To understand the stability of microbubbles, the shrinkage and growth of microbubbles under variation of a pressure field are observed with a CCD camera. The influence of gas diffusion is investigated for two kinds of microbubbles; one is Levovist which is an air microbubble coated by palmitic acid, and the other is Imavist which is a PFC gas microbubble covered by lipid and surfactant. It is shown that when the ambient liquid pressure increases, a tiny microbubble shrinks accompanied with surface depression, and does not return to the initial size even after the pressure is reduced. On the other hand, a large microbubble shrinks nearly spherically. The depression of bubble surface suggests the formation of multilayers of surfactant or lipid on bubble surface. It is also shown that air diffusion enhances the growth of the Imavist. A bubble model is also constructed by considering dynamic surface tension and gas permeation resistance of surfactant or lipid layers. The previous experimental results in which a microbubble was trapped with a laser trapping technique are compared with simulations based on the model. The results show that the rate of adsorption of surfactant is much faster than the shrinking speed of microbubbles. The decrease of surface tension due to the decrease of the surface area of a microbubble is a significant factor to determine the bubble profile. Also, the present experimental results are compared with simulations. The simulations considering the gas permeation resistance are in good agreement with the experiments for nearly spherical bubbles. The results also show that the increase of the permeation resistance during bubble shrinkage stabilizes microbubbles.

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