Yusuke Asakuma

Influence of Microwaves on the Water Surface Tension

In this study, microwave irradiation was applied to hanging droplets of both water and ethylene glycol. Once the irradiation had ceased and the droplet was allowed to return to its original temperature, it was found that the surface tension of ethylene glycol returned to its original value. In contrast, the water surface tension remained well below its original value for an extended period of time. Similar observations have been reported for magnetically treated water, but this is the first time that such a lasting effect has been reported for microwave irradiation. The effect can be attributed to the unique hydrogen bonds of interfacial water molecules. While the irradiation intensities used in this study are well above those in household devices, there is certainly the potential to apply the methodology to industrial applications where the manipulation of surface tension is required without the use of chemical addition.

Nucleation of ammonium salts in aqueous solution by high pressure

We demonstrate a new apparatus for optical observation under high pressure conditions with 4-way optical glasses. We use the proposed apparatus to study solubility of ammonium salts in aqueous solutions and to measure the nucleation pressure for different concentrations of salts in the solution. Despite the fact that the solubility of electrolytes usually increases as the pressure increases, it is found that the solubility of ammonium salts decreases as the pressure increases. In both cases, high pressure can be used towards a crystallization process. Finally, we observe high pressure induced nucleation of electrolytes in an aqueous solution.

Determination of liquid viscosity at high pressure by DLS

The movement of particles with a size smaller than few microns is governed by random Brownian motion. This motion causes the fluid to flow around the particles. The force acting upon Brownian particles as well as their velocities are measured by using the dynamic light scattering (DLS) technique. It provides the relationship between fluid shear stress and shear rate over the Brownian particle and determines the viscosity properties of the fluid. In this study, we propose a new rheometer which is widely applicable to fluid viscosity measurements at both normal and high pressure levels for Newtonian and non- Newtonian fluids.

Surface tension profiles of nanofluid containing surfactant during microwave irradiation

Manipulation of the surface tension is useful in improving heat and mass transfer performances of nanofluids in thermal systems. In our previous study, the effect of microwave irradiation on the reduction of surface tension of nanofluids (Fe2O3) was found even after it was turned off. In this study, a synergistic effect of microwave irradiation and surfactant addition (SDS) was investigated to obtain further surface tension reduction of nanofluid. Experimental results indicate that surfactant addition is effective for wider particle number density in reducing surface tension, and the reduction level strongly depends on the surfactant concentration. On the other hand, effect of the number density on the surface tension reduction is less significant for the same concentration of surfactant. From the obtained data, a combination of microwave irradiation and surfactant addition shows potential to be used as a promising method to manipulate surface tension of nanofluids.

Promotion of nucleation for nano-particle formation by two-stage microwave irradiation

Microwave has been widely used for nano-particle synthesis because rapid growth and mono-dispersed particle size can be obtained. In our previous work, it was found that bubble formation during the irradiation is greatly affected by particle size and suspension density. Nevertheless, the underlying mechanism has not been clearly understood, especially pertinent to superheat behavior caused by the higher power when colloidal particle of ferric hydroxide was produced by heating ferric chloride solution under the irradiation. In this study, to prevent superheat behavior, two-stage irradiation was proposed in nano-particle formation process. Based on in-situ measurement data, such as the profiles of bubble size, final particle size and brightness of scattering light of suspension, it is evident that nucleation of nano-particle is promoted by higher power of the first irradiation. As a result, particle number density became higher, and then microwave absorbance energy was evenly distributed to each particle. Due to suppression of heat generation in a particle, bubble size became smaller. Two-stage irradiation became more advantageous in obtaining smaller particle than continuous irradiation because lower power of the second irradiation prevents superheat behavior.

Fine particle formation by microwave irradiation: Prevention of superheat behavior by addition of alcohol

Microwave has been often used for nano-particle synthesis because rapid growth and mono-dispersed particle size are obtained. However, superheat behavior causes larger bubble and disorders temperature distribution when higher power of microwave is irradiated. Accordingly, superheat must be prevented for stable operation of nano-particle process. In previous study [1], we found that bubble formation happened during particle formation at the same time when FeCl3 aqueous solution is heated by microwave. According to the results through in-situ particle size measurement during the irradiation, superheat behavior occurred at the condition of the higher power and lower suspension density. In this study, ethylene glycol with lower electric constant was added to promote particle nucleation and to enlarge particle number density. Moreover, the solute is not dissolved in ethylene glycol as anti-solvent although ethylene glycol is dissolved in water. The results showed that bubble size during the irradiation and particle size after the irradiation became smaller by the ethylene glycol addition. As a result, operation by higher power became possible, and higher particle number density was obtained as synergy effect of anti-solvent effect and quick thermal response by microwave. Finally, addition of alcohol became a good method for nano-particle formation process to prevent superheat behavior during higher power microwave irradiation.

Mechanism on bubble size formation by two-stage microwave irradiation

In previous study, it was found that bubble sizes in dispersion medium were controlled by two-stage microwave irradiation. These results indicated that particle number density was the most important factor for the maximum bubble size when PSL particle, which is not active for microwave, was used. In this study, to understand the mechanism of controlling bubble size and to find optimum operation of microwave, the effect of the particle number density and surfactant addition on bubble profiles was investigated under microwave. For example, in the case of lower particle number density of magnetic particle, bubble size obtained by two-stage irradiation became larger than that of continuous mode. Power of first irradiation of two-stage irradiation was higher because bubble nucleation was caused by the quick thermal response of microwave. Accordingly, because heat generation of magnetic particle became larger, the much energy of microwave absorbance was used for the evaporation. However, as the particle number density was higher, two-stage irradiation became more influential for getting fine bubbles. Finally, two-stage irradiation should be a useful tool for nano-particle formation process because fine bubble was obtained in suspension with magnetic particle.

Convection and surface tension profiles for aqueous droplet under microwave radiation

Application of microwave irradiation for chemical processes, such as emulsification and polymerization, has been reported [1,2]. Surfactant free emulsion can be produced with the help of microwave irradiation. Surface tension is an important property for the industrial process such as foaming/defoaming, wetting/dewetting and flotation. Similarly, the interfacial tension plays crucial role in separation and mixing process of two immiscible liquids, which are important unit operations of the fundamental chemical engineering. In practice, surface and interfacial tensions are often altered by introducing surfactants. In our previous research [3,4], specific property for surface tension of water droplet with salt under microwave radiation was found. For example, lower surface tension after the radiation was measured. The formation of nano-bubble will explain this behavior. Normally, the surface tension of aqueous solution increases with the salt concentration because cation and anion collect water molecule more strongly as a solvation. However, the exact mechanism of surface tension reduction by microwave radiation is not clear. We tried not only measurement of surface tension but also convection in the droplet during microwave radiation. This study investigates the influence of microwave on surface tension of aqueous solution. Moreover, relation between the concentration, temperature and droplet shape, which are related with surface tension.

In-situ observation of convection in droplet under microwave radiation by PIV

In this study, microwave irradiation is applied to a liquid droplet and the surface tension, the circulation flow and temperature of water droplet are measured dynamically under the irradiation. The droplet was allowed to return to its original temperature after the irradiation, it was found that water surface tension remained well below its original value for an extended period of time. Surface tension reduction shown similar effect of ”impurity“ at molecular level during the microwave, and some “memory” after microwave, which might be caused by nano-bubble. On the other hand, microwave can introduce the circulation flow of higher rotation speed and will be expected to be applied for non-contact stirring method.