Shock wave emission from a hemispherical cloud of bubbles in non-Newtonian fluids

• We investigate shock wave emission from a cloud of bubbles in non-Newtonian fluids.
• Shock wave pressure and duration is diminished in the elastic fluid.
• The reduction is attributed to an increase of extensional viscosity of the fluid.
• Shock pressure for a bubble cloud is larger than for a single bubble.
• Implications in ultrasound-mediated surgery are discussed.

Shock wave emission from a hemispherical cloud of bubbles, situated in non-Newtonian fluids, is investigated by high-speed photography, with up to 20 million frames/s and an exposure time of 5 ns, and acoustic measurements. The non-Newtonian fluids consist of a 0.5% polyacrylamide (PAM) aqueous solution, with a strong elastic component, and a 0.5% carboxymethylcellulose (CMC) aqueous solution, with a weak elastic component. In the relatively inelastic CMC solution, the maximum amplitude and the duration of the shock wave emitted during bubble cloud rebound are almost identical to the case of water. A significant reduction of the shock wave pressure was found in the elastic PAM solution. This difference ranges from a factor of 2, for a maximum radius of the bubble cloud Rmax ≈ 800 μm, up to a factor of 3, at Rmax ≈ 270 μm. A decrease of the shock wave duration was also observed in the elastic PAM solution. The observed reduction is attributed to an increased resistance to extensional flow which is conferred upon the liquid by the polymer additive and to an increase of the cavitation threshold of the liquid. At a maximum radius of about 400 μm, the shock pressure for a bubble cloud, situated in water and 0.5% CMC solution, is with a factor of six larger than the value measured in the case of individual cavitation bubbles. This difference is smaller in the case of a 0.5% PAM solution where the shock pressure for a bubble cloud is only three times larger than for a single bubble. The results are discussed with respect to cavitation erosion in polymer solutions and collateral effects in pulsed high-intensity focused ultrasound surgery, such as histotripsy used for the destruction of blood clots.

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