Direct Measurements Of Secondary Flow Structures Induced By A Single Particle And Particle Trains In Confined Microchannels

Inertial focusing has been proved to be a promising technique in various microfluidics-based applications such as self-assemble of particles,cell separation and manipulation.Research has indicated that the location of the equilibrium positions are strongly influenced by the microscale flow structure around particles.Previous studies on this flow mainly relied on numerical simulation.The lack of direct experimental measurements limited further understanding of this important microscale transport phenomenon.In this study,we first set up a Micro-PIV measurement system which allows quantitative and direct measurement of the flow structure around a single partilce and particle trains in a confined microchannel.The main contents of present work are as follows:(1)Since the focused particle keeps moving forward along the microchannel in experiments,the measured fluid domain contains a moving boundary.In order to eliminate the influence of the moving boundary in the Micro-PIV measurement,the flow filed is calculated in the reference frame fixed to the particle center by the method of moving reference frame.By using this method,the flow structure around a single particle in inertial focusing has been measured with Re from 21 to 525.Results show that reversing flow regions exist around the particle with Re equals to 21.As Re increases,reversing flow in front of the particle turns into vortex structure.And the vortex in front of the particle get strengthened significantly as Re increases,which means the influence of the particle on main flow becomes more significant.Moreover,the velocity lag of the particle increases as Re increases,which leading to the enhancement of the vortex.(2)The flow structure around particle train on one side of the microchannel and two particle trains with staggered spacing has been measured with Re equals to 21 and 105.When Re equals to 21,the vortex structure appears in the region between adjacent particles in particle train on one side of microchannel,and the center of vortex locates in the middle of two adjacent particles.However,in the alternating pattern of particle trains,two vortices appears between adjacent particles on the same side of microchannel.The particle on the other side of microchannel squeezes fluid between adjacent particles,which makes the vortex in the middle of two adjacent particles split into two vortices.When Re equals to 105,the vortex structure between adjacent particles in particle train on one side of microchannel is similar to the one around a single particle.In the alternating pattern of particle trains,the vortex between adjacent particles on the same side slightly curves as the result of the squeeze from the particle on the other side.Only one vortex exists between adjacent particles,as the center of the vortex is close to the behind particle.(3)Dynamics of particles in inertial focusing is also investigated with Re varies from 21 to 525.It is found that the rotation rate of particle is proportional to the Reynolds number,which indicates that the shear forces also increases linearly with the Reynolds number.The large velocity gradient inside the microchannel impose strong shear forces on the particles,which induces strong rotations.The equilibrium position of inertial focused particles moves towards to the wall first and then away from the wall as Reynolds number increases.A new microfluidics chip has been developed which is used for particle 3D focusing by combining inertial focusing with sheath flow.