Oscillatory dynamics of a charged microbubble under ultrasound

Abstract

Nonlinear oscillations of a bubble carrying a constant charge and suspended in a fluid, undergoing periodic forcing due to incident ultrasound are studied. The system exhibits period-doubling route to chaos and the presence of charge has the effect of advancing these bifurcations. The minimum magnitude of the charge Qmin above which the bubble's radial oscillations can occur above a certain velocity c1 is found to be related by a simple power law to the driving frequency omega of the acoustic wave. We find the existence of a critical frequency omega_H above which uncharged bubbles necessarily have to oscillate at velocities below c1. We further find that this critical frequency crucially depends upon the amplitude Ps of the driving acoustic pressure wave. The temperature of the gas within the bubble is calculated. A critical value P_{tr} of Ps equalling the upper transient threshold pressure demarcates two distinct regions of omega dependence of the maximal radial bubble velocity v_{max} and maximal internal temperature T_{max}. Above this pressure, T_{max} and v_{max} decrease with increasing omega while below P_{tr}, they increase with omega. The dynamical effects of the charge and of the driving pressure and frequency of ultrasound on the bubble are discussed.