SONOLUMINESCENCE FROM METAL SALT SOLUTIONS

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1 Cav03-GS Fifth International Symposium on Cavitation (cav2003) Osaka, Japan, November 1-4, 2003 SONOLUMINESCENCE FROM METAL SALT SOLUTIONS Vijay H. Arakeri Department of Mechanical Engineering, Indian Institute of Science Bangalore, , India ABSTRACT Ethylene glycol is an interesting medium for conducting sonoluminescence (SL) studies with metal salt solutions. Using this medium it was possible to observe pure sodium D line emission with argon saturated sodium-metal salt solution. In the past, two possibilities for the site of such metal line emissions have been indicated. One is the liquid phase in the bulk and the second being the gas phase within the bubble. If it is the latter, an interesting question is, how the metal atoms or ions find their way into the bubble. The above issues have been examined by conducting SL studies with ethylene glycol based calcium chloride salt solutions. In this case, unlike sodium salt, there is possibility for both atomic and ionic line emissions having low excitation energies. Depending upon which line dominates the spectrum, it is possible to infer the SL site. Our present observation of dominance of the atomic line strongly suggests that SL originates from within the bubble. A model has also been proposed to explain how the metal atoms may become available within the bubble interior. INTRODUCTION Sonoluminescence [1] is the phenomenon of light emission from a cavitation bubble field generated when liquid samples are irradiated with high intensity ultrasound. In a recent study [2], we were able to observe exclusive resonance radiation as a form of multi-bubble sonoluminescence (MBSL) from a non-aqueous medium containing sodium salt. Low-resolution spectra indicated that the emission had one of the cleanest spectra, over a wide wavelength range, consisting primarily of sodium D lines (~ 589 nm). The lines showed considerable broadening (~ 4 nm) and asymmetry towards the red, thus indicating that the emission may be occurring under high-pressure environment. The spectrum along with a photograph of MBSL, so observed, is presented in Figure 1. In trying to understand the mechanism for sodium D line emission, some questions come to mind. First one is whether the emission is gas phase or liquid phase. If it is gas phase, then the next question is, how the sodium ions present in the liquid get transformed to sodium atoms and how either of these find their way into the bubble since the vapour pressure of the solute is expected to be very small. Once the atoms are within the bubble then the light emission process can be explained on the basis of invoking the familiar hot spot theory [1]. Due to the high temperatures generated within the bubble during the collapse phase, some of the sodium atoms get excited to a higher energy state and their return to the ground state is accompanied by photon emission. The other possibility is that the emission is indirect and originates within the liquid phase. In this scenario, the high temperatures within the bubble produce some reactive radicals like OH and H [3]; these diffuse into the liquid upon collapse to not only transform the ions to atoms, but also excite the atoms to a higher energy state. Thus, the emission can be ascribed to a chemiluminescence process; Flint et al. [4], proposed this route to explain a paradox wherein they found that the alkali-metal line emission broadening was insensitive to certain parameters which are expected to effect the cavitation bubble temperature. In contrast to this, Sehgal et al.[5] assuming gas phase emission used SL as a probe of acoustic cavitation temperatures. Similarly, we measured the characteristic times of MBSL flashes in the form of sodium resonance radiation to be in tens of nanoseconds [6] and modeling the sodium emission using the bubble dynamics formulation due to Kamath et al. [7] it was seen that the predicted time scales were consistent with the measured ones (see Figure 2). Therefore, on the basis of what has been presented, it is not clearly established as to whether MBSL containing a metal line emission originates in the gas phase or the liquid phase or is a combination of the two. This is a fundamental issue and is the focus of the present study. Our approach is to look at 1

2 Figure 1 Low-resolution (3 nm FWHM) spectrum and a long exposure photograph of multi-bubble sonoluminescence from argon saturated 1 N sodium chloride solution. Figure 2 Comparison of the computed (solid line) sodium D line pulse shape with that of measured one (dotted points). Taken from Arakeri and Giri [6]. 2

3 spectra of MBSL generated from an alkaline-earthmetal salt solution. If we consider a salt like calcium chloride (CaCl 2 ), some of the metal in the solution exist in the form of Ca ++ as compared to, for example, Na + in the case of sodium chloride solution. If it is liquid phase emission, then we should expect a dominant Ca + line in the MBSL spectra from a calcium salt solution since this would be analogous to Na line emission from a sodium salt solution. Anything contrary to this would suggest gas phase emission and in particular, this would be the case if lines or bands connected to Ca dominate the spectra. EXPERIMENTAL METHODS The general experimental methods used presently are fully described elsewhere [8] and hence only a brief description is provided here. The observed MBSL was generated in room temperature 100 ml samples contained in a 250 ml glass beaker and acoustically excited by a single hollow crystal attached at the bottom of the beaker. The transducer was driven at a nominal frequency of khz using a power amplifier (B&K 2713) and a sine wave generator (Wavetek 29). The spectra were obtained by allowing the emission to go directly into the entrance slit (generally set at 1 mm width) of a 0.19 m scanning monochromator (Jobin Yvon- Spex Triax 180) having a dispersion of 3.53 nm/mm. The emission was detected and processed using a PMT (Hamamatsu R928), a lock-inamplifier (SRS 530) and a PC based data acquisition system. The samples were transferred to the beaker from 0.5 N stock solutions prepared by dissolving the required amount of salt in 250 ml of ethylene glycol. Repeated degassing under vigorous agitation and purging with argon was done to prepare argon saturated solutions with minimal air content. All the experiments were done at high acoustic drive levels; the estimated maximum pressure amplitudes being of the order of 3 bar. RESULTS AND DISCUSSION We used two alkaline-earth-salts, namely: calcium chloride dihydrate (CaCl 2.2H 2 O) and barium chloride dihydrate (BaCl 2.2H 2 O) for our present experiments. Even though, emission from aqueous solution of calcium and barium salts have been reported earlier [1], no spectra have been presented. In Figure 3, a spectrum of MBSL from Ca salt solution obtained from our studies is presented. It is dominated by calcium line near 422 nm, and bands near 554 nm and 622 nm are also present. The latter are known to be associated with band systems of CaOH [9]. There is no evidence for emission near 396 or 393 nm, these being the wavelengths for Ca + line emission. Therefore, based on our earlier reasoning, the evidence for Ca emission originating from within the bubble is very strong. In contrast to Ca salt solution, we could not detect any emission from the Ba salt solution. It was verified separately that the intensity of cavitation was similar in both the salt solutions; this was done by adding small amount of NaCl in Ca and Ba salt solutions and monitoring the Na line intensity. In both cases Na line emission was easily detectable and in fact it was stronger with Ba salt solution. Therefore, the lack of emission from Ba salt solution has to be traced to differing sensitivities of emission for the two elements involved. In flame photometry, the sensitivity for Ca is reported to be an order of magnitude higher than that for Ba [10]; for example, the sensitivity reported in terms of the smallest concentration which can be detected using flame photometry is ppm for Ca and 0.3 ppm for Ba. Therefore, our observations seem to follow the trends established in flame spectroscopy or photometry. We can take this as added support for suggesting that the MBSL metal line emissions originate from thermal activity occurring within the cavitation events. Having established this, the next question to be addressed is how the metal ions or atoms find their way into the bubble since the metal salts are basically non-volatile. Here again we look for a clue from what is known in terms of metal line emissions from flames. In flame photometry, it is common practice to introduce the metal in the form of an aqueous metal salt solution. For example, sodium D line emission is readily observed by spraying aqueous NaCl solution into a flame; however, several physical processes are involved in leading to the final step of emission. These are indicated below for a general case [10]. 1. Aspiration: introduction of sample salt solution. 2. Nebulization: conversion of fraction of aspirated sample to droplets with a size distribution. 3. Desolvation: vapourization of the liquid leaving behind aerosol consisting of solid or molten salt particles. 4. Volatilization: conversion of aerosol into vapour. 5. Dissociation: conversion of vapour to basic atomic constituents. 3

4 Figure 3 Low-resolution (3.53 nm FWHM) spectrum of multi-bubble sonoluminescence from argon saturated 0.5 N calcium chloride solution. 6. Excitation: bringing metal atoms to a higher energy state. 7. Emission: return of excited metal atoms to the ground state accompanied by photon emission. Some of the processes like desolvation are relatively slow; for example, water droplets with an initial diameter of 1 µm rising at about 10 m/s require a minimum height of 0.03 mm for complete desolvation, thus requiring about 3 µs [10]. Therefore, with the time scales of MBSL estimated to be a fraction of 1 µs [6] it appears very difficult for all the above mentioned stages to be gone through before metal line emission is realized. In view of this, we suggest the following possibility in the case of MBSL. The metal salt solution is aspired into bubbles in the form of fine droplets through development of surface instabilities [11]. The droplets are heated to above critical temperature during the collapse phase and which results in the availability of salt molecules or even atomic constituents directly. Thus, through this route the stages 2,3,4 and even perhaps 5 those are involved in flame emission and indicated earlier are by-passed. We have done some studies which suggest that the dissociation process (step no.5) may not be involved. The intensity of potassium line emission near 770 nm was investigated using two different potassium salts having varying dissociation energies; the two salts chosen were, potassium chloride (dissociation energy of 4.4 ev) and potassium iodide (dissociation energy of 3.3 ev). With MBSL, comparison of intensities from two different sets of experiments is tricky; since, the intensity can vary due to variety of reasons like alteration in nucleation characteristics, bubble population density, ect. Therefore, to make comparisons meaningful we added a small amount of NaCl to both the solutions and used the sodium emission as a reference. The result of our study was that the ratio of the potassium line intensity to that of the sodium line intensity did not vary significantly with the two potassium salt solutions. 4

5 CONCLUSIONS Sonoluminescence (SL) spectrum from argon saturated calcium salt solution is dominated by Ca line near 422 nm. We take this to indicate that the SL site is the bubble interior since if it were the liquid the spectrum would have been dominated by Ca + line. It is hypothesized that the metal salt solution enters the bubble in the form of very fine droplets due to development of surface instabilities and the droplets are heated to above critical temperature that enables metal atoms to become readily available for thermal excitation and subsequent emission. More systematic investigations along with numerical simulation of droplet heating are required to put our hypothesis on a firmer footing. ACKNOWLEDGEMENTS I would like to thank K.S.Gandhi for useful discussions and Navanit V.Arakeri for his assistance in data processing and manuscript preparation. An Extra Mural Research Grant from the Council of Scientific and Industrial Research (CSIR), Govt. of India has supported this work. REFERENCES 1. Verral R E and Sehgal C M 1988 Sonoluminescence in Ultrasound: Its Chemical, Physical and Biological Effects (ed. Suslick K S), (New York:VCH), p Giri A and Arakeri V H 1998 Phys. Rev. E 58, R Prasad Naidu D V, Rajan R, Kumar R, Gandhi K S, Arakeri V H and Chandrasekaran S 1994 Chem. Engg. Sci. 49, Flint E B and Suslick K S 1991 J. Phys. Chem. 95, Sehgal C, Steer R P, Sutherland R G and Verral R E 1979 J. Phys. Chem. 70, Arakeri V H and Giri A 2001 Phys. Rev. E 63, art. no Kamath V, Prosperetti A and Egolfopouls F N 1993 J. Acoust. Soc. Am. 94, Giri A 2000 Studies on sonoluminescence in the form of resonance radiation. Ph.D. Thesis, Indian Institute of Science, India. 9. James C G and Sugden T M 1955 Nature 175, Alkemade C Th J and Herrmann R 1979 Fundamentals of Analytical Flame Spectroscopy, (Bristol:Adam Hilger). 11. Nyborg W L and Hughes D E 1967 J. Acoust. Soc. Am. 42,