Study Of Three-dimensional Measurement Of Micro Flow Field Based Light Field Microscopic Imaging

Microfluidic devices have the advantages of good controllability of liquid flow,low consumption of sample and reagent,and fast analysis speed.Therefore,microfluidic devices have wide application prospects in the field of biological,medical,electronic integrated circuits and other fields.The performance of microfluidic devices is closely related to the flow characteristics in the micro-scale channel.Accurate characterization of a three-dimensional(3D)micro-scale flow can provide reference and guidance for the design and performance improvement of microfluidic devices.Currently,micro particle image velocimetry(Micro-PIV)is one of the most effective experimental techniques for the characterization of the micro-scale flow.Based on the light field imaging technique,a single camera can record the position and direction information of light field in a single exposure,which is expected to overcome the shortcomings of existing3 D Micro-PIV techniques for the 3D position information of tracer particles,such as low time resolution and complexity in instrumentation system.In this dissertation,a 3D Micro-PIV based on the light field imaging was proposed,and a systematic and in-depth theoretical and experimental study was carried out.Based on the diffraction theory,the point spread function(PSF)calculation model of the light field microscopic imaging system was established,and the experimental verification of the PSF model was carried out.The results show that the calculated results from the PSF model are in good agreement with the experimental PSF of the light field microscopic imaging system,which verifies the accuracy of the model,and provides the basis for the imaging process simulation and deconvolution reconstruction of the light field microscopic imaging system.On the basis of the PSF model of light field microscopic imaging system,combined with LucyRichardson algorithm,a deconvolution 3D reconstruction method based on diffraction imaging is proposed.Both theoretical and experimental results show that this method can reconstruct the light intensity distribution of tracer particles at different depths of micro-scale flow field by using light field image.Furthermore,the deconvolution 3D reconstruction method is used to analyze and evaluate the performance of the light field microscopic imaging system(spatial resolution,the influence of tracer particle concentration on reconstruction quality,the reconstruction accuracy of tracer particles and the accuracy of absolute velocity measurement).Compared with the digital refocusing reconstruction method,the deconvolution 3D reconstruction method based on diffraction imaging improves the lateral resolution and axial resolution by7.5 times and 5.2 times,respectively.The effects of microlens aperture,size of CCD sensor pixel,magnification of objective lens and the spatial position of the tracer particle on the spatial resolution and reconstruction quality of the ligt field micro particle image velocimetry system(LF Micro-PIV)were analyzed systematically.The results show that compared with the region near the focal plane of the objective lens,the lateral and axial resolutions are higher in the regions far away from the focal plane of the objective lens,and is less affected by the microlens aperture,CCD sensor pixel size,and objective lens magnification.When the measurement volume is placed in the inside region close to the objective lens,it has the optimal lateral and axial resolution.In this region,the large microlens aperture and small pixel size of CCD sensor are useful to improve the lateral and axial reconstruction resolution.These results provide a theoretical basis for parameter optimization and construction of the LF Micro-PIV.Based on the optimized optical parameters,a cage light field camera was designed and assembled,and a LF Micro-PIV was further constructed.A calibration method of the cage light field camera was proposed,the calibration of the optical parameters of the cage light field camera was carred out.An application test software of LF Micro-PIV developed,which integrates PSF calculation,camera calibration,deconvolution reconstruction and 3D cross-correlation velocity vector calculation of micro-scale flow field.Based on the LF Micro-PIV,the 3D velocity fields of micro-scale flow in rectangular,tapered and T-channel chips were tested experimentally.The results show that the LF Micro-PIV can measure the 3D velocity field of laminar flow(flow in rectangular straight micro-scale channel)and complex flow(flow in tapered micro-scale channel and T-shaped micro-scale channel),and the measured results are in agreement with the typical flow characteristics in the channel.Compared with the results of Ansys simulation,except for the region near the wall,the flow velocity field distribution is in good agreement,and the error is within a reasonable range,which proves the feasibility and accuracy of light field micro particle image velocimetry technique.