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Evidence that ultrasonically-induced microbubbles carry a negative electrical charge
Evidence that ultrasonically-induced microbubbles carry a negative electrical charge D.J. W a t m o u g h , M.B. Shiran, K.M. Quan, A.P. Sarvazyant, E.P. Khizhnyakt and T.N. Pashovkint Department of Bio-Medical Physics and Bio-Engineering, University of Aberdeen, Foresterhill, Aberdeen, AB9 2ZD, UK t The Institute of Biological Physics, Soviet Academy of Sciences, Pushchino, USSR
Received 4 February 1992; revised 15 April 1992 When ultrasound, propagating in a cationic dye solution, is incident on a porous material such as paper, the diffraction pattern of the sound field is recorded by virtue of variations of dye concentration retained on the paper surface. By examining such patterns formed with 'Methylene Blue" dye and comparing with those obtained with 'Direct Sky Blue" dye, it is concluded that the mechanism of pattern formation requires the presence of an electric field to be induced 'close to the paper surface.' Using light directed at grazing incidence to illuminate the sonicated paper surface it is possible to demonstrate and record photographically a mapping of microscopic bubbles caused by sonication which has, broadly, the geometric features of a diffraction pattern. Theoretical arguments are adduced to show that such bubbles should generate a significant electric field close to the paper. In order to confirm that this field will alter dye deposition on paper, electrodes were affixed to the underside of the same paper as used to make the dye patterns. When an externally generated potential (1 kV) was applied, the Methylene Blue and Sky Blue dyes were deposited on the paper close to the negative and positive electrodes respectively. We therefore conclude that acoustically-induced gas bubbles carry a negative electric charge and for micrometresized bubbles we estimate the field to be about 7 × 105 V m -1. There is no a priori reason to expect acoustically excited bubbles to have the same charge as that on comparatively stable bubbles mechanically introduced into water, These findings raise again the question of whether phenomena such as sonoluminescence are caused by microscopic electrical discharges and whether inter-bubble forces are adequately described by current theories.
Keywords: cavitation; diffraction patterns; bubbles
The dye/paper method of recording the intensity distribution in ultrasound fields has been described by Sarvazyan et al. 1 and the mode of actioi~- has been investigated by Shiran e t al. 2 In essence, the sound field is directed through a weak dye solution (Methylene Blue was recommended by Sarvazyan et al. 1) at a paper surface, After a short exposure the paper is removed to reveal variations in dye concentration corresponding to the spatial variations of sound intensity. Shiran et al. 2 demonstrated that the sound generates an array of bubbles on the surface and showed how, theoretically the dye pattern might be attributed to microstreaming around resonant sized bubbles. An expression for the natural frequency of volume pulsation of gas bubbles in water was first derived by Minnaert 3. Elder 4 described the complexity of streaming patterns in the surrounding fluid which are induced when bubbles are sonicated at their natural frequency, and measured the magnitude of these fluid flow velocities as a function of incident sound pressure. It was shown 2 that streaming velocities should 0041-624X/ 92/ 050325-07 (~ 1992 Butterworth-HeinemannLtd
be linearly proportional to local sound intensity. It was postulated that streaming, by virtue of the stirring action it causes, prevents formation of a dye depletion layer next to the paper surface, thereby increasing the concentration close to resonant bubbles. Depletion layers in solutions close to porous boundary walls have been investigated theoretically by Crank 5. In the absence of bubbles (i.e. where the sound intensity is small or zero) the dye concentration would be low (i.e. background level). It is possible to observe dye concentrations in the paper below background levels. This is because bubbles larger than resonant size can form and these do not give rise to significant microstreaming. Such bubbles move to acoustic pressure minima and remain in these positions even when the sound is switched off. In addition they cover a portion of the paper surface and may even inhibit normal diffusion of dye into it. In the course of further investigation, to optimize the choice of dye, instances were found where inverse patterns were produced 6. These are cases where the dye
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Microbubbles have negative charge. D.J. Watmough et al. concentration deposited in the paper is lowest at points of highest intensity. It is difficult to see how, by itself, streaming can provide a complete explanation of ultrasound-induced dye patterns. In a study of the behaviour of SOAB rubber 7, it was shown that, when sonicated, the temperature of the rubber rapidly increases and bubbles form on its surface because of gaseous supersaturation in the liquid next to the wall. This is because the equilibrium quantity of dissolved gas in water falls with increasing temperature. Thus, water initially saturated with gas, warms up because of conduction from the wall, and, becomes supersaturated. The sheet of bubbles which results from supersaturation turns the material from an acoustic absorber to an acoustic reflector. We set out in this study to confirm that an array of bubbles forms on Astralux card when exposed to an ultrasound field generated by a standard 0.75 MHz therapeutic transducer and to establish whether bubble charge has any connection with the observed dye patterns. Another objective is to show that beam profiles can be derived from the dye patterns. Degrois and Baldo s predicted that acoustically induced bubbles are negatively charged and that microscopic electrical discharges are responsible for the phenomenon termed sonoluminescence. Other authors such as Noltink and Neppiras 9 and Flynn 1° attribute light emission from bubbles, to the incandescence of gas in collapsing cavities. Gaitan and Crum la have, however, shown that in a glycerine/water mixture single stable bubbles emit pulses of light without undergoing complete collapse. Saskena and Nyborg ~2 have also reported light emission from stable cavitation.
M a t e r i a l s and m e t h o d s A sheet of white Astralux paper, 200 #m thick, (Star Paper Ltd, Blackburn, Lancashire, UK) was placed on the bottom of a perspex tank (100 x 80 x 60mm3). Under the paper an absorbing rubber sheet 4ram thick was placed. A plane circular 0.75 MHz transducer, diameter 25ram, powered by a Rank Sonacel Multiphon Plus generator, was placed in the tank with its axis vertical, and 20ram from the paper. An aqueous solution of Methylene Blue dye (20 mg litre -~) was poured into the tank to cover the face of the transducer. Exposures in the continuous wave mode, at 1 . 3 W c m -2 (spatial average) for 3 min were made. Similar procedures were followed with Methyl Violet dye at a concentration of 40 mglitre-1 but the exposure time was 5 min. Direct Sky Blue dye was also utilized at a concentration of 250 mg litre-1 with an intensity set at 0.7 W cm -2 and an exposure of 2 rain. One exposure was made with a 3 MHz transducer; the paper was placed at the last axial maximum. The settings were 2 W c m -2, and 10min exposure in Methylene Blue dye solution at a concentration of 10 mg litre- 1. The reason for the differing concentrations is due to the fact that different dyes colour materials to different extents and these values were found to give approximately the same levels of colouration. After exposure the papers were washed and left to dry, following which photographic records were made. The patterns were also imaged by means of a television camera and the signals digitized to permit computation of beam profiles. In the case of the 3 MHz transducer the exposure of
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the paper was carried out in a small chamber, with an acoustically transparent window, positioned vertically. In this case sound was directed horizontally at the card held in the vertical plane by slots milled into the walls of the chamber. The chamber and transducer were positioned in a large water-filled tank. Note: the Astralux card extended from the bottom of the chamber to about 10 mm above the liquid level in it. Two copper foil electrodes 50 #m thick were glued with Loctite to the undersurface of sheets of white Astralux card. The foils were 10 mm wide and were separated by 45 ram. The electrodes were connected to the output of a stabilized high-voltage supply, and the paper placed, white side up, in the tank described above. The dye solution was poured in and the potential, set to 1000 V, switched on. After an exposure time of 5 min the paper was removed, washed and left to dry. The same procedure was followed for both Methylene Blue and Direct Sky Blue dyes. The bubble array, which we believe is responsible for producing the dye patterns, is not easily visualized or recorded against the white background of the Astralux paper. We therefore sonicated black Astralux card in the same small tank as described above containing water initially at 18 °C, with a transducer/card separation of 20 mm. Using illumination at grazing incidence we were able to observe a series of grey cloud-like rings on the surface which resembled those of the dye patterns. Using a Minolta XG2 35 mm camera and a close-up lens we were able to image this mapping which was composed of a large number of microscopic bubbles. The cloud-like appearance of the rings is due to scattering of light from this bubble array. It is formed within a few seconds of the field being switched on but disappears only slowly after the sound is switched off. It was found that some larger bubbles remained on the surface 24 hours after ultrasound exposure. This experiment was repeated with the Astralux card replaced with a thin plastic film sprayed with matt black paint. This procedure was to see if the phenomena was a general feature of sonicated surfaces or simply a property associated with Astralux card. It ought to be possible to make the dye patterns quantitative indicators of ultrasound intensity if beam profiles can be extracted from the images. The cards with dye patterns on them were placed in front of a television camera and the images digitized. A similar procedure was carried out on a dye pattern obtained at the last axial maximum of a 3 MHz transducer driven by the Sonacel generator, to demonstrate that a characteristic profile would result and to show how the Kossoff13 conical approximation might be employed to obtain a value for spatial peak intensity. The dye pattern was digitized and displayed on a grey scale with 64 levels. The variation of level 9 (0-63) across a diameter of the pattern should give the required beam profile. However, the greater the dye concentration on the paper, the lower is the 9 value. Thus, the data have to be manipulated before being plotted out. First the beam profile (consisting of 400 points) is smoothed out using a mask filter for which
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Microbubbles have negative charge: D.J. Watmough et al. The beam profile is then obtained using the relation; g{i}" = ( g ' { m a x } -- g'{i})/(g'{max} -- g ' { m i n } ) This procedure allows the data points of the beam profile to be obtained from the variations in dye concentration and also normalizes the profile to the range 0 to 1.
Results Figure la shows the diffraction pattern obtained with the transducer operating at 0.75 MHz in the continuous wave mode using the solution of Methylene Blue dye. Figure lb is the corresponding pattern obtained with Methyl Violet and Figure lc with Direct Sky Blue dye. The third pattern is depleted of dye in those parts of the paper where, in the first and second patterns, it is enhanced. It is noteworthy that the Methylene Blue and Methyl Violet dyes are cationic (colour carried on the positive dye ion) whereas the Direct Sky Blue dye is anionic, see Venkataram 14. The ring structure of the patterns is broadly as described by Wells ~5, and it is useful to examine Figure 2.2, page 28, of Wells 15 which also explains how the diffraction pattern can be considered
to arise because of waves stemming from a uniform collection of Huygen's sources. Figures 2a, 2b, 2c and 2d show the beam profiles computed from the dye patterns, taken in each case through the centre of symmetry. The example seen in Figure 2d is shown together, with the profile computed by Quan 16. The measured profile is broader than expected from theoretical analysis, suggesting that bulk streaming makes a contribution to the amount of dye impregnating the paper. Despite this limitation, the curve can be used to find the radius of a cone {best fitted to the profile} formed by rotation of the profile. This value of the radius permits a value for spatial peak intensity to be obtained if the spatial average intensity is known, (e.g. from a measurement of radiation force). This method is the well-known Kossoffx3 conical approximation. The bubble array formed after 3 min exposure at 0.65 W cm- 2 and 0.75 MHz on black Astralux paper is seen in Figure 3a and it shows a variation in the bubble density of a ring structure similar to that of the dye patterns. Figure 3b is a pattern on a surface coated with matt black paint. The choice of the paint is because it contains carbon which is known to be an efficient material for trapping gas. We interpret our findings to imply that the local heating effect of ultrasound causes this trapped gas to provide the nucleation sites for the growth of myriads of microscopic bubbles. The shape of the array will, in turn, be determined by the ultrasound diffraction pattern. Using the Astralux card with electrodes attached it was found that the Methylene Blue dye colour appeared at the negative electrode after a 5 min exposure with an applied potential of 1000 V. The Direct Sky Blue dye produced colouration at the positive electrode under the same conditions. Figures 4a and 4b are the dye distributions thus produced.
Discussion
d Figure I (a) The dye/paper pattern obtained with Astralux card and an aqueous solution of Methylene blue dye. It shows four rings surrounding a central spot. The arrow shows an effect due to secondary reflection from the paper and transducer. (b) The pattern obtained under similar conditions but using Methyl violet dye solution. The pattern is very similar to that of Figure la but the secondary feature is not apparent. (c) This pattern was also produced under the same acoustic and geometrical conditions as Figures la and lb but using an aqueous solution of Direct Sky blue dye. The area corresponding to the diffraction pattern of Figures la and Ib is largely depleted of dye although there are some fine blue rings. This finding suggests a short range effect (force) inhibits dye penetration into the paper. Otherwise the paper would have a circular area of dye depletion devoid of any fine features. (d) Here the pattern was made in a small chamber positioned vertically so that the paper was at the last axial maximum of a 3 MHz transducer also powered by the Sonacel generator. As expected there is a well-defined area of increased dye concentration which then fades away to background level. The intensity was 2 W cm 2 and exposure time 10 min. Another feature of this figure is the line of enhanced dye concentration at the water/air interface, where there is known to be an electrical double layer. Bars indicate 10 mm
It would appear that microstreaming cannot by itself account for the inverse patterns found with anionic dyes. The evidence described here suggests that the sound field generates a mapping of bubbles on an absorbing surface similar to that reported by Shiran et al. 2 and Quan and Watmough 7. Moreover, the paper used in this study contains holes which trap gas. This gas will expand under the influence of the sound field due to two properties; namely, rectified diffusion and the effect of the temperature rise in the paper. It has long been known that gas bubbles mechanically introduced into water carry a negative charge, see Alty et al. 17, Whybrew et alJ a. We can therefore surmise that a surface on which ultrasound is incident will become charged whenever gas bubbles are generated on the surface. The effect of temperature rise has previously been shown to explain how rubber, which is normally a good acoustic absorber, can become reflecting at an intensity of only 0.3 W cm- 2. The charged bubbles would thus be expected to attract the cationic dye ions and thus increase the local concentration of dye. Microstreaming would then carry the dye towards, if not into, the paper. Anionic dye ions, on the other hand, would be repelled by the electric field set up by the microbubbles so that microstreaming would carry dye-depleted liquid toward the paper. Accordng to Venkataram ~4, cellulose, which is a major component of paper, is negatively charged
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Microbubbles have negative charge. D.J. Watmough et 1.0
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F i g u r e 2 ( a ) ( b ) ( c ) are the beam profiles computed as outlined in the text. In each case a line t h r o u g h the centre of the pattern is chosen for the analysis. Close to the transducer ( d = 20 ram) the intensity is a rapidly varying function of position. The interference pattern has a g o o d degree o f circular symmetry. In Figure 2c the profile s h o w s the dye concentration in the central portion to be l o w e r than the background level. Figure 2d s h o w s the expected features of the profile at the last axial maximum, but is broader than the theoretical curve consistent with bulk streaming, c o n t r i b u t i n g to a blurring effect on the pattern. The c o n t i n u o u s curve was computed for a plane circular piston transducer ( f = 3 MHz, d = 31 cm)
when in contact with water so the dye patterns recorded with cationic and anionic dyes would accord with the observed degree of asymmetry between patterns. The experiments with electrodes attached to paper seem to be broadly consistent with an electric field having a role in the pattern formation. There is another possible explanation of how the dye concentration in the paper is increased. Again, this depends on the electric field of bubbles increasing the local dye ion concentration in the fluid near the paper. The dye concentration actually recorded in the paper could depend on the formation of liquid jets caused by collapsing cavitation bubbles as first described by Lauterborn tg. S uslick 2° has used high-speed photography
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to record the behaviour of collapsing bubbles and has shown that the fluid velocity of such a jet can reach 100m s-1. If the electrified bubble is surrounded by concentrated dye solution then the jet will force dye into the paper. Moreover, one can speculate that the addition of dye solution to a cavitating bubble field will make its collapse, as opposed to the occurrence of stable cavitation, more likely. This is because a charged bubble would tend to resist collapse since a reduction in surface area of the bubble would increase the charge density. The effect of adding a cationic dye would be to neutralize the bubble charge thus making collapse more probable. It can be envisaged that this process, whereby dye enters the paper via a jet, would occur repeatedly because cyclic
Microbubbles have negative charge: D.J. Watmough et al. discharges within oscillating or collapsing bubbles has been proposed as a theoretical explanation of the phenomenon called sonoluminescence (see Harvey 23, Degrois and Baldo s, Watmough et al.6). This explanation is not universally accepted; some authors believe light emission from cavities is a result of the rise in temperature of the enclosed gas to incandescence (3000 K). Theories of cavitation such as that of Noltingk and Neppiras 9 do not take charge density into account. Turner 24 has measured the period over which bubbles persist in water and found excess attenuation, due to bubbles, still present after periods of 100 hours. Surface tension should cause bubbles, which do not rise rapidly under bouyancy forces, to disappear in periods of a few minutes. Fox and Herzfeld 25 postulated that organic skins generated by impurities could stabilize them against collapse. The circumstantial evidence obtained in this present study suggests that the effect of surface charge on the stability of bubbles should be examined theoretically.
Theoretical considerations The experiments point to the significance of the electric field close to the bubble surface. It is interesting to estimate the magnitude of the field and to this end we
Figure 3 (a) The figure shows the pattern of bubbles on the surface of black Astralux card 200/~m thick after 3 min sonication at 0.65 W cm -2. Note that to produce these patterns water saturated with gas initially at a temperature of 18 °C was used. The light at grazing incidence was situated on the right. (b) In this case thin plastic film pre-treated with matt black paint replaced the Astralux card. The pattern was photographed about 30s after sound at 0.65 W cm 2 was switched on. The same 0.75 MHz transducer was used with d set to 20 mm
cavitation, previously described by Neppiras and Fill 21, would lead to formation of new cavities from fragments of the initial one. Adam 22 described how, at an air/water interface, there is an electric double layer, giving rise to a negative potential. In Figure ld a line of enhanced dye concentration at the interface (arrowed) may be discerned, although the dye intensity at this position is much weaker than that of the pattern caused by the sound field. This difference of dye concentration may be due either to the increased surface area of the microbubbles, or possibly the charge density itself increases while the sound is switched on. A third possibility is that microstreaming by itself enhances the dye concentration in the paper, but this possibility seems to be ruled out because when Eosin dye, for example, is used, which does not ionize, no pattern is produced. Also, if paper is sonicated in water and dye is added after the sound is switched off, then no diffraction pattern is recorded even though the bubble mapping is present. The phenomenon seems, therefore, to depend on the simultaneous presence of both an electric field and either microstreaming or liquid jet formation. There is no a priori reason to believe that the magnitude of the charge on acoustically-induced bubbles is the same as that on those generated by other means. Electrical
Figure 4 (a) Astralux card to which electrodes have been attached and to which a potential of 1000 V have been applied for 3 rain in a solution of Methylene blue dye. Note the concentration of dye near the negative electrode and depletion of dye at the positive electrode. (b) Card to which electrodes have been attached. Direct Sky Blue dye was used and in this case the dye is concentrated near the positive electrode and, at the negative electrode, depletion of dye is observed.
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Microbubbles have negative charge." D.J. Watmough et al. 5
6 7
8 9 10 11
12 13 14 15
Crank, J. The Mathematics of Diffusion 2nd edition, Clarendon Press, Oxford Watmough, D.J., Shiran, M. and Quan, K.M. Circumstantial evidence showing that acoustically excited gas bubbles carry a negative charge Acoust Bull (1992) 17 2 5-8 Quan, K.M. and Watmough, D.J. Ultrasonically induced gas bubbles on the surface of SOAB rubber; the consequences for the measurement of power output from ultrasonic instruments Meas Sci Technol (1991) 2 352-357 Degrois, M. and Baldo, P. A new electrical hypothesis explaining Sonoluminescence, chemical actions and other effects produced in gaseous cavitation Ultrasonics (1974) 12 25-28 Noltingk, B.E. and Neppiras, E.A. Cavitation produced by ultrasonics Proc Phys Soc London (1950) B63 674-685 Flynn, H.G. Physics of acoustic cavitation in liquids Physical Acoustics Vol 2b, (Ed Mason, W.P.) Academic Press, New York (1964) 57-172 Gaitan, D.F. and Crum, L.A. Observation of sonoluminescence from a single, stable cavitation bubble in a water/glycerine mixture Proc 12th International Symposium on Non-Linear Acoustics Austin Texas, August (1988) Saskena, T.K. and Nyborg, W.L Sonoluminescence from stable cavitation J Chem Phys (1970) 53 1722-1734 Kossoff, G. Calibration of ultrasonic therapy equipment Acoustica (1962) 12 84-90 Venkataram, K. The Chemistry of Synthetic Dyes Vol 2, Academic Press, New York (1952) Wells, P.N.T. Biomedical Ultrasonics Academic Press, London and New York (1974)
16 17
18 19 20 21 22 23 24 25 26 27
Quan, K.M. An experimental and theoretical study of ultrasound fields with reference to their use in physiotherapy and hyperthermia, PhD Thesis, University of Aberdeen (1991) AIty, T. The cataphoresis of gas bubbles in water Proc Roy Soc London (1924) 106 315-340 Whybrew,W.E., Kinzer,G.D. and Gunn, R. Electrification of small air bubbles in water J Geophysical Res (1952) 57 459-471 Lauterborn, W. General and basic aspects of cavitation, in: Finite Amplitude Wave Effects in Fluids (Ed Bjorno, L.) IPC Science and Technology Press Ltd., Guildford, UK (1974) Susliek, K.S. The chemical effects of ultrasound Scientific American February (1989) 62-68 Neppiras, E.A. and Fill, E.E. A cyclic cavitation process J Acoust Soc Am (1969) 46 1264-1271 Adam, N.K. The Physics and Chemistry of Surfaces Oxford University Press, London (1941) Harvey, E.N. Sonoluminescence and sonic chemiluminescence d Am Chem Soc (1939) 61 2392-2398 Turner, W.R. Microbubble persistence in fresh water d Acoust Soc Am (1961) 33 1223-1233 Fox, F.E. and Hertzfeld, K.F. Organic skins on the surface of cavitation bubbles J Acoust Soc Am (1954) 26 984-989 Bleaney, B.I. and Bleaney, B. Electricity and Magnetism 3rd edition, Oxford University Press (1976)
Crum, LA., Walton, A.J., Mortimer, A., Dyson, M., Crawford, D.C. and Gaitan, D.F. Free radical production in amniotic fluid and blood plasma by medical ultrasound J Ultrasound Med (1987) 6 643-647
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Received 4 February 1992; revised 15 April 1992 When ultrasound, propagating in a cationic dye solution, is incident on a porous material such as paper, the diffraction pattern of the sound field is recorded by virtue of variations of dye concentration retained on the paper surface. By examining such patterns formed with 'Methylene Blue" dye and comparing with those obtained with 'Direct Sky Blue" dye, it is concluded that the mechanism of pattern formation requires the presence of an electric field to be induced 'close to the paper surface.' Using light directed at grazing incidence to illuminate the sonicated paper surface it is possible to demonstrate and record photographically a mapping of microscopic bubbles caused by sonication which has, broadly, the geometric features of a diffraction pattern. Theoretical arguments are adduced to show that such bubbles should generate a significant electric field close to the paper. In order to confirm that this field will alter dye deposition on paper, electrodes were affixed to the underside of the same paper as used to make the dye patterns. When an externally generated potential (1 kV) was applied, the Methylene Blue and Sky Blue dyes were deposited on the paper close to the negative and positive electrodes respectively. We therefore conclude that acoustically-induced gas bubbles carry a negative electric charge and for micrometresized bubbles we estimate the field to be about 7 × 105 V m -1. There is no a priori reason to expect acoustically excited bubbles to have the same charge as that on comparatively stable bubbles mechanically introduced into water, These findings raise again the question of whether phenomena such as sonoluminescence are caused by microscopic electrical discharges and whether inter-bubble forces are adequately described by current theories.
Keywords: cavitation; diffraction patterns; bubbles
The dye/paper method of recording the intensity distribution in ultrasound fields has been described by Sarvazyan et al. 1 and the mode of actioi~- has been investigated by Shiran e t al. 2 In essence, the sound field is directed through a weak dye solution (Methylene Blue was recommended by Sarvazyan et al. 1) at a paper surface, After a short exposure the paper is removed to reveal variations in dye concentration corresponding to the spatial variations of sound intensity. Shiran et al. 2 demonstrated that the sound generates an array of bubbles on the surface and showed how, theoretically the dye pattern might be attributed to microstreaming around resonant sized bubbles. An expression for the natural frequency of volume pulsation of gas bubbles in water was first derived by Minnaert 3. Elder 4 described the complexity of streaming patterns in the surrounding fluid which are induced when bubbles are sonicated at their natural frequency, and measured the magnitude of these fluid flow velocities as a function of incident sound pressure. It was shown 2 that streaming velocities should 0041-624X/ 92/ 050325-07 (~ 1992 Butterworth-HeinemannLtd
be linearly proportional to local sound intensity. It was postulated that streaming, by virtue of the stirring action it causes, prevents formation of a dye depletion layer next to the paper surface, thereby increasing the concentration close to resonant bubbles. Depletion layers in solutions close to porous boundary walls have been investigated theoretically by Crank 5. In the absence of bubbles (i.e. where the sound intensity is small or zero) the dye concentration would be low (i.e. background level). It is possible to observe dye concentrations in the paper below background levels. This is because bubbles larger than resonant size can form and these do not give rise to significant microstreaming. Such bubbles move to acoustic pressure minima and remain in these positions even when the sound is switched off. In addition they cover a portion of the paper surface and may even inhibit normal diffusion of dye into it. In the course of further investigation, to optimize the choice of dye, instances were found where inverse patterns were produced 6. These are cases where the dye
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Microbubbles have negative charge. D.J. Watmough et al. concentration deposited in the paper is lowest at points of highest intensity. It is difficult to see how, by itself, streaming can provide a complete explanation of ultrasound-induced dye patterns. In a study of the behaviour of SOAB rubber 7, it was shown that, when sonicated, the temperature of the rubber rapidly increases and bubbles form on its surface because of gaseous supersaturation in the liquid next to the wall. This is because the equilibrium quantity of dissolved gas in water falls with increasing temperature. Thus, water initially saturated with gas, warms up because of conduction from the wall, and, becomes supersaturated. The sheet of bubbles which results from supersaturation turns the material from an acoustic absorber to an acoustic reflector. We set out in this study to confirm that an array of bubbles forms on Astralux card when exposed to an ultrasound field generated by a standard 0.75 MHz therapeutic transducer and to establish whether bubble charge has any connection with the observed dye patterns. Another objective is to show that beam profiles can be derived from the dye patterns. Degrois and Baldo s predicted that acoustically induced bubbles are negatively charged and that microscopic electrical discharges are responsible for the phenomenon termed sonoluminescence. Other authors such as Noltink and Neppiras 9 and Flynn 1° attribute light emission from bubbles, to the incandescence of gas in collapsing cavities. Gaitan and Crum la have, however, shown that in a glycerine/water mixture single stable bubbles emit pulses of light without undergoing complete collapse. Saskena and Nyborg ~2 have also reported light emission from stable cavitation.
M a t e r i a l s and m e t h o d s A sheet of white Astralux paper, 200 #m thick, (Star Paper Ltd, Blackburn, Lancashire, UK) was placed on the bottom of a perspex tank (100 x 80 x 60mm3). Under the paper an absorbing rubber sheet 4ram thick was placed. A plane circular 0.75 MHz transducer, diameter 25ram, powered by a Rank Sonacel Multiphon Plus generator, was placed in the tank with its axis vertical, and 20ram from the paper. An aqueous solution of Methylene Blue dye (20 mg litre -~) was poured into the tank to cover the face of the transducer. Exposures in the continuous wave mode, at 1 . 3 W c m -2 (spatial average) for 3 min were made. Similar procedures were followed with Methyl Violet dye at a concentration of 40 mglitre-1 but the exposure time was 5 min. Direct Sky Blue dye was also utilized at a concentration of 250 mg litre-1 with an intensity set at 0.7 W cm -2 and an exposure of 2 rain. One exposure was made with a 3 MHz transducer; the paper was placed at the last axial maximum. The settings were 2 W c m -2, and 10min exposure in Methylene Blue dye solution at a concentration of 10 mg litre- 1. The reason for the differing concentrations is due to the fact that different dyes colour materials to different extents and these values were found to give approximately the same levels of colouration. After exposure the papers were washed and left to dry, following which photographic records were made. The patterns were also imaged by means of a television camera and the signals digitized to permit computation of beam profiles. In the case of the 3 MHz transducer the exposure of
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the paper was carried out in a small chamber, with an acoustically transparent window, positioned vertically. In this case sound was directed horizontally at the card held in the vertical plane by slots milled into the walls of the chamber. The chamber and transducer were positioned in a large water-filled tank. Note: the Astralux card extended from the bottom of the chamber to about 10 mm above the liquid level in it. Two copper foil electrodes 50 #m thick were glued with Loctite to the undersurface of sheets of white Astralux card. The foils were 10 mm wide and were separated by 45 ram. The electrodes were connected to the output of a stabilized high-voltage supply, and the paper placed, white side up, in the tank described above. The dye solution was poured in and the potential, set to 1000 V, switched on. After an exposure time of 5 min the paper was removed, washed and left to dry. The same procedure was followed for both Methylene Blue and Direct Sky Blue dyes. The bubble array, which we believe is responsible for producing the dye patterns, is not easily visualized or recorded against the white background of the Astralux paper. We therefore sonicated black Astralux card in the same small tank as described above containing water initially at 18 °C, with a transducer/card separation of 20 mm. Using illumination at grazing incidence we were able to observe a series of grey cloud-like rings on the surface which resembled those of the dye patterns. Using a Minolta XG2 35 mm camera and a close-up lens we were able to image this mapping which was composed of a large number of microscopic bubbles. The cloud-like appearance of the rings is due to scattering of light from this bubble array. It is formed within a few seconds of the field being switched on but disappears only slowly after the sound is switched off. It was found that some larger bubbles remained on the surface 24 hours after ultrasound exposure. This experiment was repeated with the Astralux card replaced with a thin plastic film sprayed with matt black paint. This procedure was to see if the phenomena was a general feature of sonicated surfaces or simply a property associated with Astralux card. It ought to be possible to make the dye patterns quantitative indicators of ultrasound intensity if beam profiles can be extracted from the images. The cards with dye patterns on them were placed in front of a television camera and the images digitized. A similar procedure was carried out on a dye pattern obtained at the last axial maximum of a 3 MHz transducer driven by the Sonacel generator, to demonstrate that a characteristic profile would result and to show how the Kossoff13 conical approximation might be employed to obtain a value for spatial peak intensity. The dye pattern was digitized and displayed on a grey scale with 64 levels. The variation of level 9 (0-63) across a diameter of the pattern should give the required beam profile. However, the greater the dye concentration on the paper, the lower is the 9 value. Thus, the data have to be manipulated before being plotted out. First the beam profile (consisting of 400 points) is smoothed out using a mask filter for which
g{i}'=(Zg{i}
+g{i+
1}+9{i--
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where i refers to the position number of the data point and the g {i} are the smoothed data values. The maximum and minimum values of the grey levels among 9 { i} namely y {max } and g { min } are determined.
Microbubbles have negative charge: D.J. Watmough et al. The beam profile is then obtained using the relation; g{i}" = ( g ' { m a x } -- g'{i})/(g'{max} -- g ' { m i n } ) This procedure allows the data points of the beam profile to be obtained from the variations in dye concentration and also normalizes the profile to the range 0 to 1.
Results Figure la shows the diffraction pattern obtained with the transducer operating at 0.75 MHz in the continuous wave mode using the solution of Methylene Blue dye. Figure lb is the corresponding pattern obtained with Methyl Violet and Figure lc with Direct Sky Blue dye. The third pattern is depleted of dye in those parts of the paper where, in the first and second patterns, it is enhanced. It is noteworthy that the Methylene Blue and Methyl Violet dyes are cationic (colour carried on the positive dye ion) whereas the Direct Sky Blue dye is anionic, see Venkataram 14. The ring structure of the patterns is broadly as described by Wells ~5, and it is useful to examine Figure 2.2, page 28, of Wells 15 which also explains how the diffraction pattern can be considered
to arise because of waves stemming from a uniform collection of Huygen's sources. Figures 2a, 2b, 2c and 2d show the beam profiles computed from the dye patterns, taken in each case through the centre of symmetry. The example seen in Figure 2d is shown together, with the profile computed by Quan 16. The measured profile is broader than expected from theoretical analysis, suggesting that bulk streaming makes a contribution to the amount of dye impregnating the paper. Despite this limitation, the curve can be used to find the radius of a cone {best fitted to the profile} formed by rotation of the profile. This value of the radius permits a value for spatial peak intensity to be obtained if the spatial average intensity is known, (e.g. from a measurement of radiation force). This method is the well-known Kossoffx3 conical approximation. The bubble array formed after 3 min exposure at 0.65 W cm- 2 and 0.75 MHz on black Astralux paper is seen in Figure 3a and it shows a variation in the bubble density of a ring structure similar to that of the dye patterns. Figure 3b is a pattern on a surface coated with matt black paint. The choice of the paint is because it contains carbon which is known to be an efficient material for trapping gas. We interpret our findings to imply that the local heating effect of ultrasound causes this trapped gas to provide the nucleation sites for the growth of myriads of microscopic bubbles. The shape of the array will, in turn, be determined by the ultrasound diffraction pattern. Using the Astralux card with electrodes attached it was found that the Methylene Blue dye colour appeared at the negative electrode after a 5 min exposure with an applied potential of 1000 V. The Direct Sky Blue dye produced colouration at the positive electrode under the same conditions. Figures 4a and 4b are the dye distributions thus produced.
Discussion
d Figure I (a) The dye/paper pattern obtained with Astralux card and an aqueous solution of Methylene blue dye. It shows four rings surrounding a central spot. The arrow shows an effect due to secondary reflection from the paper and transducer. (b) The pattern obtained under similar conditions but using Methyl violet dye solution. The pattern is very similar to that of Figure la but the secondary feature is not apparent. (c) This pattern was also produced under the same acoustic and geometrical conditions as Figures la and lb but using an aqueous solution of Direct Sky blue dye. The area corresponding to the diffraction pattern of Figures la and Ib is largely depleted of dye although there are some fine blue rings. This finding suggests a short range effect (force) inhibits dye penetration into the paper. Otherwise the paper would have a circular area of dye depletion devoid of any fine features. (d) Here the pattern was made in a small chamber positioned vertically so that the paper was at the last axial maximum of a 3 MHz transducer also powered by the Sonacel generator. As expected there is a well-defined area of increased dye concentration which then fades away to background level. The intensity was 2 W cm 2 and exposure time 10 min. Another feature of this figure is the line of enhanced dye concentration at the water/air interface, where there is known to be an electrical double layer. Bars indicate 10 mm
It would appear that microstreaming cannot by itself account for the inverse patterns found with anionic dyes. The evidence described here suggests that the sound field generates a mapping of bubbles on an absorbing surface similar to that reported by Shiran et al. 2 and Quan and Watmough 7. Moreover, the paper used in this study contains holes which trap gas. This gas will expand under the influence of the sound field due to two properties; namely, rectified diffusion and the effect of the temperature rise in the paper. It has long been known that gas bubbles mechanically introduced into water carry a negative charge, see Alty et al. 17, Whybrew et alJ a. We can therefore surmise that a surface on which ultrasound is incident will become charged whenever gas bubbles are generated on the surface. The effect of temperature rise has previously been shown to explain how rubber, which is normally a good acoustic absorber, can become reflecting at an intensity of only 0.3 W cm- 2. The charged bubbles would thus be expected to attract the cationic dye ions and thus increase the local concentration of dye. Microstreaming would then carry the dye towards, if not into, the paper. Anionic dye ions, on the other hand, would be repelled by the electric field set up by the microbubbles so that microstreaming would carry dye-depleted liquid toward the paper. Accordng to Venkataram ~4, cellulose, which is a major component of paper, is negatively charged
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Microbubbles have negative charge. D.J. Watmough et 1.0
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F i g u r e 2 ( a ) ( b ) ( c ) are the beam profiles computed as outlined in the text. In each case a line t h r o u g h the centre of the pattern is chosen for the analysis. Close to the transducer ( d = 20 ram) the intensity is a rapidly varying function of position. The interference pattern has a g o o d degree o f circular symmetry. In Figure 2c the profile s h o w s the dye concentration in the central portion to be l o w e r than the background level. Figure 2d s h o w s the expected features of the profile at the last axial maximum, but is broader than the theoretical curve consistent with bulk streaming, c o n t r i b u t i n g to a blurring effect on the pattern. The c o n t i n u o u s curve was computed for a plane circular piston transducer ( f = 3 MHz, d = 31 cm)
when in contact with water so the dye patterns recorded with cationic and anionic dyes would accord with the observed degree of asymmetry between patterns. The experiments with electrodes attached to paper seem to be broadly consistent with an electric field having a role in the pattern formation. There is another possible explanation of how the dye concentration in the paper is increased. Again, this depends on the electric field of bubbles increasing the local dye ion concentration in the fluid near the paper. The dye concentration actually recorded in the paper could depend on the formation of liquid jets caused by collapsing cavitation bubbles as first described by Lauterborn tg. S uslick 2° has used high-speed photography
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to record the behaviour of collapsing bubbles and has shown that the fluid velocity of such a jet can reach 100m s-1. If the electrified bubble is surrounded by concentrated dye solution then the jet will force dye into the paper. Moreover, one can speculate that the addition of dye solution to a cavitating bubble field will make its collapse, as opposed to the occurrence of stable cavitation, more likely. This is because a charged bubble would tend to resist collapse since a reduction in surface area of the bubble would increase the charge density. The effect of adding a cationic dye would be to neutralize the bubble charge thus making collapse more probable. It can be envisaged that this process, whereby dye enters the paper via a jet, would occur repeatedly because cyclic
Microbubbles have negative charge: D.J. Watmough et al. discharges within oscillating or collapsing bubbles has been proposed as a theoretical explanation of the phenomenon called sonoluminescence (see Harvey 23, Degrois and Baldo s, Watmough et al.6). This explanation is not universally accepted; some authors believe light emission from cavities is a result of the rise in temperature of the enclosed gas to incandescence (3000 K). Theories of cavitation such as that of Noltingk and Neppiras 9 do not take charge density into account. Turner 24 has measured the period over which bubbles persist in water and found excess attenuation, due to bubbles, still present after periods of 100 hours. Surface tension should cause bubbles, which do not rise rapidly under bouyancy forces, to disappear in periods of a few minutes. Fox and Herzfeld 25 postulated that organic skins generated by impurities could stabilize them against collapse. The circumstantial evidence obtained in this present study suggests that the effect of surface charge on the stability of bubbles should be examined theoretically.
Theoretical considerations The experiments point to the significance of the electric field close to the bubble surface. It is interesting to estimate the magnitude of the field and to this end we
Figure 3 (a) The figure shows the pattern of bubbles on the surface of black Astralux card 200/~m thick after 3 min sonication at 0.65 W cm -2. Note that to produce these patterns water saturated with gas initially at a temperature of 18 °C was used. The light at grazing incidence was situated on the right. (b) In this case thin plastic film pre-treated with matt black paint replaced the Astralux card. The pattern was photographed about 30s after sound at 0.65 W cm 2 was switched on. The same 0.75 MHz transducer was used with d set to 20 mm
cavitation, previously described by Neppiras and Fill 21, would lead to formation of new cavities from fragments of the initial one. Adam 22 described how, at an air/water interface, there is an electric double layer, giving rise to a negative potential. In Figure ld a line of enhanced dye concentration at the interface (arrowed) may be discerned, although the dye intensity at this position is much weaker than that of the pattern caused by the sound field. This difference of dye concentration may be due either to the increased surface area of the microbubbles, or possibly the charge density itself increases while the sound is switched on. A third possibility is that microstreaming by itself enhances the dye concentration in the paper, but this possibility seems to be ruled out because when Eosin dye, for example, is used, which does not ionize, no pattern is produced. Also, if paper is sonicated in water and dye is added after the sound is switched off, then no diffraction pattern is recorded even though the bubble mapping is present. The phenomenon seems, therefore, to depend on the simultaneous presence of both an electric field and either microstreaming or liquid jet formation. There is no a priori reason to believe that the magnitude of the charge on acoustically-induced bubbles is the same as that on those generated by other means. Electrical
Figure 4 (a) Astralux card to which electrodes have been attached and to which a potential of 1000 V have been applied for 3 rain in a solution of Methylene blue dye. Note the concentration of dye near the negative electrode and depletion of dye at the positive electrode. (b) Card to which electrodes have been attached. Direct Sky Blue dye was used and in this case the dye is concentrated near the positive electrode and, at the negative electrode, depletion of dye is observed.
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Microbubbles have negative charge." D.J. Watmough et al. 5
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8 9 10 11
12 13 14 15
Crank, J. The Mathematics of Diffusion 2nd edition, Clarendon Press, Oxford Watmough, D.J., Shiran, M. and Quan, K.M. Circumstantial evidence showing that acoustically excited gas bubbles carry a negative charge Acoust Bull (1992) 17 2 5-8 Quan, K.M. and Watmough, D.J. Ultrasonically induced gas bubbles on the surface of SOAB rubber; the consequences for the measurement of power output from ultrasonic instruments Meas Sci Technol (1991) 2 352-357 Degrois, M. and Baldo, P. A new electrical hypothesis explaining Sonoluminescence, chemical actions and other effects produced in gaseous cavitation Ultrasonics (1974) 12 25-28 Noltingk, B.E. and Neppiras, E.A. Cavitation produced by ultrasonics Proc Phys Soc London (1950) B63 674-685 Flynn, H.G. Physics of acoustic cavitation in liquids Physical Acoustics Vol 2b, (Ed Mason, W.P.) Academic Press, New York (1964) 57-172 Gaitan, D.F. and Crum, L.A. Observation of sonoluminescence from a single, stable cavitation bubble in a water/glycerine mixture Proc 12th International Symposium on Non-Linear Acoustics Austin Texas, August (1988) Saskena, T.K. and Nyborg, W.L Sonoluminescence from stable cavitation J Chem Phys (1970) 53 1722-1734 Kossoff, G. Calibration of ultrasonic therapy equipment Acoustica (1962) 12 84-90 Venkataram, K. The Chemistry of Synthetic Dyes Vol 2, Academic Press, New York (1952) Wells, P.N.T. Biomedical Ultrasonics Academic Press, London and New York (1974)
16 17
18 19 20 21 22 23 24 25 26 27
Quan, K.M. An experimental and theoretical study of ultrasound fields with reference to their use in physiotherapy and hyperthermia, PhD Thesis, University of Aberdeen (1991) AIty, T. The cataphoresis of gas bubbles in water Proc Roy Soc London (1924) 106 315-340 Whybrew,W.E., Kinzer,G.D. and Gunn, R. Electrification of small air bubbles in water J Geophysical Res (1952) 57 459-471 Lauterborn, W. General and basic aspects of cavitation, in: Finite Amplitude Wave Effects in Fluids (Ed Bjorno, L.) IPC Science and Technology Press Ltd., Guildford, UK (1974) Susliek, K.S. The chemical effects of ultrasound Scientific American February (1989) 62-68 Neppiras, E.A. and Fill, E.E. A cyclic cavitation process J Acoust Soc Am (1969) 46 1264-1271 Adam, N.K. The Physics and Chemistry of Surfaces Oxford University Press, London (1941) Harvey, E.N. Sonoluminescence and sonic chemiluminescence d Am Chem Soc (1939) 61 2392-2398 Turner, W.R. Microbubble persistence in fresh water d Acoust Soc Am (1961) 33 1223-1233 Fox, F.E. and Hertzfeld, K.F. Organic skins on the surface of cavitation bubbles J Acoust Soc Am (1954) 26 984-989 Bleaney, B.I. and Bleaney, B. Electricity and Magnetism 3rd edition, Oxford University Press (1976)
Crum, LA., Walton, A.J., Mortimer, A., Dyson, M., Crawford, D.C. and Gaitan, D.F. Free radical production in amniotic fluid and blood plasma by medical ultrasound J Ultrasound Med (1987) 6 643-647
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