Dominant mode shift of bubble shape instability due to surface tension variation under MHz ultrasound

Occurrence condition for non-spherical shape mode of acoustically oscillated lipidic-coated bubble was numerically investigated by solving both the Keller–Miksis equation and dynamic equation for distortion amplitude of spherical harmonics. To simply model the lipidic-coated bubble, apparent surface tension is introduced into the numerical model. The numerical simulation indicated that the growth rate of distortion amplitude in an ultrasonic cycle is an important factor to predict the shape mode of a non-spherically oscillating bubble under MHz ultrasound. According to the occurrence condition of non-spherical oscillation modes derived from the growth of distortion amplitude, larger degree of spherical harmonics is dominant with increasing the equilibrium bubble radius, and the degree is not significantly impacted by pressure amplitude. When the apparent surface tension is reduced to model lipidic-coated bubbles, shape instability occurs in smaller bubbles, yielding numerical results that are consistent with experimental observations conducted previously by other researchers.