The shrinking and growth of microbubbles under pressure variations are observed with a CCD camera. The influence of gas diffusion on the stability of microbubbles covered with phospholipid layers is investigated. The microbubbles are made with acoustic liposomes encapsulating phosphate buffer solution and perfluoropropane gas. It is shown that when the ambient liquid pressure increases, the observed microbubbles shrink accompanied with the cyclic surface buckling and smoothing process. The bubble surface smoothing in the process shows that the excess phospholipid layers are removed from the surface, which results in the instantaneous bubble shrinkage. It is also shown that the smaller the initial radius is, the more the growth of microbubbles is reduced. The bubble model by Takahira and Ito, in which the dynamic surface tension and the gas permeation resistance of molecular layers are considered, is utilized to simulate the experiments. The simulation is in qualitative agreement with the experimental result except for the instantaneous bubble shrinkage. The model is improved so as to consider the instantaneous increase of surface tension. The instantaneous bubble shrinkage is simulated successfully with the improved model. The results suggest that the instantaneous increase of surface tension is caused by the shedding of the excess phospholipid layer material due to the zippering process proposed by Borden and Longo.

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