Claude Inserra

Feedback loop process to control acoustic cavitation.

Applications involving acoustic cavitation mechanisms, such as sonoporation, are often poorly reproducible because of the unstationary behavior of cavitation. For this purpose, this study proposes to work at a fixed cavitation level instead of a fixed acoustic intensity. A regulated cavitation generator has been developed in an in vitro configuration of standing wave field. This system implements the regulation of the cavitation level during sonication by modulating the applied acoustic intensity with a feedback loop based on acoustic measurements. The experimental setup consists of a plane piezoelectric transducer for sonication (continuous wave, frequency 445 kHz) and a hydrophone pointing to the sonicated medium. The cavitation level is quantified every 5 ms from a spectral analysis of the acoustic signal. The results show that the regulation device generates reproducible mean cavitation levels with a standard deviation lower than 1.6% in the applied intensity range (from 0.12 to 3.44 W/cm(2)), while this standard deviation can reach 76% without regulation. The feedback loop process imposes precise cavitation level even in low applied acoustic intensity.

Counterbalancing the use of ultrasound contrast agents by a cavitation-regulated system

The stochastic behavior of cavitation can lead to major problems of initiation and maintenance of cavitation during sonication, responsible of poor reproducibility of US-induced bioeffects in the context of sonoporation for instance. To overcome these disadvantages, the injection of ultrasound contrast agents as cavitation nuclei ensures fast initiation and lower acoustic intensities required for cavitation activity. More recently, regulated-cavitation devices based on the real-time modulation of the applied acoustic intensity have shown their potential to maintain a stable cavitation state during an ultrasonic shot, in continuous or pulsed wave conditions. In this paper is investigated the interest, in terms of cavitation activity, of using such regulated-cavitation device or injecting ultrasound contrast agents in the sonicated medium. When using fixed applied acoustic intensity, results showed that introducing ultrasound contrast agents increases reproducibility of cavitation activity (coefficient of variation 62% and 22% without and with UCA, respectively). Moreover, the use of the regulated-cavitation device ensures a given cavitation activity (coefficient of variation less 0.4% in presence of UCAs or not). This highlights the interest of controlling cavitation over time to free cavitation-based application from the use of UCAs. Interestingly, during a one minute sonication, while ultrasound contrast agents progressively disappear, the regulated-cavitation device counterbalance their destruction to sustain a stable inertial cavitation activity.

Numerical investigations of single bubble oscillations generated by a dual frequency excitation

The oscillations of a single bubble excited with a dual frequency acoustic field are numerically investigated. Computations are made for an air bubble in water exposed to an acoustic field with a linearly varying amplitude. The bubble response to an excitation containing two frequencies f1 = 500kHz and f2 = 400kHz at the same amplitude is compared to the monofrequency case where only f1 is present. Time-frequency representations show a sharp transition in the bifrequency case, for which the low frequency component f2 becomes resonant while the high frequency component f1 is strongly attenuated. The temporal evolution of the power spectra reveals that the resonance of the low frequency component is correlated with the time varying mean radius of the bubble. It is also observed that the total power of the bubble response in the bifrequency case can reach almost twice the power obtained in the monofrequency case, which indicates a strong enhancement of the cavitating behavior of the bubble for this specific frequency combination.

Theoretical investigation of the mechanisms involved in the modification of the cavitation threshold by multifrequency excitations

The nonlinear oscillations of a single bubble in an acoustic field are studied for both monofrequency and bifrequency excitations. An approximate solution of the Rayleigh-Plesset equation is obtained by using the method of multiple scales and is compared with direct numerical computations. Frequency-response curves show good agreement with the numerical results. For an excitation composed of two neighboring frequencies in the vicinity of the bubble primary resonance, frequency-response curves are substantially different from the monofrequency case and reveal a merging of the resonance peaks and an enhancement of the bubble response.

Numerical study of cavitation threshold by use of bifrequency excitation

Previous experiments on the influence of waveform excitation on ultrasonic cavitation threshold and activity showed that the combination of two neighbouring frequency components modifies cavitation threshold and stimulates cavitation activity beyond threshold in comparison to a classical monofrequency waveform. For high monofrequency thresholds, the bifrequency excitation reduces the threshold value. For low monofrequency thresholds, the bifrequency threshold is higher, which shows that nonlinear effects are involved in the process. In-situ measurements of the amplitudes of the difference frequency component show that this low frequency is too weak to trigger cavitation alone. Numerical simulations of the dynamics of a single bubble in a bifrequency oscillating pressure field were done. They show that the bifrequency excitation modifies the cavitation threshold for specific initial bubble radius but is not sufficient to explain experimental observations. To simulate first-order nonlinear propagation effec...