Effect of liquid viscoelasticity on the interactions between cavitation bubbles and free surfaces

The influence of liquid viscoelasticity on the interaction between cavitation bubbles and free surfaces is of great practical significance in understanding bubble dynamics in biological systems. A series of millimeter cavitation bubbles were induced by laser near the free surfaces of the water and viscoelastic polyacrylamide (PAM) solutions with different concentrations. The effects of liquid viscoelasticity on the interactions of cavitation bubbles with free surfaces are analyzed from the perspectives of the evolution of free surface and bubble dynamics. The experimental results show that as the dimensionless standoff distance increases, the evolutions of free surface behaviors in all experimental fluids can be divided into six types of water mounds, i.e., breaking wrinkles, spraying water film, crown, swallowed water spike, hillock, and slight bulge. All the critical values of the dimensionless distance dividing different types decrease with increasing concentration. The evolutions of first four types of water mounds in PAM solutions differ from those in the water. Water droplets splashing in different directions are produced around the breaking wrinkles in the water. Meanwhile, the breaking wrinkles in PAM solution move with the “liquid filaments” towards the central axis. The water spike in the pattern of spraying water film in PAM solution is more stable than that in the water. As the solution concentration increases, the water skirt in the pattern of crown contracts earlier and faster, and the rate of increase in the height of the water skirt decreases. For swallowed water spike in PAM solution, the upper part of the newly formed water spike is not significantly thicker than the middle part, and thus the water waist structure does not form. Liquid viscoelasticity inhibits the bubble growth and collapse, and the bubble migration as well, especially in the second period. Shorter and thicker cavities are formed in PAM solutions with higher concentration, while slender and stable cavities formed in the water at the same dimensionless distance. The velocity and displacement of the tip of bullet jet both decrease as the solution concentration increases.