A comparative study of the dynamics of laser and acoustically generated bubbles in viscoelastic media

Abstract

Here, we present results from experiments comparing the first-cycle growth and collapse dynamics of acoustically and laser-nucleated single bubbles in water and agarose gels of varying stiffness. Experiments were carried out in a custom-built spherical vessel which allowed both acoustically and laser-nucleated bubbles to be generated at its center. Bubbles were nucleated in water and agarose gels with stiffnesses ranging from 1.13 kPa to 570 kPa. Acoustically nucleated bubbles were generated using pulses delivered from 1 MHz transducer elements mounted in the vessel with pulse durations of <2 acoustic cycles. Laser-nucleated bubbles were generated using a pulsed Nd:YAG laser focused to the center of the vessel through an embedded window. Bubble dynamics were measured via optical imaging. The maximum radii of generated bubbles decreased with the increasing material stiffness for bubbles nucleated by both mechanisms. The growths and collapses of the bubbles occurred symmetrically (in time) about the maximum radius in water but not in gels, where the duration of growth decreased more rapidly than collapse as gel stiffness increased. Nucleation-mechanism-dependent differences in the growth-collapse asymmetry were observed which indicated that the growth duration of the laser-nucleated bubbles was longer than the acoustically nucleated bubbles in water and high water-content gels. Here, we present results from experiments comparing the first-cycle growth and collapse dynamics of acoustically and laser-nucleated single bubbles in water and agarose gels of varying stiffness. Experiments were carried out in a custom-built spherical vessel which allowed both acoustically and laser-nucleated bubbles to be generated at its center. Bubbles were nucleated in water and agarose gels with stiffnesses ranging from 1.13 kPa to 570 kPa. Acoustically nucleated bubbles were generated using pulses delivered from 1 MHz transducer elements mounted in the vessel with pulse durations of <2 acoustic cycles. Laser-nucleated bubbles were generated using a pulsed Nd:YAG laser focused to the center of the vessel through an embedded window. Bubble dynamics were measured via optical imaging. The maximum radii of generated bubbles decreased with the increasing material stiffness for bubbles nucleated by both mechanisms. The growths and collapses of the bubbles occurred symmetrically (in time) about the maximum...