Flow Characteristics Study Of Small Particles Based On The Inertial Force In Microchannel

Inertial focus of suspending particles is a spontaneous focus phenomenon in micro-channel with limited Reynolds number and laminar flow,which could be used to sort and focus small particles in microfluidic chip,such as separating cell from blood,to solve sensing,analyzing and manipulating problems in specific fields like medical diagnosis and biochemical analysis.Due to the inertia effect of liquid in microchannel,particle flowing along the main flow direction will migrate in the radial direction(lateral direction)under the drive of inertial force(lift force),until it reaches a lateral position between pipe center and pipe wall,where the lift force acting on it is zero.Then,it will not migrate at lateral direction any more and only move along the main flowing direction.Lift force plays a key role in the mechanism of particle migration and focus in microchannel,explaining the distribution character of lift force along radial direction(lateral direction)and studying the influential factors of it could do a favor to understand the phenomenon of particle focus,and offer a theoretical guidance in designing devices of inertial microfluidic chip.In this paper,in order to calculate the lift force on particle,a theoretical analysis of forces induced by liquid in flow field was conducted firstly,and the author holds an opinion that these forces mainly come from two components,one is pressure gradient and the other is liquid viscosity.Then the velocity and pressure distribution of liquid around the particle in two-dimensional microchannel was simulated by computational fluid dynamics software Fluent.By integrating the pressure force and viscous force acting on each small element of particle surface,the net and total force acting on the particle was calculated,and the component along radial direction is the lift force.In this paper,in order to calculate the lift force,two different simulating models were adopted,one is stationary particle in flowing field and the other is rotary particle in static field.Based on these two models,the direction and magnitude of lift force at different lateral position in microchannel were calculated,which could be used to explain the reason of inertial focus.Finally,the influence of Reynolds number and relative particle diameter on lift force and equilibrium position were demonstrated.The result can be concluded as follow:Lift force that driving particles to migrate and focus has three main components,and each component has the contribution of pressure force and viscous force.The first part comes from the uneven distribution of liquid velocity and relative motion between liquid and particle,the Saffman effect induces a lift force along radial direction that pointing to pipe center,but it decrease dramatically when the relative velocity narrows.The second part is caused by Magnus effect,because the particle in upside field is rotating in an anti-clockwise direction,the flow liquid acts on the particle a force dragging the particle to migrate to pipe wall.The third part can be explain by the effect of pipe wall,when particle moves towards the wall,it stops the particle to move too close to it,so acts on the particle a repellant force that directs to the pipe center.Actually,the lift forces caused by Magnus effect and pipe wall are more dominant to particle focus than the lift force caused by Saffman effect,and the resultant force of them determines the migration of particle in the pipe.The lift force in microchannel has an universal distribution character along the radial direction.When the particle is close to pipe center,the direction of lift force points to the pipe wall,and the magnitude increase at the first and then drop down gradually to zero with the increase of distance between particle and pipe center.When particle locates at the adjacent space of pipe wall,the direction of lift force points to the pipe center,and the value of it increases with the particle migrating to pipe wall.So there is a null point between the two and that is the ultimate equilibrium position.Also,with the increase of Reynolds number,the equilibrium position of particle will move to the pipe wall,and with the increase of particle relative diameter,the equilibrium position of particle will move to the pipe center.But when the particle relative radius is too big,the migration and focus phenomenon will become vague because the influence of the particle.