Experimental Study On Dynamics Of Cavitation Bubbles Near Elastic Boundaries In Viscoelastic Fluids

Cavitation bubbles play a key role in a variety of new medical diagnostic and therapeutic techniques using ultrasound and lasers.In these applications,cavitation bubbles grow and collapse in biological fluids such as human blood and tissue fluid.Most biological fluids are viscoelastic fluids,and the dynamics of cavitation bubbles in these fluids are different from those in Newtonian fluids.In addition,when the cavitation bubble oscillates near the biological tissue,the elasticity of the biological tissue strongly affects the dynamic behavior of the cavitation bubble.Therefore,cavitation bubbles near the elastic boundary in viscoelastic fluids were studied in this paper to provide references for the utilization and control of cavitation bubble dynamics involved in related medical processes.Firstly,a visualization test rig for laser-induced cavitation bubbles is designed and built to generate a single submillimeter cavitation bubble in water and viscoelastic fluids.The polyacrylamide(PAM)solution and carboxymethyl cellulose(CMC)solution were selected as viscoelastic fluids,and the rheological properties,surface tension and density of the two viscoelastic fluids at different concentrations were measured.PAM gel was prepared as an elastic boundary material to simulate biological tissue.The elastic modulus of the gel was changed by changing the water content of the gel,and the elastic modulus of the gel with different water content was measured.Next,in order to study the influence of boundary elasticity,the cavitation bubble dynamics near the rigid and elastic boundary in water was studied by high-speed photography method.The morphology evolution of cavitation bubble,jet behavior,oscillation time,migration characteristics,elastic boundary deformation and bubble splitting behavior were analyzed.The results show that both the migration of cavitation bubbles and the axial jets near the rigid boundary point to the boundary direction,while the cavitation bubbles near the elastic boundary show a mushroom shape at the later stage of collapse with a specific elastic modulus and dimensionless distance,and the cavitation bubbles are then split into two sub-bubbles by the annular jets.The rigid boundary in water has an obvious delay effect on the oscillation period of cavitation bubble,while the elastic boundary has a weak delay effect on the oscillation period.In addition,the maximum deformation of the elastic boundary is negatively correlated with the dimensionless distance of cavitation bubble and the elastic modulus of the boundary.At last,in order to investigate the influence of viscoelastic effect on cavitation bubbles,the morphology evolution,jet behavior,oscillation time and migration characteristics of cavitation bubbles near the infinite region,rigid boundary and elastic boundary of 85% water content in two kinds of viscoelastic fluids were studied.The results show that the viscoelastic effect of fluid has no obvious influence on the morphologic evolution of cavitation bubbles in the infinite area and near the rigid boundary,and has a strong influence on the second period of cavitation bubbles near the elastic boundary of 85% water content.PAM solution has obvious inhibitory effect on the axial jet flow of cavitation bubble,while the axial jet flow of cavitation bubble in CMC solution is basically consistent with the water condition.The delay effect of CMC solution on the oscillation period of the above three kinds of cavitation bubbles increases with the increase of the concentration,while the effect of PAM solution on the oscillation time of the three kinds of cavitation bubbles is different.In addition,the two kinds of viscoelastic fluids have no obvious effect on the cavitation bubble migration in the first week near the rigid boundary and the elastic boundary with 85% water content.