Parametrical Investigations On Laser Induced Fluorescence Photobleaching Anemometer And System Development On A New Fluorescence Photobleaching Velocimetry

Micro/nano-scale flow phenomena commonly exist in natural and engineering fields,e.g.life science,energy and chemistry industry,micro/nanofluidics and biomedical engineering,etc.For example,blood transport in microcapillary,ion transport through cell membrane and fluid mixing in microreactor are all typical micro/nano-scale flow phenomena.Measuring the velocity variation of micro-nano-fluids is a necessary means to understand the flow phenomenon and reveal the flow mechanism,and is also the core content of hydrodynamics and micro-nano-fluidics.To uncover the complex flow phenomena at micro and nano scales,Wang developed in 2005 the Laser Induced Fluorescence Photobleaching Anemometer(LIFPA)technology with high spatial and temporal resolution.However,as a speed measurement technology based on fluorescence bleaching,there are still some unsolved mechanical and technical problems.For example,the relationship between the photobleaching time constant τ,which determines the time resolution of LIFPA system,and the photobleaching reaction of fluorescent solution in the process of transport is not clear.In this manuscript,based on the LIFPA system,the fluorescent bleaching velocimetry technology and its mechanism are studied based on the transport and reaction process of fluorescent dyes through the laser focus area.Firstly,based on convection-diffusion-reaction equation,a theoretical model of fluorescence bleaching velocimetry for LIFPA is proposed.Through the finite element simulation software COMSOL Multiphysics,combined with the experimental results of LIFPA,it is found that there is a nonlinear relationship between the photochemical reaction coefficient K and the photobleaching time constant τ.At the same time,Péclet Number(Pe)and dimensionless parameters Zpd were used to study the effect of photobleaching on the effective concentration and fluorescence intensity distribution of fluorescent dyes.It is found that as Pe increases and Zpd remains constant,the comet broadening of the effective concentration distribution of the fluorescent dye induced by photobleaching decreases,and the peak position of the corresponding fluorescence intensity moves towards the center of the focus region along the flow direction.While as Pe remains constant and Zpd increases,the effective dye concentration and fluorescence intensity in the laser focus region decrease,and the peak position of the fluorescence intensity moves upstream along the axis until it approaches the edge of the laser focus region.Then,the effective spatial resolution deff of LIFPA is proposed based on the fluorescence signal distribution of the excitation spot,and deff is defined as the distance from If,local,1 to 0.05 If,local,1 in the fluorescence detection region.It is found that when the photochemical reaction coefficient K=8×106 l/s,the effective spatial resolution doff=66 nm is obtained.Based on these results,two possible ways to further improve the spatial resolution and break through the optical diffraction limit in the LIFPA system constructed with a confocal microscope are proposed:(1)increasing the photochemical reaction coefficient K,which can also increase the temporal resolution of the system at the same time;and(2)reducing the width of the fluorescence detection region dcl.Numerical simulations show that when the width of the detection region dcl is reduced to 0.5 df,the deff of the system can be further reduced to 54 nm.Finally,based on the fluorescence bleaching velocimetry model,an imaging based microfluidic velocimetry technique,Fluorescence Photobleaching Velocimetry(FPV),is proposed.This technique solves the problem that the LIFPA technique can not identify the velocity direction,and can measure the velocity and direction of several detecting points in the flow field synchronously.We have preliminarily completed the FPV experimental device and the speed measurement software design.In this manuscript,the mechanism of fluorescence bleaching velocimetry is studied.On the one hand,it provides a feasible direction to improve the spatial and temporal resolution of LIFPA.On the other hand,the FPV technology proposed in this paper makes up for the shortages of LIFPA technology in velocity direction identification and velocity measurement in large-scale flow field.It promotes the development of fluid velocity diagnosis technology,and helps us to explore the microscopic fluid world.