Memoli Gianluca, Baxter Kate O, Jones Helen G, Mingard Ken P, Zeqiri Bajram
School of Engineering and Informatics, University of Sussex, BN1 9QJ Falmer, UK.
National Physical Laboratory, TW11 0LW Teddington, UK.
Micromachines (Basel). 2018 Aug 15;9(8):404. doi: 10.3390/mi9080404.
The acoustically-driven dynamics of isolated particle-like objects in microfluidic environments is a well-characterised phenomenon, which has been the subject of many studies. Conversely, very few acoustofluidic researchers looked at coated microbubbles, despite their widespread use in diagnostic imaging and the need for a precise characterisation of their acoustically-driven behaviour, underpinning therapeutic applications. The main reason is that microbubbles behave differently, due to their larger compressibility, exhibiting much stronger interactions with the unperturbed acoustic field (primary Bjerknes forces) or with other bubbles (secondary Bjerknes forces). In this paper, we study the translational dynamics of commercially-available polymer-coated microbubbles in a standing-wave acoustofluidic device. At increasing acoustic driving pressures, we measure acoustic forces on isolated bubbles, quantify bubble-bubble interaction forces during doublet formation and study the occurrence of sub-wavelength structures during aggregation. We present a dynamic characterisation of microbubble compressibility with acoustic pressure, highlighting a threshold pressure below which bubbles can be treated as uncoated. Thanks to benchmarking measurements under a scanning electron microscope, we interpret this threshold as the onset of buckling, providing a quantitative measurement of this parameter at the single-bubble level. For acoustofluidic applications, our results highlight the limitations of treating microbubbles as a special case of solid particles. Our findings will impact applications where knowing the buckling pressure of coated microbubbles has a key role, like diagnostics and drug delivery.
在微流体环境中，孤立的类颗粒物体的声学驱动动力学是一种已得到充分表征的现象，并且已经成为许多研究的主题。相反，尽管涂层微泡在诊断成像中广泛应用，并且在治疗应用中需要精确表征其声学驱动行为，但很少有声流体研究人员关注它们。主要原因是微泡的行为有所不同，这是由于它们具有更大的可压缩性，与未受干扰的声场（一级 Bjerknes 力）或与其他气泡（二级 Bjerknes 力）表现出更强的相互作用。在本文中，我们研究了市售聚合物涂层微泡在驻波声流体装置中的平移动力学。在不断增加的声驱动压力下，我们测量了孤立气泡上的声学力，量化了双泡形成过程中的气泡 - 气泡相互作用力，并研究了聚集过程中亚波长结构的出现。我们展示了微泡可压缩性随声压的动态表征，突出了一个阈值压力，低于该压力时气泡可被视为未涂层的。借助扫描电子显微镜下的基准测量，我们将此阈值解释为屈曲的开始，从而在单泡水平上对该参数进行了定量测量。对于声流体应用，我们的结果突出了将微泡视为固体颗粒特殊情况的局限性。我们的发现将影响那些了解涂层微泡屈曲压力起关键作用的应用，如诊断和药物递送。
