Molecular dynamics simulations were carried out for a nanoscale spherical bubble in water at room temperature. The pressure difference between inside and outside of the bubble was investigated and the surface tension was evaluated with assumption of the Young-Laplace (Y-L) equation. The obtained surface tension shows little dependence on the bubble size and agrees with that of flat surface. Thus it is confirmed that the Y-L equation holds for nanobubbles in water. Based on the Y-L equation and the density-dependence of the liquid pressure, mechanical stability of a bubble in a finite system was discussed. The existence of mechanical instability leads to a mechanical definition of critical nucleus size in cavitation nucleation.
The shrinkage and growth of microbubbles coated by palmitic acid under variation of a pressure field are observed with a CCD camera. We investigate the influence of gas diffusion on the stability of microbubbles. It is shown that when the ambient liquid pressure increases, a tiny microbubble with a radius of about 5 μm shrinks accompanied with large surface depression, and does not return to the initial size even after the pressure is reduced. The depression suggests the formation of multilayers of surfactant on the bubble surface. A bubble model is constructed by taking the dynamic surface tension and the gas permeation resistance of surfactant layers into account. The simulations show that the hysteresis of the surface tension is an important factor in determining the bubble response to the pressure change; the bubble profiles in the experiments are simulated qualitatively by considering the hysteresis of the surface tension. Also, the simulations considering the variable permeation resistance are in good agreement with the experimental results. The results also show that the permeation resistance increases during bubble shrinkage and stabilizes the microbubble. The smaller the initial bubble radius becomes, the larger the permeation resistance becomes during bubble shrinkage. The present simulations support the experimental results that the surfactant multilayers are formed when the bubble is compressed under the pressure increase.
A mathematical model is developed to simulate the radial motion of cavitation bubbles. The heat and mass transports including phase change are formulated precisely. In order to reduce the computational cost without loss of the important thermo-fluid phenomena, two simplifications are employed: time-dependent bubble radius is described using the Rayleigh-Plesset equation; the pressure in the bubble is assumed to be uniform in space. For validation of the model, the transient radial motion of an air bubble in water is observed experimentally. A shock tube is used to make the sudden pressure reduction from atmospheric to below the saturated vapor pressure. The bubble radius is measured by high-speed photography, in which an interferomtric laser imaging technique is used for accurate determination of the initial bubble radius. The radial motion is successfully predicted by using this model. The temperature reduction at the bubble wall is a predominant factor on the bubble growth rate under superheated conditions, even if the liquid temperature is close to room temperature. The numerical result indicates that the growth rate is very sensitive to initial bubble radius, ambient pressure, and liquid temperature.
This paper describes a novel measurement technique for three-dimensional shape of micro droplet utilizing micro LIF (Laser Induced Fluorescence) technique. A measurement system consisted of microscope equipped with 10x lens, high sensitive CCD camera, mercury lamp, optical filters and so on. Calibration curve between thickness of micro droplet and emission of fluorescence dye solution was obtained by the use of the PDMS (polydimethylsiloxane) microchip, whose depth varied from 10 to 95 μm fabricated using photo lithography technique. The microchip based calibration method provides high measurement accuracy and eliminate photobleaching effect. Measurement accuracy of the present method in depth direction was about 0.57 μm assessed using laser con-focal microscope and the spatial resolution in the horizontal plane became 6.7 μm. The technique is useful to investigate a gas-liquid two-phase flow in micro scale.
Laser induced streaming is one of the interesting phenomena. When we provide a highly concentrated energy in a small region with laser beam, an induced streaming, such as micro shock wave or laser induced thermal acoustics is observed in the region. In the present paper, a micro shock wave in air which was induced by laser irradiation was observed by the optical techniques. The shock wave was generated on the metal surface due to the laser ablation. In the present experiment, we focused on a relatively low irradiation intensity region. As the laser power decreased, the plasma plume was not observed clearly on the metal target, but a strong micro shock was still induced. The trajectory of shock front was recorded with the shadowgraphic measurement and the Mach number was calculated experimentally. Alternatively, the induced density behind the shock wave was measured by speckle photographic technique. The shock Mach number and density distribution was analyzed and discussed with self-similar theory.
Effects of the relative size of a particle to the diameter of a micro-tube on velocity profiles obtained by the micro-PIV measurement of the flow through the micro-tube were investigated to achieve the accuracy of measurements of the flow in a proboscis of a mosquito. The velocity profiles obtained for different diameter-ratios of a fluorescent particle and fused silica micro-tube were compared with the exact solution of the velocity profile of flow through a circular pipe at a low Reynolds number. A suitable diameter-ratio was found to be less than 0.04. The velocity profile of flow through the proboscis of a mosquito measured by using the suitable micro-PIV method satisfying the above condition was found to be different from the exact solution of the circular-pipe flow near the center of the proboscis. The velocity profile in the proboscis was similar to that of a rectangular cross section which was verified by a SEM observation.
Flows of polymer solutions through a 3.7:1 abrupt contraction were observed in a micro- and a regular-size channels with rectangular cross sections. The test fluids are three kinds of 0.2 wt% aqueous solutions of polyacrylamide whose rheological properties are different with each other. In the previous works, apparent slips were observed on the channel wall for the polymer solutions and the slip levels of these solutions are considered to be different. In the present paper, the growth of the salient corner vortex is discussed as a function of the shear rate. Consequently, the vortex size in the microchannel is larger than that in the regular-size channel for the flow of polymer solution whose slip level is weak, whereas the vortex size in the microchannel is smaller than that in the regular-size channel for the flow of polymer solution whose slip level is strong.
A valveless micropump was realized with a diffuser/nozzle shaped channel and a variable volume actuator which produces an oscillating flow. One-way flow may be realized in the nozzle direction since the pressure loss in a nozzle channel is lower than that in a diffuser channel. Pump characteristics were measured for various angles of a diffuser/nozzle element and positions of the actuator to investigate the effect of pump geometry on characteristics. The experimental results showed an optimal diffuser/nozzle angle for pump efficiency, and the optimal actuator position. The frequency characteristics and the pump characteristics were measured. Dimensionless variables were introduced to rearrange the measured data and to understand the physical mechanisms of the micropump.
Performance of a split-and-recombine micromixer (Tan et al., 2005) is assessed by means of numerical simulation. The micromixer is designed for micro cell sorting systems using immunomagnetic beads, of which working principle is lamination of fluid layers. The fluid motion in a complex three-dimensional geometry is simulated by using the finite difference method and the motion of beads is dealt with the one-way coupling Lagrangian particle tracking method. Although the thickness of laminated layers should ideally reduce by the factor of 2n (where n is the number of unit-mixers connected in series), the present simulation with a realistic geometry shows that the thickness does not reduce that much. Relatively large unmixed regions are formed in the central region, which deteriorate the mixing performance. The size of the unmixed regions and the degree of mixing are quantitatively investigated. Based on the observation, an improved mixer arrangement is proposed. While the original arrangement proposed by Tan et al. (2005) was to simply connect identical unit-mixers, the present arrangement is to alternately connect the original unit-mixer and one with the mirrored geometry. With this modified arrangement, the unmixed region is suppressed and better mixing is achieved.
The interface characteristics in a millimeter-scale channel are investigated in order to find a technique to speed up fluid mixing. The channel is fabricated on an acrylic-resin plate, where a cavity is attached in the downstream region of a T-shaped conduit. The mixing effect is evaluated using a blue dye and a colorless liquid, which are alternately injected into the channel by two syringe pumps. The important factors for highly efficient mixing under the combination of alternate inflow and cavity are investigated. Two-dimensional and three-dimensional numerical simulations are also performed, and the results are compared those of the experiment. It is confirmed that the fractal dimension and interface length inside a cavity can be used as indicators to predict the mixing level in the downstream region, although they are not perfect. The importance of focusing on both the stretching rate of the interface and the concentration gradient where the stretching of the interface occurs is presented.
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