Experiments in Fluids

, 59:21 | Cite as

Ratiometric, single-dye, pH-sensitive inhibited laser-induced fluorescence for the characterization of mixing and mass transfer

  • Tom LacassagneEmail author
  • Serge Simoëns
  • Mahmoud El Hajem
  • Jean-Yves Champagne
Research Article


Inhibited planar laser-induced fluorescence (I-PLIF) techniques are widely used for heat and mass transfer studies in fluid mechanics. They allow the visualization of instantaneous two-dimensional field of a passive or reactive scalar, providing that this scalar acts as an inhibitor to the fluorescence of a specific molecule, and that this molecule is homogeneously mixed in the fluid at a known concentration. Local scalar values are deduced from fluorescence recordings thanks to preliminary calibration procedure. When confronted with non-optically thin systems, however, the knowledge of the excitation intensity distribution in the region of interest is also required, and this information is most of the time hard to obtain. To overcome that problem, two-color ratiometric PLIF techniques (\(\text {I}^\text {r}\)-PLIF) have been developed. In these methods, the ratio of two different fluorescence wavelengths triggered by the same excitation is used as an indicator of the scalar value. Such techniques have been used for temperature measurements in several studies but never, to the author’s knowledge, for pH tracking and acid–base mixing, despite the frequent use of the one-color version in mass transfer studies. In the present work, a ratiometric pH-sensitive-inhibited PLIF technique (\(\text {I}_\text {pH}^\text {r}\)-PLIF) using fluorescein sodium as a single dye and applicable to complex geometries and flows is developed. Theoretical considerations show that the ratio of the two-color fluorescence intensities should only depend on the dye’s spectral quantum yield, itself pH-dependent. A detailed spectrofluorimetric study of fluorescein reveals that this ratio strictly increases with the pH for two well-chosen spectral bands (fluorescence colors). A similar trend is found when using sCmos cameras equipped with optical filters to record fluorescence signals. The method is then experimented on a test flow, a turbulent acidic jet injected in an initially pH-neutral volume of fluid. The results obtained using the ratiometric version are consistent with single-color technique measurements, but excitation intensity heterogeneity is more efficiently accounted for, with a much smaller time needed for data treatment and without requiring the knowledge of laser paths across the fluid. This new technique is also able to reduce the impact of some unwanted experimental features such as time-varying excitation intensity or reflections at interfaces. It can be of great interest for further applications to multiphase mass transfer studies.

List of symbols

\(( . )_\text {rms}\)

Root-mean-square (RMS) operator


Molar concentration

\(\delta I_1\)

Intensity collected from spectral band 1

\(\delta I_2\)

Intensity collected from spectral band 2


Fluorescein molar extinction coefficient

\(\lambda _\text {e}\)

Laser excitation wavelength

\(\lambda _\text {f}\)

(fluorescence) Wavelength

\(\overline{ . }\)

Temporal averaging


Fluorescence quantum yield

\(\tau _\text {L}\)

Turbulence integral timescale


Optical constant


Fluorescent dye concentration

\(C_\text {A}\)

Acid concentration

\(C_{\text {A},\infty }\)

Bulk acid concentration

\(C_\text {A,inj}\)

Injection acid concentration


Nozzle diameter

\({\rm d}I_{\rm a}\)

Local absorbed intensity

\(\text {d}I_\text {f}\)

Local fluoresced intensity

\(\text {d}t\)

Camera’s exposure time

\(e_\text {L}\)

Laser sheet thickness


Lens focal length

\(f_\text {r}\)

Camera recording frequency


Laser output intensity

\(I_\text {f}\)

Fluoresced intensity

\(I_\text {i}\)

Incident intensity

\(I_\text {r}\)

Received fluoresced light intensity

\(L_\text {s}\)

Length of fluid sample crossed by the incident beam

\(L_\text {obs}\)

Distance between a point in the fluid and the sensor


Mass of dissolved fluorescein


Jet flow rate


Spectral bands ratio


Wavelengths ratio

\(R_\text {n}\)

Normalized intensity ratio

\(R_\text {r}\)

Ratio submitted to re-absorption


Jet Reynolds number


Spectral quantum yield

\(s_\text {px}\)

Surface of the laser sheet corresponding to one camera pixel




Jet velocity


Main tank volume


Acid tank volume


Reservoir volume

\(V_\text {s}\)

Sample volume



The authors gratefully thank Jean-Marie Bluet and the INL laboratory for providing the spectrofluorimeter used for the fluorescence studies.


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Copyright information

© Springer-Verlag GmbH Germany, part of Springer Nature 2017

Authors and Affiliations

  1. 1.Univ Lyon, INSA de Lyon, Ecole Centrale de Lyon, Université Lyon 1, CNRS, LMFA UMR 5509Villeurbanne CedexFrance

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