Analysis of Nonlinear Dynamics of Microbubbles Considering Adsorption and Desorption of Surfactant

用CCD摄像机观察了激光捕获法捕获的微泡和附着在盖玻片下的微泡的收缩和生长。研究了两种表面活性剂微泡（Levovist和Imavist）对气体扩散的影响。观察到两种类型的平衡半径激光捕获收缩微泡。第一个与表面活性剂的平衡表面浓度有关。另一个是由于在最大表面活性剂浓度下的表面积的压缩的表面张力的降低有关。结果表明，随着环境压力的增加，微小气泡的收缩伴随着较大的表面变形，并且即使压力降低后也不会恢复到初始尺寸。另一方面，大的微泡收缩成接近球形。气泡表面的凹陷表明在气泡表面形成了表面活性剂或脂质的多层膜。考虑表面活性剂的吸附/脱附、表面活性剂层或脂质层的动态表面张力、气体渗透阻力等因素，建立了气泡模型。激光捕获的微气泡的轮廓进行了比较与模拟的基础上的模型。动态表面张力的模拟结果与实验结果一致。模拟结果表明，表面活性剂的吸附速度远快于微泡的收缩速度。由于微泡表面积的减小而引起的表面张力的减小是决定气泡轮廓的重要因素。考虑气体渗透阻力的模拟结果与实验结果吻合较好。结果还表明，在气泡收缩过程中渗透阻力的增加稳定了微气泡。

The shrinkage and growth of microbubbles that were trapped with laser-trapping technique or attached underneath a cover glass were observed with a CCD camera. The influence of gas diffusion was investigated for two kinds of surfactant microbubbles; one was Levovist and the other was Imavist. Two kinds of equilibrium radii were observed for laser-trapped shrinking microbubbles. The first one was related to the equilibrium surface concentration of surfactant. The other was related to the decrease of the surface tension due to the compression of the surface area at the maximum surfactant concentration. It was shown that when the ambient liquid pressure increased, a tiny microbubble shrank accompanied with large surface deformation, and did not return to the initial size even after the pressure was reduced. On the other hand, a large microbubble shrank nearly spherically. The depression of bubble surface suggests the formation of multilayers of surfactant or lipid on bubble surface. It was also shown that air diffusion enhanced the growth of the Imavist.A bubble model was also constructed by considering the adsorption/desorption of surfactant, the dynamic surface tension, and the gas permeation resistance of surfactant or lipid layers. The profiles of laser-trapped microbubbles were compared with simulations based on the model. The simulations with the dynamic surface tension agreed with the experimental results. The simulations showed that the rate of adsorption of surfactant was much faster than the shrinking speed of microbubbles. The decrease of surface tension due to the decrease of the surface area of a microbubble was a significant factor to determine the bubble profile. The simulations considering the gas permeation resistance were in good agreement with the experiments for nearly spherical bubbles attached to the cover glass. The results also showed that the increase of the permeation resistance during bubble shrinkage stabilized microbubbles.