Abstract
It has recently observed that microbubbles coated with a lipid monolayer on their surface oscillate non-spherically under driving acoustic pressure. In addition, the oscillations have been proved to make substantial effects, in particular, on acoustic scatterings, collapse phenomena, and life spans of the bubbles. However, it is so complicated to take mechanical properties of a lipid coating into consideration that further study focusing on the lipid coating is needed. Notably, surface tension varies due to the existence of the lipid coating. In this study, firstly we investigate parametrical instability of a free gas bubble oscillating spherically. In general, the coupling of the radial oscillation and the surface oscillation is one-way coupling so that the two systems exchange energy each other when the surface oscillations are parametrically excited. Secondly, how the effective surface tension changes the unstable regions in which surface oscillations are parametrically excited is discussed, using numerical simulation.
