ULTRASONIC WAVES IN COLLOlD CHEMISTRY' KARL SOLLNERl Department of Agronomy, New York State College of Agriculture, Cornell University, Ithaca, New York Received July 14, 1958
The main phenomena of interest for the colloid chemist caused by ultrasonic waves (2,6, 13, 14,28) are so well known that they need not be considered extensively. Ultrasonics may cause all sorts of disintegration phenomena, e.g., the formation of emulsions and of fogs, and they may bring about coagulation, Although these facts have been well known for ten years, the underlying mechanisms have been obscure; accordingly several years ago we undertook their investigation. This paper gives a short account of the work. The experimental technique described by Wood and Loomis (28) was used. Plates about 7 cm. in diameter and 1 cm. thick, cut from quartz crystal, are placed on a lead electrode and covered with a brass ring electrode. The whole system is immersed in transformer oil, an alternating field is applied, and the quartz, which is pronouncedly piezoelectric, starts vibrating. Since the electrical frequency is the same as the mechanical frequency of the quartz plate, the vibrations are so pronounced that the oil over the quartz is set into motion and a fountain of oil rises above it. Although the quartz discs used (11)had frequencies of about 200,000 cycles a second, it must be emphasized that the frequency employed is immaterial. If a test tube containing water and mercury or water and an immiscible organic liquid is dipped into the oil fountain, emulsification occurs a t once. Gray clouds of very fine mercury droplets are thrown into the water from the water-mercury interface, and white clouds of dispersed water or organic liquid, respectively, are produced where the two liquids meet; soon more or less concentrated emulsions are obtained. These emulsions do not shour any special features. In non-protected systems under our standard conditions 6 g. of mercury per liter and 50 to 60 g. of benzene or similar substances are dispersed, when the equilibrium state is reached. This occurs under our experimental conditions in approximately 1 min. In the presence of suitable emulsifiers very high concentrations can be obtained. Presented at the Fifteenth Colloid Symposium, held a t Cambridge, Massachusetts, June 9-11, 1938. New address: Department of Physiology, University of Minnesota, Minneapolis, Minnesota. 1071
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The size of the particles (5, 7, 28) varies somewhat with the metal dispersed and the medium of dispersion; low concentration and a fluid medium of dispersion favor a higher degree of dispersion, from a few tenths of a micron to several micra, although the truly colloidal fraction in many cases is rather small. In 1929 Richards (20) emphasized the differences in the nature of the emulsification of mercury and in the dispersion of organic liquids in water. For liquid and molten metals the mechanism of dispersion, according to him, is as follomrs (4,20) : When brought into the oil fountain the violent transversal vibrations of the glass tube pump small quantities of water into the liquid metal; the water droplets rising in its interior reach the interface metal-water covered with a thin film of metal. When this film breaks, a cloud of minute metal droplets is thrown into the liquid. This process, as described for the macro-interface metal-water, obviously occurs also between the water droplets in the interior of the liquid metal, thus accentuating the effect. Figure 1 demonstrates the whole mechanism rather convincingly. A low-melting alloy was irradiated with water and cooled down during the irradiation. Its sponge-like nature is apparent from its cross section and from its surface covered with little blisters. For a reason which may be understood later, it must be stressed that this dispersion of metals occurs equally well in vacuo or when high external pressure is applied. Kow let us turn our attention to the mechanism of emulsification in oilmater systems3 (3), which was shown t o hold quite generally, e.g., for the peptization of gels (12) or the formation of fogs (23) of organic or aqueous liquids, and for the dispersion of solids in liquids ( 2 5 ) . The only exception is the formation of metal emulsions, discussed above. The facts concerning emulsification in non-metallic systems, as described first by Kewton Harvey (13), are as follows: Emulsification, Le., disintegration, in general, occurs neither in vacuo nor when sufficiently high outside pressure is applied, the liquids in the latter case being gas-free or saturated with gas at a lower pressure only. This rule applies as well in the presence of even the best emulsifiers, where mere shaking may easily produce stable and fine-grained emulsions ( 3 ) . At equilibrium pressure emulsification always occurs, provided this pressure is neither too high nor too lorn-. The lower limit is much more sensitive in toluene-water systems, e.g., a gas pressure of a t least 100 mm. of mercury is necessary. 8 L. A Chambers and M. Kewton Gaines must be given credit for prior publication of an explanation of their experiments, similar to ours, on emulsification with low frequencies. This work (J. Cellular Comp. Physiol. 1 , 4 5 1 (1932)) unfortunately did not come to our attention until after the whole series of our investigations on ultrasonics had been published.
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CHICMISTHI
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1
4 fii .$ l m - ~ n d ~:dly i w I' I:L(:, I. b:n~ulsifir.~~tion nf jl mc.t.iil hy ultrwu~>ic~s. dwing iiriidiation. Ahout t w i w I(I.Lu:LI i i i t , . FK;.2. 1:mulsification hy ultrasonics. The influrner of tlic presence of L( foreign! ym upon emulsification. About one-third actual sire. R c . 3. Coagulation of an emulsion by ultrssmics. About one-quarter actual size
FIG.4. Stationary wave patterns. (a) Toluene in water; (b) quarts in Water: (e) toluene and quartz in water. About 1.5 times actual sire.
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How striking this difference is may be gathered from figure 2. The pressure experiments were performed in long test tubes. The column of the upper liquid is rather high, so that no saturation can occur in the critical region, the interface liquid-liquid, when an outside pressure is applied by means of a compressed gas. How do the sound waves cause what a t first sight appears to be such complicated phenomena, thc effects being substantially the same whether performed a t a few thousand or at millions of cycles a second (3)? A sound wave travelling through a liquid compresses and stretches it periodically. If the stretch is moderate and the irradiated liquid is free of gas, nothing qpectacularoccurs, but if the liquid is saturated with gas, gas bubbles appear, as was shown in detail by Boyle (6) and Newton Harvey (13) and their coworkers. What happens if the liquid is stretched unduly was desrribed more than sixty years ago as a curiosity by Kundt and Lehmann (17) (working at low frequencies). They said: “While the whole system was vibrating violently the water close to the end of the vibrating rod turned turbid during the vibration. Since it was entirely free of air, these small bubbles causing the turbidity could only be due to the disruption of the water under the influence of the intense vibrations.” When irradiating with ultrasonics (9,24) in a long test tube containing a carefully degassed liquid, such as benzene, toluene, or slightly warmed water, zones of a slight and somewhat glittering opacity were always formed. Although no bubbles rose to the surface, a hissing noise was always heard. Thus it is clearly indicated that the liquid disrupts under the stretch of sound waves, hence a high hydrostatic pressure prevents the whole phenomenon. If the same experiment is performed in a vacuum even a t low energies, the liquid bubbles and boils but no hissing noise is heard. Under neither of these conditions does emulsification occur. Strong mechanical action by ultrasonics is always accompanied by the hissing noise, its loudness being so characteristic that it is used as an indicator of their efficiency. I t is apparent that, although the formation of cavities does not give rise to destructive action, their disappearancr, always connerted with thr hissing qound, doeti. Lord Rayleigh (19) ralriilated thr p r ~ w i r e swhich may O P ~ U Pwhen A vapor bubble rollapses in i\. liquid and fotmd that many thousands of atmospheres may hr obtaincd locally iii this way. Thv collapse of ravitie. at t hr rear of steamship proprller blades may (+awethe rapid destrurtioii of the blades. In engineering the entire phenomenon, involving the formation of cavities and their rehement collapse due to outside pressure, is called “cavitation.” This term will be used in this sense. It may be easily demonstrated that the collapse of steam bubbles produces emulsification when ocrurring at the interface of two liquids. The collapse of the steam bubbles when accompanied by a rattling sound causes
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rapid emulsification (3). This method of emulsification is substantially different from the older one in which the vapor of an organic liquid is introduced into water, forming an emulsion on condensation. We may safely conclude that the emulsification by ultrasonics is due to “cavitation” as defined above (3). This accounts for the fact that no emulsification occurs in a vacuum or when a high outside pressure is applied. I n a vacuum cavities may be formed, but lack of outside pressure prevents their violent collapse. On the other hand, a sufficiently high pressure prevents the formation of cavities. The prevention of cavitation by applying an outside pressure is well known in engineering. The influence of dissolved gases will not be discussed here in detail; one of their main functions is to act as weak spots when the liquid is stretched, thus promoting cavitation (3). Similarly, cavitation is favored at the interface water-oil, a fact promoting the dispersing action. The influence of pressure and vacuum is not observed when dispersing liquid metals since, as already mentioned, their dispersion is not due to cavitation. The dispersion of gels (10, 11, 12), e.g., rubber in benzene, or the liquefaction of thixotropic gels, is due to the same mechanism as emulsification in oil-water systems, occurring under exactly the same conditions. This is also true for the dispersion of solid bodies in liquids (25). An efficient dispersion of this kind has not as yet been observed with metallic or other substances of great cohesion, although oxide films, etc., may be easily removed by ultrasonics. On the other hand softer substances, especially of high cleavage (such as graphite or mica), may be readily dispersed. This process proceeds a t a remarkably rapid rate when semicolloidal or microscopic suspensions, but not macrocrystals, are irradiated, colloidal solutions being readily obtained (25). It can be demonstrated also that the formation of fog (23) by ultrasonics, as described by Wood and Loomis, is due to cavitation. A beaker or test tube containing a volatile liquid, such as benzene or toluene, is rapidly filled with a white cloud or fog when irradiated. With less volatile liquids it is necessary to use high energies or, more advantageously, to concentrate the energy by means of a special collector, a test tube drawn out at the middle to a thin-walled constriction. When brought into the oil fountain the constriction vibrates violently, and even liquids of a very viscous nature and of a high boiling point are dispersed into the air, if they are allowed to run down through the constriction. Air currents set up by the rectifying action (18) of the transversally vibrating collector transport the fog droplets away from it. The very suggestive idea that one is dealing with evaporation on account of the great heat developed at the critical spot can readily be shown to be wrong. When an oil containing a non-volatile dyestuff is used, a colored
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cloud is formed around the collector, which proves that the dispersion is a direct one. Here, too, experiments at varying pressures show that cavitation causes the dispersion. On close inspection one may see how the surface becomes rippled at energies just sufficient to start fog formation. It looks as if the surface were hit from above,-indeed it is hit from above by the collapse of the cavities formed a t the surface (23). How active the interface liquid--gas is may also be seen from the follotving experiment: A strip of rubber partially dipped into benzene is, on irradiation, attacked most near the surface of the benzene, and may be eaten away entirely a t this point, whereas its lower end, further away from the surface of the benzene, is practically free from attack.
We shall consider next the opposite of the phenomena discussed, the coagulating action of ultrasonics (21). Ultrasonics must be able to coagulate at a high speed, since a stationary state is soon reached in preparing an emulsion. It is easily demonstrated that this coagulation is not due to the streaming or stirring which is always observed during strong irradiation. Rapid coagulation occurs also a t rather low energies where such a movement is never observed. It may be shown that coagulation is due to the formation of stationary wave patterns,-Kundt’s dust figures in liquids (21). When an emiilsion is irradiated (the phenomenon is best seen in high columns of liquid), more whitish zones appear in the lower and middle part of the test tube, where the droplets of the emulsion accumulate. How such a test tube looks after a few minutes of irradiation may be seen from figure 3. Near the surface, where cavitation is strongest, the emulsion is not much affected, whereas in the lower parts all the dispersed material is accumulated, The process of coagulation may be observed microscopically (21). M7ithlow energies practically complete coagulation may be brought about, since the opposite process, emulsification, nerds energies exceeding a rather well defined limiting value. The coagulation of suspensions is quite analogous. In unstable systems, for instance quartz powder in organic liquids, it is very impressive. On slight irradiation the particles accumulate and stick to each other, thus forming big lumps which sink rapidly to the bottom of the tube; the whole liquid is free from particles after a few seconds. To study the stationary wave patterns more closely, capillaries filled with an emulsion or suspension were used. They were closed at one end with picein, leaving a small air gap between the emulsion and the piceill stopper, to prcvent any appreciable movement of the liquid. When the oscillations are started the dispersed phase forms zones of accumulation which rapidly get very sharp; one has the impression of small disks standing upright in the capillary (figure 4). The distance between each two of the zones of
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accumulation can be calculated rather exactly from the sound velocity of the liquid and the known frequency of the oscillation. In some cases the dispersed phase is collected in the nodes; in other cases in the loops, The main decisive factor-there must be some other ones too-seems to be the ratio of the specific gravities of the dispersed phase and the medium of dispersion. If the dispersed phase is lighter, it accumulates in the nodes; if it is heavier than the medium of dispersion, in the antinodes (16). This means that in the former case the meniscus becomes free from dispersed phase; in the latter case, there is a locus of accumulation (figure 4a, 4b). When a mixed system containing, e.g., dispersed toluene and quartz powder is irradiated the two dispersed phases separate and accumulate a t their proper places, in the nodes and loops, respectively (figure 4c). Larger particles are much more readily collected, particles several micra in diameter accumulate practically instantaneously, whereas smaller particles, below 0.51 or so in diameter, are hardly affected a t moderate energies and the frequency employed. The coagulating action of the ultrasonics is now easily understood: firstly, the particles are accumulated and, as one knows, the rate of spontaneous coagulation increases rapidly as the concentration increases; secondly, particles of different size migrate with different velocities towards the zones of accumulation, thus being liable to additional collisions, a kind of orthokinetic coagulation (26, 27) ; thirdly, the particles do not migrate only in one direction, their macroscopic movements merely being the result of the asymmetry of the oscillations which they perform according to their (different) size; this too causes a kind of orthokinetic coagulation. Xuch very interesting work has been done by other investigators (1, 15) on the coagulation of fogs and smokes. There is considerable analogy with the coagulation in liquid systems, orthokinetic coagulation being particularly pronounced with such systems (22). In conclusion, mention should be made of the Orientation of anisometric, Le., rod- and plate-like, particles by ultrasonics (8). When a very dilute suspension of finely ground mica or graphite is irradiated even with very weak energies, brilliantly glittering zones are observed at once in transmitted or reflected light, indicating an orientation of the (anisometric) particles, perpendicular to the axis of the tube, Le., perpendicular to the flux of energy. I n this experiment the energy applied may be so weak as to cause no appreciable accumulation even. over long periods. This orientation is found with all rod- and plate-like substances of macroscopic and microscopic dimensions; it was also found with some truly colloidal systems, e.g., vanadium pentoxide sols or ferric oxide sols containing anisometric particles, ultramicroscopic in all three dimensions. The orientation with these sols is studied most conveniently with polarized light,
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the orientation of the particles being in all cases perpendicular to the flux of energy; this interesting phenomenon undoubtedly needs further investigation. (1) (2) (3) (4) (5) (6) (7) (8) (9) (10) (11) (12) (13) (14) (15) (16) (17) (18) (19) (20) (21) (22) (23) (24) (25) (26) (27) (28)
REFERENCES ANDRADE:Trans. Faraday SOC.32, 1111 (1936). B E R G M ~ NDer N : Ultraschall. VDI Verlag, Berlin (1937). BONDY AND SOLLNER: Trans. Faraday SOC.31,835 (1935). AND SOLLNER: Trans. Faraday SOC.31,843 (1935). BONDY BONDY AND SOLLNER: Trans. Faraday SOC. 32, 556 (1936). BOYLE:Science Progress 23, 75 (1928); (Gives further references). BULLAND SOLLNER:Kolloid-Z. 60,263 (1932). BURGERAND SOLLNER: Trans. Faraday SOC.32, 1598 (1936). AND GAINES:J. Cellular Conip. Physiol. 1, 451 (1932). CHAMBERS FREUNDLICH, ROGOWSKI, AND SOLLNER: Z. physik. Chem. A160, 469 (1932). FREUNDLICH, ROGOWSKI, AND SOLLNER: Kolloid-Beihefte 37, 223 (1933). FREUNDLICH AND SOLLNER:Trans. Faraday SOC.32,966 (1936). HARVEY:Biol. Bull. 69, 306 (1930) (further references given). HIEDEMAXN: Ergeb. exakt. Katurw. 14,201-63 (1935). HIEDEMANN AND COWORKERS: Trans. Faraday SOC. 32, 1101 (1936); Kolloid-Z. 77, 103, 168 (1936). KING:Proc. Roy. SOC.(London) 147A, 212 (1934). KUNDT .4ND LEHhL4NN: POgg. Ann. 163, 1 (1874). Cf., e.g., MEISSNER:Physik. Z. 28, 621 (1927); Naturwissenschaften 17, 25 (1929). LORDRAYLEIGH: Phil. Mag. [SI 34, 94 (1917). RICHARDS: J. Am. Chem. SOC.61, 1724 (1929). AND BONDY: Trans. Faraday SOC.32, 616 (1936). SOLLNER SOLLNER:Trans. Faraday SOC.32, 1119 (1936). SOLLNER: Trans. Faraday SOC.32, 1532 (1936). SOLLNER: Trans. Faraday SOC.32, 1537 (1936). S ~ L L N ETrans. R : Faraday SOC.,in press. T n o R I L A : Kolloid-Bcihefte 24, 1 (1927). WIEGNER:J. SOC.Chem. Ind. 60, 55 (1931). WOODA N D LOOMIS:Phil. Mag. [7] 4, 417 (1927).
