Nonlinear Dynamics of Microbubbles and Its Influence on a Tissue Surface

1.激光捕获法分析微气泡的动力学我们采用激光捕获法对微气泡的动力学进行了实验研究。主要研究结果如下：（1）测量了作用于微泡的光阱力。在110 mW的净激光功率下，垂直力约为90 pN。(2)微气泡运动轨迹的模拟结果与实验结果吻合较好。(3)我们在固体边界附近进行了激光捕获，并模拟了剪切流中气泡从激光阱中逃逸的轨迹。轨迹在很大程度上依赖于气泡半径。(4)我们研究了气体扩散和表面张力对微泡稳定性的影响。结果表明，超声微泡的表面张力比空气-水气泡小约800倍。当被捕获或合并的微泡半径大于由表面张力和溶解气体浓度确定的临界气泡半径时，气泡迅速长大.微气泡溃灭的边界元法数值分析本文发展了一种考虑内部气体传热的环形气泡动力学数值计算方法。我们将此方法应用于代表组织表面的固体壁或可变形壁附近的气泡塌陷。结果表明：（1）内部气体的传热加速了微泡的溃灭。液体射流穿过气泡表面后，在射流冲击点周围观察到极高压区。(2)热扩散系数的有效多变指数对定性预测气泡运动是有用的。然而，为了计算最大壁压，我们需要直接处理传热。(3)气泡平移的中性坍缩条件很大程度上取决于内部气体的热扩散。

1. Analysis for the dynamics of microbubbles using laser trapping methodWe applied a laser trapping method to investigate the microbubble dynamics experimentally. The results are summarized as follows:(1) We measured optical trapping forces acting on microbubbles. The vertical force was about 90 pN under the net laser power of 110mW.(2) The simulated trajectories of microbubbles were in good agreements with experiments.(3) We performed the laser trapping near a solid boundary and simulated the bubble trajectories that escaped from the laser trap in a shear flow. The trajectory was much dependent of bubble radii.(4) We investigated the influence of gas diffusion and surface tension on the stability of microbubbles. It was shown that the surface tension of microbubbles for ultrasonography was about 800 times smaller than the air-water bubble. When the radii of trapped or merged microbubbles were larger than a critical bubble radius that was determined from the surface tension and the concentration of dissolved gas, the bubbles grew rapidly.2. Numerical analysis for the collapse of microbubbles with the boundary element method.We developed a numerical method to compute the toroidal bubble dynamics by considering the heat transfer of internal gas. We applied this method to the bubble collapse near a solid wall or a deformable wall that represents the tissue surface. The results are summarized as follows:(1) The microbubble collapse is accelerated due to the heat transfer of internal gas. After the liquid-jet threads the bubble surface, extremely high-pressure area is observed around the jet-impact point.(2) The effective polytropic index for thermal diffusivity is useful to predict bubble motions qualitatively. However, to evaluate the maximum wall pressure, we need to deal with the heat transfer directly.(3) The condition of the neutral collapse for bubble translation is much dependent on the thermal diffusion of internal gas.