均一粒径を持つ極微細気泡生成技術の開発

利用高速摄像机通过显微镜，以50万帧/秒的帧率获得超声场中毛细波破碎的连续图像。实验结果表明，在运动粘度为5 ~ 100 mm^2/s的高粘性液体中注入少量气体，会以恒定的周期速率生成直径为4 ~ 15 μm的微气泡。结果表明：(1)微气泡稳定生成的条件受激发频率、液体表面张力、粘度和针尖尺寸的影响；(2)稳定生成的两个控制参数为韦伯数（在>^3/σ中We = ρf^2d_<，其中ρ为液体密度，f为激发频率，dm为针尖内径，a-为表面张力）和沃默斯利数(在>(flv)^<1/2>中Wo+= d_<，其中ν为液体的运动粘度),(3)当We < 300, 2 < Wo < 5时稳定生成等径微气泡。利用边界元方法，我们也模拟了这种气液界面行为。虽然模拟模型简单，因为假设针周围的流场是轴对称和无旋转的，但模拟结果表明，针内液体粘度和气体压力的影响与实验结果定性一致。模拟和实验结果表明，微气泡是由表面波从针的外部传播到针的内部所产生的投影的顶部脱离而产生的。表面波是由针壁附近的速度差产生的，然后由表面张力传播。由于液体粘度的影响，气液界面收敛于周期性行为，产生的微气泡的大小变得均匀。

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.