Ultrasound driven microbubble tunneling through soft material

Experiments show that focused ultrasound in tissues and hydrogels can cause small bubbles cavitated in the material to translate, leaving tunnel-like trail of degraded material behind. This has been recognized to cause collateral damage to healthy tissue outside a treatment zone in therapeutic procedures involving strong acoustic waves. It might also be therapeutically useful in some cases. An axisymmetric simulation model is used to study the tunneling mechanisms. A sharp-interface method robustly resolves the driven bubble as it undergoes extreme compression, expansion, and a potential jetting collapse. The material is represented by a high-viscosity fluid that degrades to a lower-viscosity fluid under high strain. This simple model is sufficient to reproduce the key observation of slow steady tunneling over hundreds of ultrasound cycles. Tunnels are found to grow by a two-step process: the tunnel extends during the bubble expansion, and the bubble translates along it due to its asymmetric environment during its collapse. Ultrasound and material parameters mediate the details, including a curious threshold behavior, but the overall mechanism is robust. The observed steady tunneling does not depend on a bubble collapse jetting instability, as has been speculated.