Behaviour of a bubble cluster in an ultrasound field

Ultrasound medical applications, such as an ultrasound imaging with micro-bubble contrast agents, and high intensity focused ultrasound (HIFU) therapy have attracted much attention in recent years. These applications have a close relation to the motion of micro-bubbles, so that it is essential to understand their dynamics. The bubble motion is influenced by the internal phenomena and its interaction with the surrounding medium, such as the thermal diffusion, the mist formation, the mass diffusion, the heat and mass transfer through the bubble wall. It is important that the oscillation of micro bubbles generates a strong acoustic pressure for these medical applications. The strong pressure also appears in the case of a bubble cluster, like a cloud cavitation. As these dynamics of bubbles are strongly influenced by the thermal phenomena inside them, it is necessary to construct the model taking these phenomena into account to analyse the behaviour of the bubble cluster precisely. Thus, the following effects are considered: the evaporation and condensation of the liquid at the bubble wall, the heat transfer through the bubble wall, and the compressibility of the liquid. Then the spherical bubble cluster is numerically simulated. When the frequency of the ultrasound is sufficiently high, the bubble cluster hardly oscillates. On the contrary, when the frequency of the ultrasound is at the resonance of the bubble cluster, the pressure wave generates the shock wave and it focuses to the cluster centre. As a result, the pressure inside the bubble at the cluster centre becomes much higher than that of a single bubble. Though this extreme high pressure causes the severe cavitation erosion, it is thought that this high energy concentration has the potential to be utilized for medical applications. Copyright (C) 2004 John Wiley Sons, Ltd.