Shock wave emission and cavitation bubble dynamics by femtosecond optical breakdown in polymer solutions

Shock wave emission and the dynamics of micrometer-sized cavitation bubbles generated after femtosecond optical breakdown in viscoelastic fluids were investigated experimentally using high-speed photography with 5 million frames/s and acoustic measurements. The viscoelastic fluids consisted in a 0.5% polyacrylamide solution, with a strong elastic component, and a 0.5% carboxymethylcelullose, with a weak elastic component. Breakdown in water and both polymer solutions generated a purely compressive pressure wave. The maximum amplitude and the duration of the breakdown shock wave as well as the maximum bubble radius were not affected by the polymer additives. The notable influence of the polymer additives was found during the collapse phase of the bubble and is manifested by a prolongation of the oscillation time of the bubble and a reduction of the maximum pressure of the shock wave emitted during bubble collapse. A sizeable attenuation of the bubble collapse was found in the elastic polyacrylamide solution. The present results are consistent with an interpretation which invokes the effect of enhanced levels of uniaxial extensional viscosity on the collapse of micrometer-sized cavitation bubbles. The consequences for cavitation in blood are also discussed.