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

用CCD相机观察了激光捕获技术捕获的微气泡或附着在盖板玻璃下的微气泡的收缩和生长情况。研究了两种表面活性剂微泡对气体扩散的影响；一个是左倾派，另一个是意象派。激光捕获的收缩微泡有两种平衡半径。第一个与表面活性剂的平衡表面浓度有关。另一个与表面活性剂浓度最大时，表面面积受到压缩而导致表面张力下降有关。结果表明：当环境液体压力增大时，微小的微泡收缩并伴有较大的表面变形，即使降低压力也不会恢复到初始尺寸；另一方面，一个大的微泡几乎呈球形收缩。气泡表面凹陷表明气泡表面形成了多层表面活性剂或脂质。空气的扩散促进了Imavist的生长。考虑表面活性剂的吸附/解吸、动态表面张力以及表面活性剂或脂质层的气体渗透阻力，建立了气泡模型。将激光捕获的微泡形貌与基于该模型的模拟结果进行了比较。动态表面张力的模拟结果与实验结果吻合。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降低是决定微泡轮廓的一个重要因素。考虑气体渗透阻力的模拟结果与近球形气泡附着在盖板玻璃上的实验结果吻合较好。结果还表明，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.