Parametric Investigations On The Emission Spectra And Photobleaching Time Constants Of Fluorescent Dye In Laser Induced Fluorescence Photobleaching Anemometer(LIFPA) Applications

Developing high efficiency microfluidic chips with desired functions and performance is the dream of worldwide researchers.However,the development of functional microfluidic devices requires deep understanding on the flow dynamics involved.Especially,when the dimensions of flow decreases to nanoscale,many new flow phenomena have been observed and remain unsolved.To reveal the puzzle in micro/nanofluidics,discover new phenomena and make comprehensive understandings on the flow dynamics of micro/nanofluidics,proper flow diagnostic techniques on velocity measurement is required.In this study,the factors,e.g.fluorescent excitation and emission spectra and photobleaching time constant that may affect laser induced fluorescence photobleaching anemometer(LIFPA)measurements,have been investigated with coumarin 102 fluorescent dye in a confocal-microscope based system.We found that the peak position of excitation spectrum shifted to the long wave direction with the increase of dye concentration in the measurement of excitation spectrum without photobleaching,so called-“red shift”,In the case of photobleaching,the emission spectrum of Coumarin 102 solution shifted to the short wave direction,so called,“blue shift”,and the blue shift degree increased with the increase of dye concentration.We tried to explain the bleaching phenomenon mechanism with photochemical reaction.In the meanwhile,different photobleaching time constants are also observed in different bands of the emission spectrum.All of the photobleaching time constants are below 3 ?s,which is nearly an order smaller than what have been reported before.Among all of the spectral bands,for studying an unsteady flow with small velocity fluctuations and relatively lower frequency components,the bands at 470-492 nm can provide a higher fluorescent intensity and better accuracy for velocity measurements.When studying a fast transient process with a high frequency,the bands at 430 and 460 nm can provide a higher temporal resolution.Finally,by improving the Lifpa device,a single optical fiber or pinhole in the traditional confocal microscopy is replaced by a fiber bundle,so that the intensity distribution of the fluorescent spot can be measured simultaneously while spatial filtering,further,according to the distribution law of fluorescence intensity decreasing along the flow direction,we can simultaneously measure the flow size and direction on the plane perpendicular to the optical axis,that is,to measure the two-dimensional flow velocity vector.