Mechanism Study Of Single Cavitation Bubble Impacting On The Rigid Wall In A Low-frequency Ultrasonic Field

Ultrasonic cavitation,an important phenomenon in acoustic hydrodynamics,holds great significance and has been widely used in the industrial production(e.g.,pharmaceuticals produced by emulsification),the daily life(e.g.,ultrasonic toothbrushes that use high pressure shock during bubble collapse),the clinical medicine(e.g.,the low-frequency ultrasound-based skin permeabilization technology for the transdermal drug delivery)and the cell biology(e.g.,the ultrasonic cell perforation for the targeted administration).In these applications,the collapse process of bubbles induced by ultrasonic pressure,especially the bubbles' non-spherical collapse near the boundaries,plays a major role.However,how the mechanism of a single ultrasonic cavitation bubble,the basic form of ultrasonic cavitation in applications,impacts on the boundary is still unclear.This would directly affect the application efficiency and safety of ultrasonic cavitation.The combined experimental and theoretical method was applied to investigate the typical dynamics of a single bubble in a low-frequency ultrasonic field and the effects of various initial conditions on the bubble's dynamics.The relationship between the dynamics of a single bubble and the impact on the rigid wall was analyzed,and the mechanism of a single cavitation bubble impacting on a rigid wall was explored in a low-frequency ultrasonic field.First,a boundary element method(BEM)based on the potential flow theory was used to simulate the dynamics of a single bubble near a rigid wall in an ultrasonic field in the present work,in which the surface tension and the viscosity of the liquid were considered.In addition,the effects of ultrasonic harmonics were considered for the first time in the theoretical model by adding related coefficient equations into the governing equations.The actual situations in the applications of the ultrasonic cavitation were considered in the modified boundary element simulation model.The theoretical results matched the experimental results well,and the modified model would provide a better theoretical support for the study of the impacting mechanism of the ultrasonic cavitation bubbles on the rigid wall.Second,a method based on the synchronous photographic imaging was proposed to monitor the transient dynamics of micro-bubbles in a microfluidic occasion.Based on this method,a synchronously high-speed observation system for the single cavitation bubble driven by low-frequency ultrasound was set up,which consisted of a bubble generation apparatus,an ultrasonic generator and a high-speed camera.The experimental system was triggered to work in synchronization.In the present experiments,the precise control of the size and position of a generated single bubble in micrometer scale was achieved firstly,then the rapid morphological changes of a single bubble in a low-frequency ultrasonic field were clearly recorded,and the dynamic characteristics of a single bubble near a rigid wall in a low-frequency ultrasound field were characterized.After that,the typical dynamic behaviors of a single micro-bubble were investigated,and the process of the bubble motion could be divided into four phases:oscillation,movement,collapse and rebound.Meanwhile,the effects of various parameters on the single bubble's dynamics in a low-frequency ultrasonic field were studied,and some important results were obtained.First,the intensity of ultrasound had a significant effect on the dynamics of the single bubble.As the intensity of the ultrasound increased,the movement of the single bubble became more intense: the collapse of the bubble appeared earlier,the bubble vibration amplitude and the microjet velocity increased,and the dynamic behaviors of the single bubble after its first collapse differed.Second,the initial radius of the bubble and the distance between the bubble and the rigid wall obviously affected the dynamics of the bubble in a lowfrequency ultrasonic field.Under suitable initial conditions,the bubble's motion was the most violent: the bubble collapsed earlier with higher liquid jets compared to other initial conditions.Third,the effects of liquid characteristics on the dynamics of a single bubble were studied.It was found that the decrease in the surface tension of the liquid would increase the instability of the bubble motion,and lead to the earlier appearance of the bubble collapse and the change of the collapse position.The velocities of the micro-jets were also influenced by the viscosity of the liquid.The increase in the viscosity of the liquid significantly delayed the collapse time of the bubble,and reduced the vibration amplitude of the bubble margin and the velocities of the micro-jets.Last,the present work experimentally explored the magnitude of the impact of various bubble dynamics on the rigid wall,studied the relationship between the dynamic characteristics of the single bubble and the impact of the bubble on the rigid wall,and analyzed the effects of various mechanisms on the impact of a single bubble on the rigid wall.The different application fields corresponding to the various impacting mechanisms were also discussed.The results exhibit great significance for the practical applications of the ultrasonic cavitation.