均一粒径を持つ極微細気泡生成技術の開発

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Consecutive images of the fragmentation of capillary waves in an ultrasonic field were obtained using a high-speed video camera through a microscope at a frame rate of 500,000 frames per second. The images showed that micro bubbles of uniform diameter from 4 to 15 μm were generated at a constant periodic rate when a small amount of gas was introduced into a highly viscous liquid whose kinematic viscosity was between 5 and 100 mm^2/s. Results revealed that (1) conditions for stable generation of micro bubbles are affected by excitation frequency, surface tension and viscosity of the liquid, and dimensions of the needle, (2) two controlling parameters for stable generation are the Weber number ( We = ρf^2d_<in>^3/σ, where ρ is density of the liquid, fis excitation frequency, dm is inner diameter of the needle, and a-is surface tension) and Womersley number (Wo+= d_<in>(flv)^<1/2>, where ν is the kinematic viscosity of liquid), and (3) uniform-diameter micro bubbles are generated stably when We < 300 and 2 < Wo < 5. Using a boundary element method, we also simulated this gas-liquid interface behaviour. Although the simulation model is simple because the flow field around the needle is assumed axisymmetrical and irrotational, the simulation results of the periodic interface behaviour and of the effects of liquid viscosity and gas pressure inside the needle qualitatively agreed with experimental results. Simulation and experimental results revealed that a micro gas bubble is generated by detachment of the top of a projection produced by propagation of a surface wave travelling from the outside to inside of the needle. The surface wave is produced by the difference in velocity near the needle wall, and is then propagated by surface tension. The size of the generated micro gas bubbles becomes uniform because the gas-liquid interface converges on periodic behaviour due to the influence of liquid viscosity.