Flow Visualization And Numerical Simulation Study Of Particle Motion Driven By Micro-Scale Double Bubble Coalescence

Micro-nano motors are one of the ways to establish fluid microenvironment and macro manipulation,and the advantage of high driving speed of bubble-driven micro motors makes them promising in practical applications such as environmental pollution degradation,in vivo targeted drug delivery and biosensors.The common tubular bubble micromotor is not limited in its application scenarios because it generates bubbles inside the tube and relies on the motion of the bubbles rebounding from the tube to drive the micromotor.However,the driving efficiency of tubular bubble micromotors is low,only10^-10 order of magnitude.The single-bubble driven Janus microsphere motor can compensate for the low efficiency of tubular micromotor but is only applicable near the gas-liquid interface.In view of this,this paper proposes a new system of Janus microsphere motor driven by double bubble aggregation,which can reconcile the high driving efficiency with the interface limitation.The scalar law of the evolution of the microscale double-bubble aggregation neck and its intrinsic mechanism are also revealed along with the study of the double-bubble aggregation driving method.The growth process of the neck of the double bubble aggregation was recorded experimentally with the help of high-speed camera,and the growth process of the bubble phase contact neck was influenced by the surface tension,and the viscous and inertial forces of the external solution also played a role.In addition,the process of double bubble aggregation driving the micromotor was also recorded,where the bubbles in the immediate vicinity of the microsphere undergo aggregation and significantly drive the microsphere motion by the released energy,and the fusion process is a unique gas-liquid interface evolution problem under the limitation of the movable curved wall.The effects of bubble/particle size ratio and other factors on microsphere displacement and initial kinetic energy conversion rate are given.Relying on the experimental results,numerical simulations are further combined with the pseudo-potential lattice Boltzmann method in order to better understand the hydrodynamic mechanism driven by bubble aggregation.The results reveal the detailed characteristics of bubble aggregation and flow field at different times,and confirm the driving mechanism of surface energy release by double bubble aggregation.The applicability of this numerical simulation method to such problems is verified by comparing the morphological evolution and time through the bubble neck growth pattern in the experiment.This is used to explore the neck growth at smaller time scales.The experimental results are analyzed against the numerical simulation results,revealing that the main cause of the micromotor motion is the interfacial oscillation generated by the bubble aggregation,and the energy conversion rate of this driving method is slightly higher than that of the tubular micromotor driven by bubble bouncing and slightly lower than that of the Janus microsphere motor where the bubble fuses with the gas-liquid interface and disappears.The effect of bubble/particle size ratio on particle displacement and initial kinetic energy conversion rate was investigated,and it was concluded that when the bubble-particle size ratio was in the range of 0.7～1.5,the displacement of bubble aggregation and driven particles increased with the increase of bubble-to-particle size ratio,and the initial kinetic energy conversion rate was basically the same.The main time periods for generating net displacement were clarified,and in the solution with small viscosity,the main stages for bubble aggregation and driving particles to generate net displacement were the results of interfacial pushing after the bubble aggregation neck growth was completed and interfacial pushing after the interface was pushed out;in the solution with relatively large viscosity,the main stages for bubble aggregation and driving particles to generate net displacement were only the results of interfacial pushing after the bubble aggregation neck growth was completed.