Particulate Fluids Processing Centre, School of Chemistry, University of Melbourne, VIC 3010, Australia.
J Phys Chem B. 2010 Sep 2;114(34):11010-6. doi: 10.1021/jp105618q.
Various fundamental properties of acoustic cavitation bubbles have been investigated in single- and dual-frequency sound fields. It was found that the relative extent of bubble coalescence in the dual-frequency field correlated strongly with the synergistic enhancement of the sonochemical reaction rates. Both the relative extent of coalescence and the sonochemical synergy observed were enhanced through the addition of coalescence-inhibiting solutes. This was attributed to greater nucleation in the dual-frequency mode compared with the single-frequency modes, producing a very localized and high-density bubble field. The acoustic bubble size, compared with that measured at 355 kHz alone, was found to increase upon the application of synchronous 20 kHz pulses but was reduced dramatically when the low frequency was applied as a continuous wave. This trend is consistent with previous reports indicating that the bubble density and cavitation activity are relatively higher in the pulsed system and that the continuous wave application exerts a strong cancellation effect. The changes in bubble density and coalescence rates are proposed to govern the acoustic bubble size. The bubble lifetime was found to be longer in the dual-frequency field (>0.30 ms; >6 low-frequency oscillations, >100 high-frequency oscillations) compared with both single-frequency fields (0.26 ms and 5 oscillations for the low frequency; 0.22 ms and 75 oscillations for the high frequency). The confluence of a longer bubble lifetime and more asymmetric collapse conditions, the latter inferred from a more pronounced sodium atom emission in the sonoluminescence spectrum, resulted in a lower bubble collapse temperature measured in the dual-frequency system.
在单频和双频声场中研究了声空化泡的各种基本性质。研究发现,双频声场中泡的相对聚并程度与声化学反应速率的协同增强密切相关。通过添加聚并抑制剂,可以增强聚并程度和超声协同作用。这归因于与单频模式相比,双频模式下的成核更多,从而产生了非常局部化和高密度的泡场。与单独在 355 kHz 下测量的声空化泡尺寸相比,发现施加同步 20 kHz 脉冲时会增加,但当低频作为连续波施加时会急剧减小。这种趋势与先前的报告一致,表明在脉冲系统中泡的密度和空化活性相对较高,而连续波的应用会产生强烈的抵消效应。泡密度和聚并速率的变化被认为控制着声空化泡的尺寸。与单频场(低频为 0.26 ms 和 5 次振荡,高频为 0.22 ms 和 75 次振荡)相比,双频场中的泡寿命更长(>0.30 ms;>6 次低频振荡,>100 次高频振荡)。较长的泡寿命和更不对称的崩溃条件的融合,后者从声致发光光谱中更明显的钠原子发射推断得出,导致在双频系统中测量到的泡崩溃温度更低。