Smith W R, Wang Q X
School of Mathematics, University of Birmingham, Edgbaston, Birmingham B15 2TT, UK.
School of Mathematics, University of Birmingham, Edgbaston, Birmingham B15 2TT, UK.
Ultrason Sonochem. 2023 Aug;98:106473. doi: 10.1016/j.ultsonch.2023.106473. Epub 2023 Jun 10.
Rectified diffusion has wide and important applications in sonochemistry, ultrasonic cleaning and medical ultrasound. Recent experimental results have demonstrated that the addition of surfactant substantially enhances bubble growth rate. As a hypothesis, this was widely attributed to the acoustic microstreaming and mass transfer resistance caused by surfactants. In this research, the effects of the surfactant of sodium dodecyl sulphate on the rectification have been simulated by considering only the variation of the surface tension coefficient due to the surfactant. The computations are carried out using a newly developed tractable model based on the multi-scale method and the method of matched asymptotic expansions, which allows the prediction of bubble growth taking place over millions of oscillation cycles. The rate of bubble growth observed in the experiments is accurately predicted by our computations, for a range for bulk surfactant SDS concentrations less than or equal to 2.4 mM. Contrary to the widely held hypothesis in the published literature, this has demonstrated that the dominant physical mechanisms remain the shell and area effects in this range of bulk surfactant concentrations. The further enhancement of bubble growth rate provided by either acoustic microstreaming or the resistance to mass transfer is only evident at higher bulk surfactant concentrations. Therefore, the role of surface tension in rectified diffusion for aqueous surfactant solutions is more significant than previously understood. The new results also show that the bubble growth rate is sensitive to small changes in the bubble radius which may account for its unpredictability in applications of sonochemistry.
整流扩散在声化学、超声清洗和医学超声领域有着广泛而重要的应用。最近的实验结果表明，添加表面活性剂可显著提高气泡生长速率。作为一种假设，这被广泛归因于表面活性剂引起的声微流和传质阻力。在本研究中，仅考虑表面活性剂引起的表面张力系数变化，模拟了十二烷基硫酸钠表面活性剂对整流的影响。计算使用了一种新开发的易于处理的模型，该模型基于多尺度方法和匹配渐近展开法，能够预测数百万次振荡周期内的气泡生长情况。对于本体表面活性剂SDS浓度小于或等于2.4 mM的范围，我们的计算准确预测了实验中观察到的气泡生长速率。与已发表文献中广泛持有的假设相反，这表明在该本体表面活性剂浓度范围内，主要物理机制仍然是壳层效应和面积效应。只有在较高的本体表面活性剂浓度下，声微流或传质阻力对气泡生长速率的进一步增强才会明显。因此，表面张力在水性表面活性剂溶液整流扩散中的作用比以前所理解的更为显著。新结果还表明，气泡生长速率对气泡半径的微小变化很敏感，这可能解释了其在声化学应用中的不可预测性。
