Experimental And Simulation Study On The Motion Behavior Of Active Particles In Flow Field

Active particles are self-driven particles that can convert other forms of energy in the environment into mechanical energy to promote their own movement.E.coli,Bacillus subtilis and many single-celled microorganisms in nature are typical active particles.Inspired by these active microbes,researchers are developing artificial particles with similar motion mechanisms and applying them to practical engineering in fields such as biology,chemistry and medicine.However,whether biological active particles or artificial particles,the flow field directly interacting with them in the liquid environment is mostly shear flow field.In recent years,the research on the special movement behavior of active particles in shear flow field has been widely concerned.In this paper,the movement behavior of active particle E.coli in flow field was studied by combining experiment,theoretical analysis and simulation calculation,and the transverse migration mechanism of bacteria at mesoscopic scale was explored.First,choose the typical biological active particles E.coli as the research object,this paper introduces the bacteria cultivation method and configuration of e.coli suspension,using microfluidic device form controllable shear flow field,through high-speed capture bacteria in the flow field in the microscopic image acquisition system trajectory,the near the wall of e.coli in static water body and the movement behavior of shear flow field,The effects of shear rate of flow field and self-actuation of bacteria on transverse migration velocity and steering frequency of E.coli were investigated.The results show that the bacterial motion in stationary water and shear flow field can be divided into global and local parts.In stationary water,the bacterial motion in the global Angle presents a circular motion near the wall,while in the local area,the bacterial motion presents an alternating motion mode of straight motion and turn.After the introduction of shear flow field,the global motion is transformed into transverse migration along the vorticity direction of the flow field.The stronger the self-driving ability of bacteria is,the higher the shear rate of the flow field is,the faster the lateral migration speed is,and the stronger the trend of the flow field is,the steering frequency in the local motion is also accelerated with the increase of the shear rate of the flow field.Then,the angular velocity equation of E.coli movement in the flow field was constructed by coupling the shear flow field action,wall effect and flagellum chirality effect.Combined with the experimental results,the global and local movement behavior of bacteria in the flow field was deeply explored,and the critical shear rate y_c was introduced.The results showed that when the shear rate of flow field was less than y_c,the effect of flow field shear was small,and the bacteria were mainly affected by wall effect and flagellar chirality.The bacteria had a strong ability to migrate across streamlines,and the lateral migration rate increased rapidly with the increase of flow field shear rate.When the shear rate is greater than y_c,the flow field shear is dominant,the ability of bacteria to migrate along streamlines weakened,while the lateral migration rates tended to stabilize.In the local steering motion,the steering frequency was mainly affected by shear flow field,bacteria aspect ratio and flagellar chirality,among which the shear flow field played a dominant role.Therefore,the bacterial steering frequency showed a linear increase trend similar to the experimental results with the increase of shear rate.Finally,on the basis of the dissipative particle dynamics,build an e.coli movement model of active particle motion model,and in shear flow field in the mesoscopic simulation,studied the flow shear rate,bacteria from the driving speed,circular motion,angular velocity and flagella chiral induction of angular velocity of the single factor’s influence on the particle motion behavior patterns.The simulation results show that at mesoscopic scale,with the increase of shear rate,the relative velocity between active particles and solvent molecular group increases,and the dissipative resistance F^D_ij also increases.Therefore,the transverse migration velocity increases slowly at a larger shear rate.Based on the movement data of E.coli measured at 20℃and 35℃,the single-factor analysis of self-drive velocity and circular angular velocity showed that the change of self-drive velocity had a greater and dominant influence on the lateral migration of bacteria.