Study On The Instability Characteristics Of Self-propulsion Rotation Ellipsoidal Janus Particle

Active particles are a type of substance that can rely on its own independent movement by absorbing energy from the environment.Natural bacteria and algae belong to active particles,and their response to environmental impacts is more sensitive and complex at the micro-nano scale.In recent years,various micro-nano artificial active particles have been produced one after another.The typical Janus particles are composed of two parts with different physical or chemical properties.The gradient field is established by using different properties on both sides of the particles to realize the self-propulsion of the particles.The self-propulsion external environment of artificial active particles is mainly a solution environment,which shows good application prospects in water environment detection and water environment restoration.This article first introduces the research background of active particles and the mechanism of Janus particle self-propulsion movement.At low Reynolds numbers,the Stokes flow around spherical particles is symmetrical and theoretically has a linear trajectory of spherical particles.In the experiment of active particle movement,the state of self-propulsion rotation of the particles was observed.In this paper,the numerical simulation research method is used to discuss the rotation mechanism.Secondly,the theoretical basis of the numerical model and the stress analysis of the fluid particles are introduced in detail.The physical model of the concentration gradient slip boundary under the applied concentration is proposed.The slip coefficient is introduced to characterize the effect of the concentration gradient on the wall slip velocity.Finally,the COMSOL multiphysics numerical analysis platform is used for numerical simulation to study the self-propulsion translation and self-propulsion rotation of Janus particles.Translation is the basis of rotation.Through the numerical simulation of self-propulsion translation,the corresponding matching parameters of the self-propulsion rotation model are determined.Observe the phenomenon of self-propulsion movement and explain the cause of self-propulsion rotation.The research on the self-propulsion translational phenomenon of Janus particles is divided into two parts.One is to study the self-propulsion translational speed of Janus particles under steady-state conditions.A two-dimensional axisymmetric model is established according to the motion conditions,and three different types are studied.The self-propulsion speed of the shaped Janus particles,especially for oblate ellipsoids with different main curvatures,the effect of different main curvatures on the self-propulsion performance was studied.The results show that the self-propulsion speed is mainly positively related to the projected area in the axial direction.The optimal particle size of the flat ellipsoid makes its self-propulsion speed the largest,which provides a theoretical basis for the future application of particle shape factors.The second is to study the causes of low Reynolds number flow instability under transient conditions,that is,the cause of asymmetric flow of particles with symmetrical structure under low Reynolds number flow,to establish a two-dimensional model,set the dimensionless number Reu,characterizing the relative magnitude of slip force and viscous stress in the flow field at low Reynolds number,when Reu～10^-13,obvious vortex begins to occur,it can be considered that stability damage occurred at this time,and this result has nothing to do with the ellipsoid curvature and the speed of movement.The effect of the concentration gradient on the wall causes the direction and size of the particle wall sliding velocity to change continuously.The results show that the viscous stress exhibits a combination of viscous shear stress and viscous normal stress.For the study of the self-propulsion rotation of the ellipsoidal Janus particles,this paper considers the rotating laminar flow module and the dilute matter transfer module under transient conditions,establishes a two-dimensional rotating coordinate system coupling the slip velocity,and studies the rotating ellipsoid in the flow field.During rotation,the velocity field and concentration field change with time.The research results show that the O₂ concentration on the lower side of the particle,that is,the Pt catalytic surface decreases from inside to outside.The reaction starts at the initial moment,a small vortex appears on the reaction surface,and the catalytic reaction time is extremely short,the vortex disappears and the particles rotate at a constant angular velocity.The rotation angular velocity of ellipsoid particles with high aspect ratio is significantly higher than that of particles with low aspect ratio,which is consistent with the experimental measurement results.In this paper,the numerical simulation method explained the particle rotation characteristics——stability damage from the mechanism,which laid the foundation for the later research on the controlled directional movement of Janus particles.