Experimental Study Of The Interaction Between Cavitation Bubble And Suspended Microparticle

In recent years,because of the innovative structural characteristics and diverse potential applications of Janus particles,the related research as a new emerging multi-disciplinary field,has aroused great concern.Janus particles are a kind of micro and nano-particles whose surfaces are covered with two different materials,also called "double-faced microspheres".With the heterogeneity between two hemispheres of a Janus particle,a local gradient field or a microbubble will be created to generate self-propulsion.So there are usually two types of self-propulsion of Janus particles.O ne is self-propulsion by various gradient fields,including concentration gradient,magnetic field,temperature gradient or light intensity gradient;and another is bubble propulsion.Previous researches have found that gas molecules can nucleate to form microbubble by changing the particle shape or increasing its size,and then to propel Janus particle.With faster propelled velocity and more application value,the microbubble propulsion causes much widespread attention.In this paper,we first introduce the research background,and then fabricate a kind of bubble-propelled Pt-SiO2 type hollow Janus microspheres based on the advantages of bubble propulsion,which is more efficient compared to the present quasioscillatory translation mode bubble propulsion and tubular micromotor bubble propulsion.Secondly,we study the interactive action between cavitation bubble and syspended microparticles by experimental method.O n the one hand,we study the bubble-propelled motion of Pt-SiO2 hollow Janus microspheres in H2O2 solutions(concentrations between 2% to 4%).Three typ ical stages have been observed in every period of bubble's growth-collapse,which are self-diffusiophoresis,bubble growth and bubble collapse,respectively.At the bubble collapse stage,the speed propelled by a microjet is about 2 or 3 orders of magnitude higher than the speed in the other two stages.In the experiment,three scaling laws between the bubble radius and the elapsed time during the bubble growth stage are observed: Rb~t2/3,Rb~t1/2 and Rb~t1/3.Because the bubble location at the Janus microsphere's Pt side deviates from the axisymmetric point,the trajectory of the Janus microsphere takes on a circle line.O n the other hand,Micro-PIV technique is utilized to visualize the velocity field and flow pattern in bubble collapse moment.And compared with solid Janus microspheres,the influence of interfaces is considered.An intense microjet arising from the asymmetry of the medium around the Janus microsphere after bubble collapse occurs,and the instantaneous speed of the fluid may reach 0.1~1 m/s,which is a key point to improve the self-propelled velocity.The directions of microjet after bubble collapse are obviously influenced by the type of interfaces.The microjet can focus the energy on propelling the Janus microsphere,therefore,it can significantly improve the propulsion efficiency.In conclusion,we have fabricated a kind of Pt-SiO2 hollow Janus microspheres with high improved propulsion performance and taken them to study the interaction between cavitation bubble and suspended microparticle by experimental method.We show that the motion characteristics of bubble-propelled hollow Janus microsphere and reveal that the microjet after bubble collapse is a key point to realize high efficient propulsion of Janus microsphere.In view of this,this study not only further clarifies the motion mechanism of Janus microspheres,but a lso provides the valuable information for improving the speed and energy utilization of Janus micromotors in practical application.