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PHOTOCHROME DYE TRACING IN WATER FLOWST. W. FOGWELL a & C. B. HOPE aa Harwell Laboratory , Englandb Department of Chemical Engineering , Imperial College , London, EnglandPublished online: 27 Apr 2007.

To cite this article: T. W. FOGWELL & C. B. HOPE (1987) PHOTOCHROME DYE TRACING IN WATER FLOWS, ExperimentalHeat Transfer: A Journal of Thermal Energy Generation, Transport, Storage, and Conversion, 1:2, 141-154, DOI:10.1080/08916158708946337

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Experimental Heat Transfer, vol, I, pp. 141-154, 1987

PHOTOCHROMIC DYE TRACING IN WATER FLOWS

T. W. FogwellHarwell Laboratory. England

C. B. HopeDepartment of Chemical Engineering. Imperial College, London, England

The properties and uses of photochromic dyes ore presented, and the history of theuse of various photochromic dyes in fluid flow experiments is discussed. The mech­anisms and theory of the changes in properties of various types ofphotochromic dyesare presented. In response to the need for a water-soluble photochromic dye, a tri­ary/methane dye, Acid Violet 19, was selected/or use in two-phase air-water exper­iments. Its optical and chemical properties are given, and the dependence of its rateconstant on various parameters is studied. Details of the use of Acid Violet 19 influid flow experiments are presented.

INTRODUCTION

The techniques described here depend on the use of photochromic dyes. Aphotochromic dye is distinguished from other types by the fact that part of photo­chromic means that some wavelength of light causes a change in the dye. The chromicpart of photochromic signifies that the change is a change in color. The other dis­tinguishing feature of photochromic dyes is that the reaction that occurs is reversible.It can reverse instantly or it can wait for some energy input of some sort to causethe reversal of the reaction. This definition makes photochromism different from aphotochemical reaction in which the reaction is considered virtually irreversible. Alsomost photochemical reactions have the additional property that the introduction of agiven number of photons of light serves essentially as a catalyst for a reaction thattakes place not only in the molecule that receives the light but also in adjacent mol­ecules; this produces the extreme sensitivity of some photographic films. In photo­chromic reactions, it is only the light that causes the change, so the activation energynecessary for a certain change is very important in the usefulness of a given pho­tochromic dye.

Photochromic dyes have found many uses in recent years [I, 2]. The mostfamiliar perhaps is their use as eye protectors. Many companies now sell sunglassesthat change to darker colors when light falls on them. The rate at which these dyeschange color depends greatly on the matrix in which they are embedded. A com­petitive edge seems to go to the company that can produce a combination of dyeand matrix that produces color changes more rapidly. Another form of eye protectionwas developed for sudden flashes of light, which might be caused in certain militarysituations [3]. Eye protection for some of these cases using photochromic dyes wasdeveloped at Aldermaston 14].

Probably the most active current area of research and development on photo­chromic dyes is for their use in information storage and retrieval [5-7]. Theoreti­cally, large amounts of data can be stored in a very small space by using light to

Copyright © 1987 by Hemisphere Publishing Corporation 141

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142 T. W. FOGWELL AND C. B. HOPE

change a color at a certain point and then using a color detection device to read thepattern of the changed color [8]. These effects can be made reversible by using onewavelength of light to cause the original color change and a different wavelength tochange it back.

MECHANISMS AND THEORY

The most important feature of photochromic changes is the change in absorp­tion spectrum from one form of the dye to another form. An example of this is shownin Fig. I. The most effective wavelength for the initial absorption is usually at themaximum absorption in the original spectrum. Sometimes, however, the dye is ca­pable of several different types of changes, which can be selectively triggered byinjection of energy at certain wavelengths. Then, in order to get the molecule to acertain state, it is necessary to use two or more wavelengths at the same time so thatas the transition is made at one of the wavelengths, the other transition is immediatelyfacilitated by the second wavelengths.

Many things can happen to a molecule when a photon of light hits it. Themolecule can remain unaffected or can absorb the energy of the light, depending onthe wavelength. When the energy is absorbed, it can cause several things to happen.The energy might be used to change the orbital state of the electrons. Then fromthis higher energy state the electrons might fall back to some lower state, not nec­essarily the ground state. As they do so, the molecule can emit light. If the electronsdrop from an excited singlet state to the ground state the result is fluorescence. Ifthe drop is from a triplet state to the ground, the result is phosphorescence. Theemission spectrum of the resulting fluorescence or phosphorescence is charactersticof the particular molecule.

If the next change after the excitation of the electrons in the molecule is notrelaxation to the ground state but instead a change in the chemical nature of themolecule, a photochemical reaction takes place. The chemical change can result ina molecule with an absorption spectrum different from the first one. If this change

700600

r-.. \I .

f \! \I •. \I .

• II .i \! \

I \i \

i \\! \\._.~/ \

400 500Wavelength (nm)

300200

"uc'".0~

o

".0<t

">~

'""'"

Fig. 1 Absorption spectra of colorless (solid curve) and colored (brokencurve) solutions of 5,7-dichloro-6-nitroBIPS in ethanol at 20'C. FromBrown [2].

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PHOTOCHROMIC DYE TRACING IN WATER FLOWS 143

is reversible, it is photochromism. Some of the types of chemical reactions possibleare given by Brown [2]. They include heterolytic cleavage, homolytic cleavage, cis­trans isomerization, tautomerism, lattice defects, ionization, and various biochemicalchanges.

TECHNIQUES IN FLOW VISUALIZATIONAND VELOCITY MEASUREMENTS

Flow visualization is an extremely powerful means of establishing the velocityfields within a flow system. Generally the process relies on the addition of foreignmaterial into the flow field, rendering the region of interest visually perceptible.Several established techniques that have been applied to the study of liquid flowseemingly disclose the true nature of the flow but are limited in accuracy becauseof their very presence. Dye has been injected into the system upstream of the pointof interest and its progress followed using a movie camera. Similarly, injection ofan aqueous solution of fluorescein sodium, which is highly fluorescent on irradiationwith ultraviolet light, has been used. It is, however, obvious that no matter howcarefully the injection process is performed, it still undoubtedly disturbs the flow.For water systems a technique of injecting air or oxygen bubbles from probes po­sitioned within the flow has been used. This technique was further refined to the useof hydrogen bubbles produced electrolytically at electrodes situated within the flowstream. The problem here is twofold. Not only does the presence of the injectionprobe (or electrodes for hydrogen production) seriously disrupt the flow but also thedensity mismatch between the bubble and the displaced fluid produces buoyancyeffects such that the bubble does not follow the path the displaced fluid would havetaken. Solid particles such as polystyrene spheres have been added to the fluid understudy, but again the density difference leads to an erroneous representation of thetrue velocity field.

Use of Photochromic Dyes

A technique that does not suffer from the above disadvantages, in that it isnondisturbing, is that of photochromic dye tracing. The first application of the tech­nique may be attributed to Goldish et al. [9], although the process was not strictlyphotochromic in the sense that it was irreversible. Goldish et al. applied an adaptedversion of the blueprint reaction to the study of axial dispersion in laminar flow ina round tube. Their solution (clear yellow-green), on irradiation with light in therange blue to ultraviolet, produced ferrous ions, which reacted with ferricyanide toproduce ferrous berlinate, known as Trunbull's blue, a dark blue colloidal suspen­sion. The irradiation of the solution was performed using a high-intensity xenon flashlamp in the form of a helical coil around the test tube. This produced a "plug" ofcolor that could be followed by means of colorimetry. Although nondisturbing, thisprocess is unsuitable for the study of velocity profiles because the solution requiresa fairly long period of irradiation to secure the color change.

An improvement on this technique, and the first application of a truly photo­chromic reaction, was made by Popovich and Hummel [10]. The dye type used waspyridine, specifically 2-(2,4-dinitrobenzyl)-pyridine (DNBP), dissolved in a 95% ethyl

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144 T. W. FOG WELL AND C. B. HOPE

alcohol solution to a concentration of 0.1 % by weight. A parabolic mirror was usedto focus the light from a xenon flashtube, which forced the solution to undergo the

H

tautomeric shift producing the dark blue trace in less than 3 u.s. The method wasapplied to the study of flow conditions in the viscous sublayer in turbulent pipe flow.

A further refinement was made by Frantisak et al. [II], who replaed the flash­tube with a laser, producing, after frequency doubling, a well-collimated high-energyultraviolet beam. This markedly improved the coherence and contrast of the trace.The improvement was complemented by the use of a high-speed cine camera (10,000frames/s) to record the trace deflection, allowing the technique to be applied toturbulent flow conditions, whereas previously it had been limited to viscous flowstudies.

The major drawback of the technique at that stage, as noted by Zolotorofe andScheele [12], was that after irradiation the lifetime of the DNBP trace in 95% alcoholwas very short. Alternative solvents were found to yield an even shorter trace life.Zolotorofe and Scheele needed to measure laminar velocity distributions of low Rey­nolds numbers in an investigation of hydrodynamic stabilility in a heated verticalpipe. The existing technique was inadequate in that at the low flow rates involvedthe trace deflection, in the time that the trace remained visible, was insufficient toafford accurate measurements. This induced the search for an alternative dye witha longer life span. Zolotorofe and Scheele were advised to use a spiropyran dye inan aromatic solvent. The specific dye employed was 6-nitro-1 ,2,3-trimethyl­spiro(2H-benzopyran-2,2-indoline) (TMINBPS). The forward reaction in the case ofthis dye type relies on the ultraviolet radiation breaking the carbon-oxygen bond inthe spiropyran ring:

TMINBPS has been found to produce an effective color change In a number of

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PHOTOCHROMIC DYE TRACING IN WATER FLOWS 145

solvents, including benzene, toluene, xylene, acetone, and carbon tetrachloride. In­deed, the color of the solution in the excited and unexcited states varies dependingon the solvent.

Many studies have been effectively conducted using the TMINBPS dye andhave yielded interesting if not important results. For example, Fowlis [13] at NASAcarried out preliminary studies of free convection within fluids, using this dye. Theultimate application was carried out under the near zero gravity conditions of Space­lab. The technique is, however, severely limited by the insolubility of TMINBPS inwater. This directly limits the scale of application (on economic and safety grounds)and indirectly limits the flow range that may be employed. The toxicity and generalobnoxious nature of the available solvents have meant that the majority of the flowsstudied up to the present have been gravity-induced. The necessity to operate overa wide range of Reynolds numbers and on a comparatively large scale was the in­centive for the current investigation.

PROPERTIES OF TRIARYLMETHANE DYE-ACID VIOLET 19

For fluid flow experiments there are advantages to having a photochromic dyethat is water-soluble. One advantage is that of safety; it is much safer to work withwater in flow experiments than it is to work with organic fluids. Another is that mostprevious measurements and experiments have been conducted using water; so thereis a vast store of data on water systems that can be used for comparison. Also, manyapplications of fluid mechanics require the use of water.

One method of obtaining a water-soluble dye is to take a dye that is solubleonly in organic solvents and sulfonate it. A water-soluble compound of this type waspatented in Japan as patent No. 48-23787, dated March 1973 [14]. Harwell attemptedto obtain samples of the dye from the assignees of the patent but were told that itwas unavailable from them. It is a spiropyran dye that is sulfonated. Chemists atHarwell made and tested samples of the dye, which turned out not to have the pho­tochromic properties required. This was probably due to the polar nature of wateras a solvent. In an organic solvent this type of dye changes color as a result ofphotoactivation by the mechanism of heterolytic cleavage. In a highly polar solvent,this cleavage is triggered by the solvent itself at fairly low temperatures.

A more promising attempt using the same approach and some fulgide dyesdeveloped by Heller ([15]; H. G. Heller, personal communication) is being fundedby NASA. The advantage of these fulgide photochromic dyes is that their activationwavelength is about 350 nm. Preliminary tests seem to indicate that they maintaintheir photochromic properties in highly polar solvents. A water-soluble photochromicdye was manufactured by researchers in the Chemical Engineering Department atthe University of Toronto. Although the dye was water-soluble, it had the difficultythat the activation wavelength required was well below 275 nm (M. Ojha; personalcommunication). Very few materials will transmit UV light at such low wavelengths.The dye had a p-nitrobenzyl structure with a carboxylate ion substituent ortho to thealkyl group and changed color as a result of an alkyl-carboxylate H transfer. Thecolor change was not very easily observed, either, for this dye. We also looked intothe possibility of using long-duration phosphorescence, but although there are water-

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146 T. W. FOGWELL AND C. B. HOPE

soluble phosphorescent materials, the presence of oxygen quenches the phospho­rescence.

The dyes that we found to be the best for our purposes were ones that under­went ionization when activated by the right UV wavelength of light. They are calledtriarylmethane dyes, and their ionic form looks like the following:

R R

-, /

/R

c+

where the R's are usually aromatic ring structures. The ionic form is colored andthe bleached form is usually colorless. The bleaching can be done by either eN orHS03• We selected Acid Violet 19 and used HS03 as the bleaching agent for safetyreasons. The reaction is given as follows:

R, /RC

/' +R

+ H503 - -

The absorption spectra for the bleached and unbleached forms are shown in Figs. 2and 3. There is a local peak in the absorption spectrum of the bleached dye at about310 nm. We use an Xe-C1 excimer laser, which emits UV at about 308 nrn, for thelight activation source. This converts the bleached nonionic form of the dye to thecolored ionic form. A time history of the relative absorbance of the dye is shownin Fig. 4 for a pH of 9.0 and dye concentration of 4.0 x 10-6 M with a salt con­centration of 0.14 M, buffered by borax.

7BIJ BBB

I ••

g•

••7.

~ 5•...u~ sa'"0:0 ••~

'"-c ,.Zll

10

2.. "'. ••• "'" 50.

VAVELENCTHC....)

Fig. 2 Acid Violet 19 (bleached pH-5.6).

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PHOTOCHROMIC DYE TRACING IN WATER FLOWS

,"..88

7.

tl .."'uz

58<CDa:0 ••'"CD-c

3B

2.

,.2•• 3•• '00 5" s00 ". ...

YAVElENG TH (n",)

Fig.3 Acid Violet 19 (unbleached).

The structure of pure Acid Violet 19 is

147

The method of preparation of a working solution of this dye is different fromthat for the pyridine or spiropyran dyes. On dissolution in water the dye is in thecolored ionized state. Decoloration of the solution is then effected by the additionof anions. On exposure to ultraviolet light the starting color returns and then grad­ually fades, the fading rate depending on a number of factors, principally the pH.

Dye Concentration

An excimer laser operating with a proportionate xenon-chloride gas mixturewas employed as the ultraviolet light soure. The output of the laser was of a variable­frequency intermittent nature, the radiation wavelength being 308 nm. The energyper pulse and pulse duration were 6 mJ and 7 ns, respectively. The 308 nm wave­length corresponded almost exactly to a local peak absorption of the colorless so­lution. It was found that with a dye concentration of 0.01 % by weight the laserproduced a highly contrasting trace extending approximately 20 mm into the solu­tion. Although this penetration was sufficient for the present study it was by no

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t48 T. W. FOG WELL AND C. B. HOPE

XI~-l

.·/5,,- -----_-------------_-------,

!'-...._ ....._ ....I----------'-~r~--..._'l'i-_"""........-----. --------

r /Mf( l'flS 1

Fig. 4 Graph of relative absorbance versus time.

means a maximum. With a fixed irradiating power the depth of penetration may beincreased by reducing the dye concentration, but with a loss of trace contrast. Al­ternatively, with a fixed dye concentration the penetration can be increased by in­creasing the irradiating power, with no loss of trace contrast. It should be obviousin this respect that power losses should be kept to a minimum and hence windowmaterials are extremely important. This is discussed below.

Operating pH

It was recommended that the solution should be activated while in the pH range5 to 6. This was found to produce a favorable trace, although provided the solutionremained bleached a colored trace could be induced with the pH set below 5 andpossibly as high as 7. The pH was found to be the main factor controlling the rateof fade of the colored trace. The lower the pH the slower the trace faded; the timefor the trace to fade ranged from about 5 to lOs with the pH between 5 and 6 tomore than a minute with the pH below 3. A problem with operating at a low pHwas that over a period of time the solution tended to revert to the starting color ofthe dye; hence for optimal results it was necessary to maintain the pH between 5and 6.

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PHOTOCHROMIC DYE TRACING IN WATER FLOWS 149

Bleaching

As mentioned, the decoloration of the dye solution is effected by the additionof anions, especially cyanide, hydroxide, sulfite, or bisulfite. The pH of the dyesolution prior to the addition of bleach was ideally positioned between 5 and 6,specifically at 5.4. It was found that addition of the bleaching anions displaced thepH away from this optimum range, either to the left or right depending on the specificanion used. By employing the correct proportionate amounts of two bleaching agentsthat had opposite effects on the pH, it was possible, by taking advantage of thecanceling effect, to maintain the pH in the range required. The bleaching agents usedwere potassium sulfite (K ZS03) , which on its own had a tendency to raise the pH,and sodium metabisulfite (NazSzOs) , which tended to lower the pH. The requiredconcentration of K ZS0 3 used on its own was 20 g per gram of dye. Through directmolecular proportioning, taking NazSzO s to be twice as powerful an agent as K ZS0 3 ,

it was calculated that the equivalent amount of NazSzOs required was 12 g per gramof dye. Extensive experimentation revealed that using 60% KZS0 3 with 40% NazSzO,(i.e., 12 g K ZS0 3/g dye + 4.8 g Na2S20,/g dye) yielded a pH for the resultingsolution of just below 7. The reason for selecting that particular pH is discussedbelow.

Stability of the Bleached Solution

It was found that over a period of time the pH of the solution dropped (to aslow as 2) and the color returned. The gradual acidity increase of the solution was aconsequence of the inherent nature of sulfite to form sulfate in the presence of ox­ygen. The solution could be rebleached by restoring the pH to between 5 and 6 withthe addition of an alkali (NaOH); however, the subsequent time for the pH to dropwas significantly reduced. It may be thought that using K ZS0 3 (which raises the pHto approximately 9) on its own would be advantageous in this respect; however,experience has shown that if the pH of the solution is in an alkaline sense, it willgradually drop until it is just above 7 and will stabilize there. Addition of acid wouldthen be required, which is an awkward process to control in this particular pH range(5-7). Initial setting of the pH just below 7 by manipulation of the bleaching agentconcentrations went some way to increasing the working life of the solution, butclearly in a practical sense the stability remained unsatisfactory.

Buffer

As a means of stabilizing the pH a number of buffer solutions were investi­gated. Because of concerns over surface-active properties, the buffers available forconsideration were limited to those that were inorganic. The successful candidate,in terms of maintaining the pH while not appreciably affecting the surface tension,was a mixture of disodium hydrogen orthophosphate (NazHPO.) and sodium dihy­drogen orthophospate (NaHzPO.). The pH was set using proportionate amounts ofeach in a fixed volume of water. The dye and bleach were then added to this solution.Specifically, for a pH of 5.8 (the lowest possible with this particular buffer) the

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ISO T. W. FOGWELL AND C. B. HOPE

!!;<,<

Z.J

concentrations required were 1.136 g Na2HPO. plus 14.357 g NaH2PO. per liter ofsolvent.

Important work on the kinetics of the back reaction was carried out at Uni­versity College of Swansea by Peter Douglas with his student, G. Hickel. The data(P. Douglas, personal communication) are consistent with the reaction equation

for rate constant K. At high SO~- concentrations this becomes fairly constant andthe rate equation is

d[dye]-- = kob,[dye]

dt

where

This first-order reaction behavior is corroborated by the data shown in Fig. 5. A plotof Kob' versus SO~- conentration is given in Fig. 6. The effect of dye concentrationon the rate constant for the first-order reaction seemed to be negligible. A change

-5.G~..-- ~ ---,

-5.28 WAVELENGTH=S11 om

-2.813

~.(J8 t:ITl--------s .08

XHr"1TIM[(m~)

Fig. S First-order treatment of data.

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PHOTOCHROMIC DYE TRACING IN WATER FLOWS 151

(50,'- I • 10) I mol r'Fig. 6 Kotn versus SO~- concentration.

of ionic strength did, however, have an effect on the rate constant, which decreasedwith increasing ionic strength. This is supportive of the assumption that the actualreaction is

c+

+ HS03 - _ c

The rate constant decreases as the pH is lowered, as indicated by the plot in Fig. 7.

'2°6:-------!,-------!----:------:'::----":------:':,---------,J

pH

Fig. 7 Plot of observed rate constant versus pH.

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152 T. W. FOGWELL AND C. B. HOPE

OPTICAL CONSIDERATIONS

Wavelengths

As mentioned above, the dye solution was activated using the ultraviolet lightfrom a gas-filled excimer laser. The advantage of using laser light is the ability toproduce a single-wavelength beam that corresponds to the maximum absorptionwavelength of the bleached dye. This will yield the maximum possible optical den­sity for the produced trace. This, however, is not always optimal, because the depthof penetration and the optical density of the trace are inversely related. Where thelaser power is limited it is a question of optimizing the depth of penetration andoptical density for the specific application.

Window Materials

Obviously, the power loss of the ultraviolet beam between the source and targetshould be kept to a minimum. This requires that the vessel containing the dye so­lution and any additional material employed to improve the visualization should beas transmissively efficient, at the particular wavelength of ultraviolet used, as pos­sible. A study of window materials for flow visualization has been made by Fogwell[ 161.

Fluorinated ethylene propylene (FEP) has a refractive index very close to thatof water. Also, its transmittance for light with a wavelength of 308 nm is greaterthan 90%, hence it makes an ideal tubing material for water studies. Fused silicawas found to be an ideal window material for the water jacket as it transmits almost100% of light from the ultraviolet though the visible to the infrared. It can thus beused in the viewing port, illumination port, and entry port for the laser beam withpractically no power attenuation.

Filming

High-speed cine films have proved equally effective when made in both blackand white and color. In both cases diffuse light from a 250 W bulb has been usedas the illumination source. For black-and-white films the trace contrast has beenoptimized by using a narrowband interference filter on the back light, which passesonly a narrow wavelength band corresponding to the maximum absorption of thedye solution in the colored form. Thus the trace appears effectively black. With colorfilms, however, the opposite is the case. The highest contrast is achieved using back­ground lighting that covers as wide a wavelength range as possible, hence diffuselight with no filtering produces the best results. The filming rate for optimal resultsdepends on the nature of the event being studied, but with equipment at Harwell itcan be anything up to 10,000 frames/s, allowing intricate details of high-speed eventsto be studied.

Viewing Trough Surfaces

As mentioned, the geometric arrangement for flow visualization is all-impor­tant. Curved boundaries and mismatch of refractive indices can lead to loss of in­formation through image distortion. Although it is possible to compensate for thedistortions in calculations, no amount of mathematical manipulation can restore the

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PHOTOCHROMIC DYE TRACING IN WATER FLOWS 153

Cam,rax

Back light

Ilrullhnation SOl,ltee

Circular tubewoll

xLant

F'IOI w,ndowsvr rcc es

Fig. 8 Recommended geometry.

information that is not only distorted but lost altogether. Curved boundaries can be"eliminated" if the refractive index (RI) of the tube material matches that of thesolution under study and the outside of the tube is surrounded by a jacket containingthe solution and having plane surfaces. The viewing, illumination, and laser entrywindows (see Fig. 8) should be at 90° to the hardware associated with their functions.This will eliminate RI mismatch effects at the window-air and window-solvent in-

Fig. 9 Photochromic dye tracing in air-water system.

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154 T. W. FOG WELL AND C. B. HOPE

terfaces. Transmission loss by reflection at the interfaces can be minimized by ap­plying certain coatings to the window surfaces [16].

CONCLUSION AND SUMMARY

There are many considerations in the selection of an appropriate photochromicdye for use in flow visualization experiments. First, the dye must be soluble in theright kinds of fluids. Second, the activation wavelength should be as close to thevisible part of the spectrum as possible. This makes the materials problems muchsimpler. The energy required to produce a given color change should be as low aspossible. Finally, the contrast in color of the two forms of the dye should be as greatas possible.

At present, most flow visualization studies with photochromic dyes are carriedout with spiropyran dyes in organic solutions. There have been some attempts inCanada to use DNB for water flow experiments. At Harwell the existing techniquefor photochromic dye tracing has been extended. A method for preparing a water­based solution is given. The use of the triarylmethane dye in an aqueous solutiongreatly improves the flexibility of the technique over that with the spiropyran dye,which is limited to use in organic solvents.

A successful application of the water-based solution has been made at Harwellto the study of air-water two-phase flow, Fig. 9. The search is still on, however, toimprove on the current dye, with hopes that a good water-soluble fulgide dye canbe developed in the near future.

REFERENCES

I. J. B. Birks, Photophysics of Aromatic Molecules.2. G. H. Brown, Photochromism-s-Techniques of Chemistry, vol. 3, Wiley-Interscicnce,

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1970.13. W. W. Fowlis, Opt. Eng., vol. 18, pp. 281-286, 1979.14. T. Yoshihara, et aI., Kokai Tokkyo KOHO Patent No. 48-23787, Japan, 1973.15. A. P. Glaze, et aI., J. Chern. Soc. Perkin Trans. I, 1985.16. T. W. Fogwell, AERE Rept. No. R. 12313, 1986.17. J. B. Birks, Organic Molecular Photophysics, vol. I, Wiley, New York, 1973.18. J. B. Birks, Organic Molecular Photophysics, vol. 2, Wiley, New York, 1975.19. J. C. Chuang, et aI., J. Mechanochem. Cell Motif., vol. 2, pp. 105-112, 1973.20: V. A. Krongauz and A. A. Parshutkin, Photochem. Photobiol ., vol. 15, pp. 503-507,

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