Mechanisms of nuclei growth in ultrasound bubble nucleation

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 Φ12/Φ2 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

Investigation of the Effect of Ultrasound Parameters on Continuous Sonocrystallization in a Millifluidic Device

© 2016 American Chemical Society. Continuous-flow crystallization of adipic acid in a millichannel chip equipped with a piezoelectric element is presented and investigated experimentally and numerically. A single, straight channel chip (cross section: 2 mm × 5 mm, length: 76 mm) made of glass, which is ultrasonically transparent, was designed and fabricated. The piezoelectric element allows studying the effect of different ultrasound frequencies in the kHz to MHz range. Ultrasound was applied in burst mode to reduce heating; this allowed operating at higher levels of input power. To accurately control the temperature of the fluid, Peltier elements were used to cool the bottom and top surfaces of the chip. Crystallization was performed in isothermal conditions, ensuring that the temperature and in turn the supersaturation were kept uniform along the channel. The effect of ultrasound frequency and sonication time was studied. Crystal size distributions at different operating conditions were obtained by laser diffraction. The distributions were narrow, with coefficients of variation â0.5, while the mean sizes were small (â30 μm) and decreased when the sonication time increased. The crystal production rate increased by increasing the sonication time; this suggests that ultrasound enhances nucleation. On the other hand, in crystal breakage experiments, no difference in the size distribution of the seed crystals entering and leaving the device was observed, and hence, in this setup, ultrasound does not cause breakage. Numerical simulations of wave propagation in aqueous solution were utilized to predict the probability of cavitation, adopting a suitable cavitation threshold. The simulations showed that high pressure amplitudes are achievable inside the channel at low frequencies. The size range of bubbles which undergo violent collapse at different pressure amplitudes and frequencies was quantified. By increasing the frequency in the simulations, it was observed that the probability of transient cavitation decreases. The theoretical prediction of negligible transient cavitation at higher frequencies, in conjunction with the absence of crystals at such frequencies, indicates a strong link between transient cavitation and sonocrystallization

Benchmarking preconditioned boundary integral formulations for acoustics

The boundary element method (BEM) is an efficient numerical method for simulating harmonic wave scattering. It uses boundary integral formulations of the Helmholtz equation at the interfaces of piecewise homogeneous domains. The discretisation of its weak formulation leads to a dense system of linear equations, which is typically solved with an iterative linear method such as GMRES. The application of BEM to simulating wave scattering at large-scale geometries is only feasible when compression and preconditioning techniques reduce the computational footprint. Furthermore, many different boundary integral equations exist that solve the same boundary value problem. The choice of preconditioner and boundary integral formulation is often optimised for a specific configuration, depending on the geometry, material characteristics, and driving frequency. On the one hand, the design flexibility for the BEM can lead to fast and accurate schemes. On the other hand, efficient and robust algorithms are difficult to achieve without expert knowledge of the BEM intricacies. This study surveys the design of boundary integral formulations for acoustics and their acceleration with operator preconditioners. Extensive benchmarking provide valuable information on the computational characteristics of several hundred different models for multiple scattering and transmission of acoustic wave fields