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Originals Experiments in Fluids 25 (1998) 1Ð15 ( Springer-Verlag 1998 Dual emission laser induced fluorescence for direct planar scalar behavior measurements J. Coppeta, C. Rogers 1 Abstract In this paper, a new method of measuring scalar mixing measurements through the scalar pH can be dif®cult behavior in bulk aqueous ¯uid ¯ows is presented. Using due to non-uniformities in the light sheet and pH dependent a simple ratiometric scheme, laser induced ¯uorescence from absorption. These limitations can be shown by examining organic dyes can be normalized so that direct measurements of the simple case of a collimated beam of monochromatic a scalar in the ¯ow are possible. The technique dual emission light passing through a homogeneous ¯uorescent solution laser induced ¯uorescence (DELIF) relies on normalizing the (Guilbault 1973). It should be noted that the model presented ¯uorescence emission intensity of one dye with the ¯uores- below is not entirely general to pH dependent dyes under all cence emission intensity of a second dye. Since each dye pH and excitation conditions. Martin (1975) describes some of ¯uoresces at a different wavelength, one can optically separate the complications involved in ¯uorescence behavior under the emission of each dye. This paper contains an overview certain conditions of pH and excitation frequency. For the case of the basic ratiometric technique for pH and temperature of intermediate pH units and 514 nm excitation, the ¯uores- measurements as well as the spectral properties of nine water cence intensity measured at some arbitrary point along the soluble dyes. It also covers the three most signi®cant sources of excitation beam can be expressed as error in DELIF applications. To demonstrate the technique, I (b)\I (b)AULeC (1) steady state turbulent jet mixing and temperature ®elds in f e a thermal plume were quanti®ed. The accuracy was camera where I is the measured ¯uorescence intensity at a point f limited at under 3% of the ¯uorescence ratio which corres- b along the excitation beam's axis of symmetry, I is the e ponds to 0.1 pH units or 1.8 ¡C. intensity of the excitation light beam at point b, A is the fraction of ¯uorescence light collected, U is the quantum 1 ef®ciency, L is the length of the sampling volume along the Introduction path of the excitation beam, e molar absorptivity, and C is the molar concentration of the ¯uorophor. For the special case of Several studies pertaining to ¯uid mechanics have used Laser a pH dependent dye such as ¯uorescein, the molar absorptivity Induced Fluorescence (LIF) as a diagnostic technique for both is pH dependent. Therefore, Eq. (1) can be rewritten for ¯ow visualization and mixing measurements; Breidenthal ¯uorescein as follows: (1981), Koochesfahani and Dimotakis (1985), Bellerose and Rogers (1994), Cetegen and Mohammad (1993), Coppeta and I (b, pH)\I (b, pH)AUL (pH)C (2) f e e Rogers (1995), Walker (1987) and Coppeta and Rogers (1996). where the excitation intensity is now a function of both the Most of these studies used a single dye, ¯uorescein, as position and the pH ®eld through which the beam traveled a ¯uorescent tracer. Fluorescein is ubiquitous in LIF studies (again this model does not account for ¯uorescein's behavior because its physical properties are ideal; excitable with both under all possible conditions of pH and excitation frequency the 488 nm and 514 nm lines of an argon ion laser, water but does ®t the behavior observed under our working condi- soluble, pH dependent emission, high quantum ef®ciency and tions; pH ranges of 5 to 10 and excitation frequencies of low cost. However, using ¯uorescein to make quantitative 488 nm and/or 514 nm). This can be shown explicitly by the following expression for the excitation intensity at some arbitrary point b: Received 7 June 1996/Accepted 17 June 1997 I (b, pH)\I e~e(1H)lC (3) J. Coppeta, C. Rogers e 0 Dept. of Mechanical Engineering, Tufts University where l is the length of solution the excitation beam traveled Medford, MA, 02155 USA through before reaching point b. In order to relate the position dependent ¯uorescence intensity to a pH value, the excitation Correspondence to: C. Rogers intensity at point b must be known. In practice, calculating the excitation intensity at an arbitrary point would involve The author would like to thank Tufts University Professor David Walt for his guidance during the initial stages of this research. We would stepping downstream along a light ray and constantly correct- also like to thank McDonnell Douglas, Intel Corp. and Cabot Corp. for ing for laser light distribution and pH dependent absorption. partial funding of this work, and Tufts University Professor Robert Simple ratioing of experimental conditions with initial condi- Bridges for use of his laboratory's spectrophotometer. tions can be misleading due to light absorption or shadowing. For instance, a ¯uorescing specie in the shadow of a pH of 10 Cylindrical will ¯uoresce less intensely than if it were in the shadow of lens a pH of 4 simply because the pH of 10 solution absorbed more Interrogation tank laser light. This absorption is ¯ow dependent and can cause Laser sheet substantial errors in the measurements. In addition, alignment issues and laser light re¯ections would further complicate the calculation. One way to bypass these issues is to normalize the ¯uores- Beam splitter cence intensity of the pH dependent dye with a pH indepen- platform Laser dent dye. Returning to the simple case of a collimated beam of beam light passing through a homogeneous ¯uorescent solution now Beam 2 containing two dyes (both with a constant concentration splitter through out the solution) the ¯uorescence ratio at any point Filters(red &yellow) can be expressed as I (b) e (pH, j)C U Video camera 1f \ 1 1 1 (4) I (b) ( )C U 2f e2 j 2 2 where I is the ¯uorescence intensity of the pH dependent dye 1f and I is the ¯uorescence intensity of a pH independent dye. 2f Fig. 1. Top view of experimental set up From Eq. (4) it is evident that the ¯uorescence ratio is only a function of a few physical properties of the dyes, not the excitation intensity. Assuming that both ¯uorophors are present in constant concentrations everywhere in the ¯uid, Figure 2 demonstrates the effectiveness of the ratiometric these physical properties can be normalized through a calib- technique. Figure 2a and 2b show an intensity image of the LIF ration of ¯uorescence ratios versus pH. That is, the ratio of the from a laser sheet which has been ®ltered for yellow and red concentration quantum ef®ciency product is a constant and light respectively. The rectangular box in Fig. 2a encloses the does not in¯uence the change in the ratios with pH. Note the image subsection which is used for analysis in the subsequent ¯uorescence intensity of the pH dependent dye contains the ®gures. Figure 2c is an intensity ``image'' of the ratio values mixing information while the ¯uorescence intensity of the obtained by dividing the red and yellow intensity image secondary dye contains the excitation intensity information at subsections. Figure 2d shows an intensity plot versus horizon- every point in the laser sheet. If the ¯uorescence intensity from tal position for both the red and yellow images. Note the each dye is measured simultaneously, the ¯uorescence ratios intensity changes by a factor of two across the light sheet but will be independent of laser light alignment, distribution, and the ratio of these horizontal cuts (Fig. 2e) does not change intensity. appreciably. A similar analysis is performed for a vertical cut Numerous combinations of dyes can be used for this ratio across the light sheet in Fig. 2f and 2g. In both cases, while the technique. In this paper, we evaluate the characteristics of nine intensity varies by as much as a factor of two, the ratios are dyes to determine their compatibility in a ratiometric system. constant to within 3% of the mean ratio value (this is within The purpose of this paper is to present a number of possible the uncertainty due to camera response). dye combinations Ð each suited for different types of experi- ments. Although some of the spectra presented here can be 3 found in the literature, they are presented here for complete- Design considerations ness. In this paper, we ®rst present a demonstration of the While the analysis above examines the ratiometric method for ratiometric technique for measuring pH, then present a num- pH dependent dyes, a similar analysis can be performed for ber of different possible dye combinations for measuring pH dyes which are dependent on the concentration of another and temperature, and then conclude with two more simple scalar such as magnesium or calcium ion concentration in demonstrations (measuring the amount of mixing and the solution. Using pH as a scalar indicator of mixing has the temperature). advantage that one can return to the initial conditions by simply neutralizing the ¯uid. In contrast, some scalars such as 2 temperature affect the emission of a dye by a slightly different Ratiometric demonstration mechanism. Although the thermal effects may affect the The experimental set up used for demonstration purposes absorption band of a dye like the scalar pH, other mechanisms in this section is shown in Fig. 1. A 514 nm laser beam is such as collisional deactivation can affect the emission expanded into a sheet before passing through an interrogation intensity (Guilbault 1990). We will not attempt to model the tank ®lled with a ¯uorescent solution at a pH of 7. The solution underlying mechanism of thermal effects but we do utilize the is made up of two ¯uorescent dyes which emit in the red and phenomenon of temperature dependent emission to quantify yellow regions of the spectrum.

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