Generation of micro gas bubbles of uniform diameter in an ultrasonic field
Generation of micro gas bubbles of uniform diameter in an ultrasonic field
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DOI：
10.1017/s0022112005007470
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发表时间：
2006-02
影响因子：
3.7
通讯作者：
T. Makuta;F. Takemura;E. Hihara;Y. Matsumoto;M. Shoji
中科院分区：
文献类型：
--
作者：
T. Makuta;F. Takemura;E. Hihara;Y. Matsumoto;M. Shoji
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。图像显示，当少量气体（通过针管）进入运动粘度在5 ～ 100 mm $^{2}\,{\rm s}^{-1}$之间的高粘性液体时，以恒定的周期速率产生直径为4 ～ 15 $\,\mu$ m的均匀微气泡。同时，通过改变针的内径在0.08 ～ 0.34 mm之间，激励频率在18.77 ～ 42.15 kHz之间，液体的运动粘度在5 ～ 100 mm $^{2}\,{\rm s}^{-1}$之间，表面张力在20 ～ 34 mN m $^{-1}$之间，研究了稳定产生均匀直径微气泡的条件。气体黏度在9.0 ～ 31.7 $\,\mu$ Pa s之间。结果表明：(1)在振荡气液界面上形成一个突起，突起尖端释放出微气泡；（ii）气体粘度对凸出物的形成有重要影响，为使母泡稳定振荡，气体粘度应在20.0 $\,\mu $ Pa s左右；（iii）微气泡稳定产生的条件还受激励频率、液体的表面张力和粘度、针的尺寸等因素的影响；（iv）稳定产生的两个控制参数为韦伯数（${\it We}\,{=}\,\rho {f}^{2}d_{\hbox{\scriptsize{\it in}}}^{3}/\sigma $，其中$\rho $为液体密度，$f$为激励频率，$d_{\hbox{\scriptsize{\it in}}}$为针的内径，$\sigma $为表面张力）和沃默斯利数（${\it Wo}\,{=}\,d_{\hbox{\scriptsize{\it in}}}(f/{\nu })^{1 / 2}$，其中$\nu $为液体的运动粘度）；(v)等径微气泡在${\it We}<300$和$2<$${\it Wo}<5$时稳定生成。在稳定产生直径均匀的微气泡的条件下，气泡直径几乎随针内气体压力的增加而线性增加。该线性函数的梯度可以表示为Wo We和归一化针外径的函数，并且随着针内径的减小或随着激励频率、液体表面张力和粘度以及针外径的增大而减小。
Consecutive images of the fragmentation of capillary waves in an ultrasonic field were obtained using a high-speed video camera through a microscope at a frame rate of 500000 frames per second. The images showed that micro bubbles of uniform diameter from 4 to 15$\,\mu$m were generated at a constant periodic rate when a small amount of gas was introduced (via a needle) into a highly viscous liquid whose kinematic viscosity was between 5 and 100 mm$^{2}\,{\rm s}^{-1}$. Conditions for stable generation of micro bubbles of uniform diameter were also studied by changing the inner diameter of the needle between 0.08 and 0.34 mm, excitation frequency of around 18.77 and 42.15 kHz, kinematic viscosity of liquid between 5 and 100 mm$^{2}\,{\rm s}^{-1}$, surface tension between 20 and 34 mN m$^{-1}$, and viscosity of gas between 9.0 and 31.7$\,\mu$Pa s. Results revealed that (i) a projection is formed on the oscillatory gas–liquid interface and micro bubbles are released from the tip of the projection; (ii) gas viscosity critically affects the formation of the projection and should be around 20.0$\,\mu $Pa s for stable mother bubble oscillation; (iii) conditions for stable generation of micro bubbles are also affected by excitation frequency, surface tension and viscosity of the liquid, and dimensions of the needle; (iv) two controlling parameters for stable generation are the Weber number (${\it We}\,{=}\,\rho {f}^{2}d_{\hbox{\scriptsize{\it in}}}^{3}/\sigma $, where $\rho $ is the density of the liquid, $f$ is the excitation frequency, $d_{\hbox{\scriptsize{\it in}}}$ is the inner diameter of the needle, and $\sigma $ is the surface tension) and the Womersley number (${\it Wo}\,{=}\,d_{\hbox{\scriptsize{\it in}}}(f/{\nu })^{1 / 2}$, where $\nu $ is the kinematic viscosity of liquid); and (v) uniform-diameter micro bubbles are generated stably when ${\it We}<300$ and $2<$ ${\it Wo}<5$. Under the conditions where micro bubbles of uniform diameter were stably generated, the bubble diameter increased almost linearly with increasing gas pressure inside the needle. The gradient of this linear function can be expressed as a function of Wo We, and the normalized outer diameter of the needle, and decreases either with decreasing inner diameter of the needle or with increasing excitation frequency, surface tension and viscosity of the liquid, and outer diameter of the needle.