The effects of surfactant on the multiscale structure of bubbly flows
The effects of surfactant on the multiscale structure of bubbly flows
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DOI：
10.1098/rsta.2008.0023
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发表时间：
2008-06
期刊：
影响因子：
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通讯作者：
S. Takagi;T. Ogasawara;Y. Matsumoto
中科院分区：
文献类型：
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作者：
S. Takagi;T. Ogasawara;Y. Matsumoto
众所周知，受污染水中的气泡上升速度比纯净水中的气泡上升速度慢得多，而且受污染系统中的上升速度可能不到净化系统中的一半。这一现象是由表面活性物质吸附在气泡表面引起的所谓的Marangoni效应解释的。换言之，当气泡上升时，表面活性物质沿气泡表面存在一个表面浓度分布，这是因为吸附的表面活性物质通过平流从前部卷走并在后部积累。由于表面活性物质在气泡的后部积累，表面张力沿表面出现变化，从而在气泡表面产生切向剪应力。这种剪应力导致了污染液体中气泡上升速度的降低。更有趣的是，这种Marangoni效应不仅影响气泡的上升速度，而且还影响气泡在平均切变存在时的横向移动。这些影响共同导致了整个泡状流结构的剧烈变化。在本文中，我们讨论了与泡状流结构的这种剧烈变化有关的一些实验结果。结果表明，在表面活性剂浓度一定的条件下，在向上的气泡槽流中观察到气泡聚集现象。随着浓度的增加，这种团簇消失。我们用作用于表面活性剂水溶液中气泡的升力来解释这一现象。结果表明，作用在1 mm污染气泡上的剪切力可以比作用在清洁气泡上的剪切力小得多。
It is well known that a bubble in contaminated water rises much slower than one in purified water, and the rising velocity in a contaminated system can be less than half that in a purified system. This phenomenon is explained by the so-called Marangoni effect caused by surfactant adsorption on the bubble surface. In other words, while a bubble is rising, there exists a surface concentration distribution of surfactant along the bubble surface because the adsorbed surfactant is swept off from the front part and accumulates in the rear part by advection. Owing to this surfactant accumulation in the rear part, a variation of surface tension appears along the surface and this causes a tangential shear stress on the bubble surface. This shear stress results in the decrease in the rising velocity of the bubble in contaminated liquid. More interestingly, this Marangoni effect influences not only the bubble's rising velocity but also its lateral migration in the presence of mean shear. Together, these influences cause a drastic change of the whole bubbly flow structures. In this paper, we discuss some experimental results related to this drastic change in bubbly flow structure. We show that bubble clustering phenomena are observed in an upward bubbly channel flow under certain conditions of surfactant concentrations. This cluster disappears with an increase in the concentration. We explain this phenomenon by reference to the lift force acting on a bubble in aqueous surfactant solutions. It is shown that the shear-induced lift force acting on a contaminated bubble of 1 mm size can be much smaller than that on a clean bubble.