Stability of Microbubbles under Variation of a Pressure Field

Abstract

The shrinkage and growth of microbubbles coated by palmitic acid under variation of a pressure field are observed with a CCD camera. We investigate the influence of gas diffusion on the stability of microbubbles. It is shown that when the ambient liquid pressure increases, a tiny microbubble with a radius of about 5 μm shrinks accompanied with large surface depression, and does not return to the initial size even after the pressure is reduced. The depression suggests the formation of multilayers of surfactant on the bubble surface. A bubble model is constructed by taking the dynamic surface tension and the gas permeation resistance of surfactant layers into account. The simulations show that the hysteresis of the surface tension is an important factor in determining the bubble response to the pressure change; the bubble profiles in the experiments are simulated qualitatively by considering the hysteresis of the surface tension. Also, the simulations considering the variable permeation resistance are in good agreement with the experimental results. The results also show that the permeation resistance increases during bubble shrinkage and stabilizes the microbubble. The smaller the initial bubble radius becomes, the larger the permeation resistance becomes during bubble shrinkage. The present simulations support the experimental results that the surfactant multilayers are formed when the bubble is compressed under the pressure increase.

Keywords

• microbubble
• microfluidics
• diffusion
• surfactant
• dynamic surface tension
• gas permeation resistance