| Microfluidic devices have been gained increasing popularization and application in the fields of biology,medicine,material,chemistr y,and so forth,over the decades,in which fields one important application of the devices is to separate the mixed suspended particles(e.g.,human cells such as blood cells and circulating cells,microorganisms such as bacteria and protists,material particles such as solid particles,droplets,and vesicles),so as to purify or detect the target particles.At present,one challenge in this field is how to separate the particles with similar size efficiently and quickly.Among the numerous particle separation techniques utilizing microfluidic devices developed,inertial microfluidics has received much concern due to its high processing efficiency arising from its utility in high-speed flow(Reynolds number Re ?100),as well as its simple structure and stable performance.Inertial microfluidics technique makes use of the inertial migration of the particles in Poiseuille flow to equilibrate the particles at specific lateral positions within the channel cross-section,and then for separation.However,correlational studies on the inertial migration of particles with different shape or deformability drew confused and even contradictory results,and corresponding mechanisms and theories still need to be improved.Thus,the present study investigates the laws and the mechanisms of the inertial migration of solid spheres,solid ellipsoids,surface tension constrained deformable particles represented by droplets,and membrane constrained deformable particles represented by vesicles and cells in plane Poiseuille flow over Re ?100,so as to probe the effects of shape and deformability on the inertial migration of the particles,and then to explore the feasibility of separating particles by employing inertial microfluidics based on shape or deformability differences of the particles.The dynamics of inertia migration of solid spheres in Poiseuille flow is analyzed at first,which clarifies the transverse forces on the spheres.The inertial migration of solid spheres in plane Poiseuille flow over Re ?100 is then simulated.It is found that the solid spheres equilibrate at lateral positions midway between the channel center and the wall,with the positions moving closer to the center as Re increases.The accuracy of the simulation results is subsequently verified by experiments and by comparison with results of other researchers.Through theoretical analysis,the lift force induced by slip-shear between the spheres and their surrounding fluid is identified as the decisive reason for the motion trend of solid spheres with variation of Re.Based on above conclusion,three methods,i.e.,varying the driving force applied on the spheres,changing the sphere density,and altering the boundary slip condition at the sphere-fluid interface,are proposed to vary the slip-shear velocity between the spheres and their surrounding fluid,which are used to manipulate the lateral equilibrium positions of the spheres within the channel.The three methods are proved by simulations.Changing the shape of the solid spheres into prolate and oblate ellipsoids,respectively,the inertial migration of the solid ellipsoids with the same volume but different shapes in plane Poiseuille flow over Re ?100 is simulated,which is to investigate the effect of the shape on the inertial migration of the solid particles.The relationship between the inertial migration and the rotation of the solid ellipsoids is also discussed.It is found that,the lateral equilibrium positions of the sol id ellipsoids are closer to the channel center than that of the solid spheres of the same volume at the same Re;the positions moving closer to the channel center as Re and the axial ratio increase;change of shape leads to various rotational attitudes of the ellipsoids,which shows significant effect on their equilibrium positions within the channel.The dynamics of inertia migration of deformable particles in Poiseuille flow is then analyzed.Choosing droplet as research target,the inertial migration of surface tension constrained deformable particles in plane Poiseuille flow over Re ?100 is studied through simulations.The effects of the flow inertia(Re),the droplet-channel size ratio,the surface tension,the droplet-medium fluid viscosity ratio,and the viscosity of the medium fluid on the lateral equilibrium position,the deformation,and the inclination angle of the droplets in the flow field are investigated,respectively;meanwhile,the effects of the deformation and the inclination angle on the lateral equilibrium position are discussed.It is found that,the lateral equilibrium positions of the droplets are closer to the channel center than that of the solid spheres of the same volume at the same Re;the positions move closer to the channel center with increasing Re;the droplet-channel size ratio reflects the restriction effect of the walls;the surface tension,the droplet-medium fluid viscosity ratio,and the viscosity of the medium fluid affect the deformation and the inclination angle of the droplets,with the increase of the deformation and the decrease of the inclination angle being conductive to reduce the wall-directed shear-gradient lift force on the droplets,which results in lateral equilibrium positions closer to the channel center for the droplets.Choosing vesicles/cells as research targets,the inertial migration of membrane constrained deformable particles in plane Poiseuille flow over Re ?100 is studied through simulations.The effects of the flow inertia(Re),the membrane shear modulus,the membrane bending rigidity,and the vesicle/cell-medium fluid viscosity ratio on the lateral equilibrium position,the deformation,and the inclination angle of the vesicles/cells in the flow fields are investigated,respectively;and the effects of the deformation and the inclination angle on the lateral equilibrium position are discussed simultaneously.It is found that,the lateral equilibrium positions of the vesicles/cells are closer to the channel center than that of the solid spheres and that of the droplets of the same volume at the same Re;the positions move closer to the channel center as Re increases;the decreases in membrane shear modulus,membrane bending rigidity,and vesicle/cell-medium fluid viscosity ratio,are in favor of the increase of the deformation and the decrease of the inclination angle for the vesicles/cells,which lead to their lateral equilibrium positions moving closer to the channel center;relative to the vesicle/cell-medium fluid viscosity ratio,the membrane mechanical properties,i.e.,the membrane shear modulus and the membrane bending rigidity,play more important roles on the inertial migration of the vesicles/cells.In conclusion,the laws and the mechanisms of the inertial migration of solid spheres,solid ellipsoids,surface tension constrained deformable particles represented by droplets,and membrane constrained deformable particles represented by vesicles and cells in plane Poiseuille flow over Re ?100 are studied,and the effects of shape and deformability on the inertial migration of particles are thus investigated.Based on the discrepancies between the particles' lateral equilibrium positions which results from their shape or deformability differ ence,an efficient and fast particle separation method utilizing inertial microfluidics is proposed. |
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