is added to the work-fluid. The parameters of the experiment are additive amount of PCM, the rotational speed of the displacer piston and the temperature of heat transfer surface. It is clarified that the increasing of engine cycle output is brought by the PCM addition. The effect of PCM addition is evaluated by output ratio which is defined from the experimental cycle output data. The requirements for acquiring the increasing effect of output by adding PCM are clarified.

In this study, we investigated the fundamental melting and solidification characteristics of mannitol, erythritol, and their mixture (70 % by mass mannitol: 30 % by mass erythritol) as potential phase-change materials (PCMs) for latent heat thermal energy storage systems, specifically those pertaining to industrial waste heat, having temperatures in the range of 100–250 °C. The melting point of erythritol and mannitol, the melting peak temperature of their mixture, and latent heat were measured using differential scanning calorimetry. The thermal performance of the mannitol mixture was determined during melting and solidification processes, using a heat storage vessel with a pipe heat exchanger. Our results indicated phase-change (fusion) temperatures of 160 °C for mannitol and 113 and 150 °C for the mannitol mixture. Nondimensional correlation equations of the average heat transfer during the solidification process, as well as the temperature and velocity efficiencies of flowing silicon oil in the pipe and the phase-change material (PCM), were derived using several nondimensional parameters.

Phase change materials show great promise as a way of thermal energy storage. A number of alkanes and these mixtures with melting point below 50 °C have been investigated to develop the microcapsules. However, there have been few investigations aimed at harnessing heat from environmental temperatures around 50 °C. Here, n-tetracosane, which has a melting point of 51 °C, was successfully microencapsulated in a calcium carbonate shell through a self-assembly technique. The structural characterization was conducted by the method of Fourier transform infrared spectroscopy and energy-dispersive X-ray spectroscopy. The encapsulation ratio of the phase change material into the shell was estimated based on the mass loss of the microcapsules due to heating and the differential scanning calorimetry analysis. A maximum encapsulation ratio of 53% was achieved, and the effects of the encapsulation ratio on the effective thermal conductivity of the microcapsules were evaluated by a steady-state method. In addition, heating–cooling cycles were demonstrated in composites of the microcapsules and building plaster. Repeatable thermal energy storage and release characteristics were achieved suggesting that this material is a potential candidate material for management of high temperatures.

The three-phase contact line motion on a superhydrophobic surface through particle-laden sessile droplet evaporation was investigated. Sample surfaces with micro- and nanoscale structures were generated by various durations of chemical treatment and SiO_{2} spherical particles with different sizes were used as additives of test liquid. The contact angle and contact radius profiles were studied, and the discontinuous motion of those profiles on micro- and nanostructured hierarchical surfaces was observed, while it was not observed on a nanostructured superhydrophobic surface. Suspensions with low particle concentration induced a relatively large contact radius jump compared to the high-concentrated condition; in contrast, the previous report showed the opposite trend for flat surfaces. In order to explain this result, a simple explanation was provided-that the stacked particles at the contact line region suppressed to the deformation of the liquid-vapor interface near the contact line. This is confirmed by side-view images of the deposition results because the contact line region after evaporation of the dense suspension showed a large contact angle compared to that of the diluted suspension. In addition, deposition at the contact line region was observed by scanning electron microscopy to discuss the effect of the characteristic length scale of the surface structure and particles on the contact line motion. We believe that these results will help one to understand the deposition phenomenon during particle-laden droplet evaporation on the superhydrophobic surface and its applications such as evaporation-driven materials deposition.