抄録
Ultrasonic medical treatments attract much attention in recent years, such as sonodynamic therapy using cavitation, ultrasound imaging with micro-bubble contrast agents and so on. These phenomena have closed relation to the motion of micro bubbles, so that it is essential to understand the dynamics in micro bubles. In sonodynamic therapy, it is important that the oscillation of micro bubbles generates the high pressure and high temperature. This phenomenon appears very strongly in the case of bubble cluster like 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 analyze the behavior of bubbles strictly. Thus the following effects are considered: the evaporation and condensation of liquid at the bubble wall, heat transfer through the bubble wall, and the compressibility of liquid. Then the spherical bubble cloud in pure water is numerically simulated. Initial cloud radius is equal to 0.5mm, bubble radius is 1.7μm, void fraction is 0.1%, ambient pressure is 101.3kPa, temperature is 293K and the amplitude of ultrasound is 50kPa. The resonant frequency of bubble cloud is about 180kHz in this case. It is about 1/10 of the case of a single bubble. When the frequency of ultrasound is sufficiently high, for example the resonant frequency of the single bubble, bubble cloud hardly oscillates. On the contrary, when the frequency of ultrasound is resonant frequency of bubble cloud, the high-pressure wave with the steep rising edge focuses on the center of bubble cloud and the pressure inside the bubble reaches about 5MPa. Meanwhile the pressure reaches no more than 0.6MPa in the case of a single bubble. Though this very high pressure relates the cavitation erosion, it is thought that it can be utilized for the field of sonochemistry, such as sonodynamic therapy.
