The Bubble Dynamics And Its Line Emissions In An Ultrasound Field

As ultrasound wave propagates through liquid containing encapsulated bubbles or cavitaing bubbles, the bubble dynamics and its energy transformation from sound to light are main research topics for people. The encapsulated bubbles peopled make are used for ultrasound imaging, drug target and curing. The cavitating bubbles gen-erate from fracture of liquid irradiating by intensive ultrasound. Under this condition, bubbles may be emitted light, which is termed sonoluminescence (SL).Centered around the subject on the dynamics and physical effects of a bubble in an ultrasound field, this thesis mainly introduces our measurements of the dynamics of an encapsulated microbubble and the spectra of SL, and our model of oscillation of two interacting bubbles. A series of important conclusions have been achieved.1. We have measured the oscillation of an encapsulated micro-bubble driven by ultrasound. Using a long-distance microscope imaging system and a lock-in phase in-tegrate technique, optical measurements demonstrate that the evolutional behaviors of a phospholipid-shelled micro-bubble, a kind of ultrasound contrast agent. Near100frames of an oscillating micro-bubble for one acoustic cycle were captured. The exper-imental data of micro-bubble radii were fit by the numerical calculations of the Hoff model, Yasui model and the Rayleigh-Plesset (R-P) model, respectively. The results show the good agreement between experimental data and the theoretical calculation in the Hoff model. In addition, the spectral analysis based on the experimental data indicates that the relative intensity of the second harmonic increases with the driving pressure amplitude.2. We have measured the SL spectra in four kinds of aqueous solutions of halide salts containing no noble gas. we observed line emissions of the trivalent terbium ion (Tb3+) and sodium (Na) atom in both single bubble SL (SBSL) and multi-bubble SL (MBSL). The Tb3+line emissions from the aqueous solution containing terbium chloride (TbCl3) includes four line emissions at the central wavelengths485,540,580and615nm, respectively. And they are attributed to the Tb3+transitions from its first excited state5D4to its ground state7FJ (J=6,5,…,0). we also observed line emissions from the Na atom at the central wavelength589nm in the three kinds of aqueous solutions dissolved without noble gas, containing sodium chloride (NaCl), sodium bromide (NaBr), and sodium iodide (Nal), respectively. These observations test that the noble gas is not necessity of line emission in SL.We were able to explain the dependence between the definition of line spectrum and the ultrasound pressure by using the framework of Bohr’s theory. For a suitable driving pressure, the collapsing cavitation bubble supplies an appropriate energy for Tb3+to jump from the ground state7FJ to the first excited state5D4with high prob-ability, leading to a bright line emissions of5D4→7FJ. For a weaker driving, the Tb3+will be excited to5D4with low probability and emit dim lines, since the bubble supplies insufficient energy. However, if the driving pressure is too strong, the bub-ble supplies too much energy, increasing the probability for the transition7FJ→5D3(the second excited state). Meanwhile, the probability for the transition7FJ→5D4decreases, and the line emissions from the return (5D4→7FJ) will become dim.3. We have proposed a model describing oscillation of two interacting bubbles in a spherical acoustical field. Based on fluid dynamics, we derived a model of aspheric oscillation of two interacting bubbles by using the perturbation theory and Bernoulli equation. This model includes two parts, spherical and aspheric components. We can analyze the stable oscillation of two bubbles by calculating a2(t) and b2(t) those describe the surface deformation of bubbles1and2, respectively. In addition, the models of multi-bubbles dynamics may be proposed under the inspiration of our model.