Numerical study of ultrasound-driven droplet-bubble compound motion near a wall

Numerical simulation is performed for the ultrasound-driven motion of a droplet-bubble compound near a wall. The compound of a therapeutic droplet and a microbubble as ultrasound contrast agent receives increasing attention in medical imaging and targeted drug delivery. The droplet and bubble surfaces are tracked by two level-set functions. The conservation equations of mass and momentum in the droplet, bubble and water regions are solved including the effect of bubble compressibility. Computations are conducted for a gas bubble in a droplet as well as a droplet in a gas bubble, called antibubble. Gaussian pressure pulses are imposed at the open boundaries to initiate the compound motion. The bubble oscillates in the radial direction depending on the pulse conditions and the droplet is deformed as the bubble oscillation is pronounced. The ultrasonic frequency that maximizes the compound oscillation compares well with the analytically predicted resonance frequency. The maximum value of the wall shear stress is observed to be almost independent of the droplet-bubble configuration of the compound, but the wall shear stress decays more quickly in the case of antibubble. The effects of bubble volume and fluid properties on the compound motion and wall shear stress are quantified.