| The transient or steady flow characteristics of fluids in microcavities with special structures in microfluidic chips and the motion of particles with limited size inside them have become the frontiers of multidisciplinary interdisciplinary.Different from macroscale,microcavities with its micro-scale flow properties effects,and have important applications in the fields of micro-mixers,micro-heat exchangers,particle and cell sorting,etc.The flow characteristics in the microcavities and the evolution behavior of the vortex have obvious effects on the particle manipulation.It is of great scientific significance and practical application value to deeply explore the interaction of the vortex-particle motion and reveal the particle accommodation mechanism of the microcavities.Experimental fluid mechanics and simulation methods are used in this thesis.On the one hand,high-speed imaging system and inverted microscope camera system are used to carry out the visualization experiment of particle orbital motion.On the other hand,combined with microscopic particle image velocimetry(Micro-PIV)and numerical simulation,the transient and steady flow field characteristics in cavities were measured,and the orbital motion mechanism of particles in the confined microcavities was further analyzed with theoretical analysis.Finally,a new type of cavity structure is optimized and designed,which realizes the application of high-capacity,instant discharge,and parallel sorting of multiple microcavities.The results have important scientific value for depth understanding of micro-scale vortex and micro-particle twophase flow,and have important reference significance for particle/CTCs sorting,biological and chemical applications and other fields.(1)Qualitative and quantitative measurement of the vortex flow field characteristics of rectangular deep cavities.The effects of inlet Re(0.1-133)and aspect ratio h/b(0.4-4)on the vortex formation,strength,center position and flow field velocity vector distribution were studied.The critical conditions for the generation of vortex in deep cavities were investigated,and the three-dimensional orbital motion of particles was further traced through multi-angle shooting.The vortex flow field structure in deep cavities is affected by both the inlet flow characteristics and the aspect ratio.The reverse double vortex was first observed in experiment at h/b(29)1 with Re(29)33,and the vortex center was constantly approaching the main channel.At the same time,the flow velocity decays exponentially away from the inlet,and the attenuation is also affected by the aspect ratio of microcavities.(2)Visualization study of the transient evolution mode of initiation flow in a circular microcavity.The effects of Re(0-266)and cavity aspect ratio(width/diameter of 1.1,1.3,1.5 and 1.7)on the evolution of boundary morphology and vortex structure were investigated.When Re = 155,three boundary lines appeared in the dye solution in sequence,and they continued to evolve with time.The developing vortex were quantitatively characterized to reveal the mechanism of boundary morphology and migration.Three evolution modes of the boundary under the combined influence of flow conditions,aspect ratio and cavity inlet width are plotted,indicating that the transient flow mode has an important effect on fluid convection and mass transfer in the initial stage of groove initiation flow.(3)Experimental tests were carried out on the particle capacity and discharge enrichment rate of the optimized microcavity structure.A circular cavity with a bottom square cavity and a drainage channel was designed,and the effects of inlet Re,cavity neck diameter,and drainage channel position on the particle capture peak capacity and enrichment efficiency were experimentally studied.Combining the internal flow field characteristics of different cavities and particle motion trajectories,the mechanism of high-capacity trapping of particles is analyzed.The optimal structural parameters for the microcavities to stably accommodate the particles and be easily released are obtained.Finally,the array chip is used to realize the continuous multi-mode sorting of particle swarms,and the flexibility of the separation cut-off diameter was realized through the inlet flow rate and the flow resistance ratio. |
