National Institute of Advanced Industrial Science and Technology (AIST), 2266-98 Anagahora, Shimoshidami, Moriyama ku, Nagoya 463-8560, Japan.
Ultrason Sonochem. 2011 Jan;18(1):92-8. doi: 10.1016/j.ultsonch.2010.03.004. Epub 2010 Mar 17.
At high ultrasound frequencies, active bubble structures are difficult to capture due to the decrease in timescale per acoustic cycle and size of bubbles with increasing frequencies. However the current study demonstrates an association between the spatial distribution of visible bubbles and that of the active bubble structure established in the path of the propagating acoustic wave. By monitoring the occurrence of these visible bubbles, the development of active bubbles can be inferred for high frequencies. A series of still images depicting the formation of visible bubble structures suggest that a strong standing wave field exists at early stages of wave propagation and weakens by the increase in the attenuation of the acoustic wave, caused by the formation of large coalesced bubbles. This attenuation is clearly demonstrated by the occurrence of a force which causes bubbles to be driven toward the liquid surface and limit standing wave fields to near the surface. This force is explained in terms of the acoustic streaming and traveling wave force. It is found that a strong standing wave field is established at 168 kHz. At 448 kHz, large coalesced bubbles can significantly attenuate the acoustic pressure amplitude and weaken the standing wave field. When the frequency is increased to 726 kHz, acoustic streaming becomes significant and is the dominant force behind the disruption of the standing wave structure. The disruption of the standing wave structure can be minimized under certain pulse ON and OFF ratios.
在高超声频率下，由于每个声循环的时间尺度减小以及气泡尺寸随频率增加而减小，因此难以捕获活动气泡结构。然而，目前的研究表明，可见气泡的空间分布与在传播声波路径中建立的活动气泡结构的空间分布之间存在关联。通过监测这些可见气泡的发生，可以推断出高频下活动气泡的发展情况。一系列描绘可见气泡结构形成的静态图像表明，在波传播的早期阶段存在强驻波场，并且随着由于形成大的聚结气泡而导致的声波衰减的增加而减弱。这种衰减通过发生的力明显地表现出来，该力会导致气泡被驱动到液体表面并将驻波场限制在表面附近。该力可以根据声流和行波力来解释。发现 168 kHz 处建立了强驻波场。在 448 kHz 时，大的聚结气泡会显著衰减声压幅度并减弱驻波场。当频率增加到 726 kHz 时，声流变得显著，并且是破坏驻波结构的主要力。在某些脉冲 ON 和 OFF 比下，可以最小化驻波结构的破坏。
