Modeling The Dynamics Of Encapsulated Bubble In Liquid Driven By Ultrasound

In liquid, bubble whose inner gas is encapsulated by a shell is called encapsulated bubble. In real world, some bubbles naturally have or spontaneously generate encapsulated shells, and some other ones are given artifical shells for application purposes. In the field of acoustic cavitation, researches show that the surface of a sonoluminescence bubble pulsating with large amplitude is not simply a gas-liquid interface, but contains a layer of encapsulation. And a kind of artificial encapsulated bubble, which is the micro gas bubble coated by an shell usually made of albumin or lipid or polymer, is used in ultrasonic medicine. It used to act as the contrast agent which can be injected into human body for enhancing the effects of ultrasonic diagnosis and imaging. Also, it serves as a vehicle of the drug in targeted drug delivery. In recent years it is even employed into ultrasonic therapy. After being injected to the spot where tumor cells locate inside human body, it is forced to oscillate nonlinearly with large amplitude by high intensity ultrasound so that its mechanic and thermal effects can help destroying the tumor cells efficaciously. This thesis introduces our models for the dynamics of the encapsulated bubble in reality and the relevant theoretical discussions. The content can be divided into four parts.1. The encapsulated bubble dynamic model for cavitation bubble Based on the experimental results, the extremely high pressure and temperature inside a cavitation bubble at collapse may turn a thin layer of liquid surrounding the bubble into a supercritical water layer which just acts as an encapsulating shell of the cavitation bubble. So, a model for the dynamics of cavitation bubble covered with a supercritical water layer is constructed based on fluid dynamics and thermal dynamics. The numerical results show that the supercritical water layer has negative effect on bubble dynamics. Because it can apparently cool the gas temperature inside the collapsing bubble and buffer the pulsation of the bubble, which just tells that the former models for cavitation bubble may overestimate the extreme physical conditions inside a cavitation bubble.2. A model for aspheric encapsulated bubble dynamics Generally, the artificial encapsulated bubbles are not strictly spherical, such as the contrast agent bubbles. Based on fluid dynamics and elastic mechanics, an analytical model for the dynamics of aspheric encapsulated bubble driven by ultrasound is proposed. The encapsulated shell is treated as viscoelastic solid. Based on the model, the pulsation of egg-like encapsulated bubble, which is usually seen in the sight of microscope, is simulated. The results tell that the shape, especially the thickness distribution, of encapsulated bubble has important influence on bubble dynamics. Where the shell is thin, there is relatively larger pulsation amplitude.3. The localized near-field pressure of the aspheric encapsulated bubble Considering the importance of the geometrical shape of encapsulated bubble, it is easy to infer that the geometrical shape may also apparently affect the effect which an encapsulated bubble having on its external environment, that is, the practicality of an encapsulated bubble. Based on the proposed model, further discussion is put on the pressure in liquid outside an aspheric encapsulated bubble. The analytical expression of the liquid pressure is derived. It can be theoretically proved that the liquid pressures at some local places of the near field of an aspheric encapsulated bubble are extremely high. And the pressures at these places can be much larger than those at corresponding places outside a spherical encapsulated bubble at the same conditions. Based on these conclusions, a revelation is that it is possible to use a relatively weaker ultrasound drive to achieve the expected medical effects if the aspheric encapsulated bubble is used.4. A model for large-amplitude, nonlinear pulsation of encapsulated bubble In the application of high intensity therapeutic ultrasound, since the encapsulated bubble pulsates nonlinearly with large amplitude, ordinally models for encapsulated bubble dynamics are not available for this situation. So, a rigorous and practical analytical model is constructed for the large-amplitude pulsationof encapsulated bubble, where the Eulerian-Almansi finite strain tensor is introduced. This model is suitable for most kinds of encapsulated bubble whose shell is viscoelastic solid. If the maximum shear stress of shell material is known, base on this model, one can numerically estimate whether an encapsulated bubble driven by an acoustical pressure will rupture. Besides, the nonlinear frequency response of an encapsulated bubble can be easily calculated based on this model. The numerical results show that as the drive pressure amplitude increases, the resonance frequency gradually decreases, and the intensity of subharmonic resonances becomes stronger and stronger. Numerical results output by this model and the corresponding experimental observations are all in good agreements.