UCL Mechanical Engineering, University College London, London, United Kingdom.
UCL Mechanical Engineering, University College London, London, United Kingdom.
Ultrason Sonochem. 2022 Aug;88:106091. doi: 10.1016/j.ultsonch.2022.106091. Epub 2022 Jul 6.
This paper interrogates the intersections between bubble dynamics and classical nucleation theory (CNT) towards constructing a model that describes intermediary nucleation events between the extrema of cavitation and boiling. We employ Zeldovich's hydrodynamic approach to obtain a description of bubble nuclei that grow simultaneously via hydrodynamic excitation by the acoustic field and vapour transport. By quantifying the relative dominance of both mechanisms, it is then possible to discern the extent to which viscosity, inertia, surface tension and vapour transport shape the growth of bubble nuclei through non-dimensional numbers that naturally arise within the theory. The first non-dimensional number Φ/Φ is analogous to the Laplace number, representing the balance between surface tension and inertial constraints to viscous effects. The second non-dimensional number δ represents how enthalpy transport into the bubble can reduce nucleation rates by cooling the surrounding liquid. This formulation adds to the current understanding of ultrasound bubble nucleation by accounting for bubble dynamics during nucleation, quantifying the physical distinctions between "boiling" and "cavitation" bubbles through non-dimensional parameters, and outlining the characteristic timescales of nucleation according to the growth mechanism of bubbles throughout the histotripsy temperature range. We observed in our simulations that viscous effects control the process of ultrasound nucleation in water-like media throughout the 0-120 °C temperature range, although this dominance decreases with increasing temperatures. Enthalpy transport was found to reduce nucleation rates for increasing temperatures. This effect becomes significant at temperatures above 30 °C and favours the creation of fewer nuclei that are larger in size. Conversely, negligible enthalpy transport at lower temperatures can enable the nucleation of dense clusters of small nuclei, such as cavitation clouds. We find that nuclei growth as modelled by the Rayleigh-Plesset equation occurs over shorter timescales than as modelled by vapour-dominated growth. This suggests that the first stage of bubble nuclei growth is hydrodynamic, and vapour transport effects can only be observed over longer timescales. Finally, we propose that this framework can be used for comparison between different experiments in bubble nucleation, towards standardisation and dosimetry of protocols.
本文探讨了气泡动力学与经典成核理论（CNT）之间的交叉点，旨在构建一个模型，以描述空化和沸腾极值之间的中间成核事件。我们采用 Zeldovich 的流体力学方法来获得描述气泡核的方法，这些气泡核通过声场的流体动力学激发和蒸汽传输同时生长。通过量化这两种机制的相对主导地位，就可以通过理论中自然出现的无量纲数来辨别粘度、惯性、表面张力和蒸汽传输如何通过非尺寸数来影响气泡核的生长。第一个无量纲数Φ/Φ 类似于拉普拉斯数，代表表面张力和惯性对粘性效应的约束之间的平衡。第二个无量纲数δ表示进入气泡的焓传输如何通过冷却周围的液体来降低成核速率。这种表述通过考虑成核过程中的气泡动力学，通过无量纲参数量化“沸腾”和“空化”气泡之间的物理区别，并根据整个 histotripsy 温度范围内气泡的生长机制概述成核的特征时间尺度，增加了对超声气泡成核的当前理解。我们在模拟中观察到，在 0-120°C 的温度范围内，粘性效应对水相似介质中的超声成核过程起控制作用，尽管这种主导地位随着温度的升高而降低。发现焓传输会随着温度的升高而降低成核速率。这种效应在温度高于 30°C 时变得显著，并有利于形成尺寸较大的较少核。相反，在较低温度下可忽略不计的焓传输可以使小核的密集簇核（例如空化云）成核。我们发现，如瑞利-普莱塞特方程所模拟的核生长发生在比蒸汽主导生长更短的时间尺度上。这表明气泡核生长的第一阶段是流体力学的，只有在较长的时间尺度上才能观察到蒸汽传输的影响。最后，我们提出该框架可用于不同实验之间的气泡成核比较，以实现协议的标准化和剂量测定。
