bondy - 1935 - on the mechanism of emulsification by ultrasonic waves

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W. JACKSON 83 5

O N T H E M E C H A N I S M OF E M U L S I F I C A T I O N BYU L T R A S O N I C W A V E S .BY C. BONDY ND K. SOLLNER.

Received 12th March, 1935.It has been known since the work of Wood and Loomis,l that ultra-sonic waves readily bring about the formation of emulsions in two-liquid systems such as water/oil (the term oil is used for organic liquidsno t miscible with water) or waterlmercury. The mechanism of thisemulsification has been investigated more than once and theoreticalexplanations have been advanced to account for it.2 But since thereis apparently as yet no really satisfactory theory, it seemed worthwhile to at tack the problem again.3Experimental results, which will be discussed later, led us to theconclusion that there is a fundamental difference in the mechanism

leading to the formation of emulsions in water/oil and waterlmercurysystems. This paper deals only with the former type of emulsions,mercury emulsions being discussed in the following paper.We further came to the conclusion that a so-called cavitation at theinterface causes emulsification.In the literature on ultrasonics the term cavitation has beenused by several writers when merely referring specially to the expulsionof dissolved gases.4 We found i t necessary to revert to it s original mean-ing as known in hydrodynamics. Since cavitation and the phenomenacorrelated with it are not generally known, an introductory note willdeal with them.1 R. W. Wood and A. L. Loomis, Phil. Mug. (7), 1927. 4, 417.2W. . Richards, J . A m . Chem. Soc., 1929, 51, 724. E. N. Harvey, Biol.Bull., 1930, 59, 306. N. Marinesco, Compt . rend., 1933, 196, 346: F. Rogowskiand K. Sollner,2. f j h y s i k . Chem. (A.), 1933, 166,428.3 The apparatus used for this work was described by H. Freundlich, F. Rogow-ski and K. Sollner, Kollch. Beih., 1933, 37, 223 ; see also the same authors, 2.f ihys ik . Chem. (A.), 1932, 160,469. F. 0.Schmitt, C. H. Johnson, and A. R. Olson, ibid., 1929, 51, 370. C. H. Johnson,J . Physiol., 1929, 67, 356.

W. T. Richards and A. L. Loomis, J . Am. C h e m . Soc., 1927,49,3086.E. N. Harvey, loc. ci t .2 .

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836 EMULSIFICATION BY ULTRASONIC WAVESThe Formation of Cavities in Liquids.

The word cavitation was introduced (in 1894)when engineers first 5became acquainted with high speed propellers and steam turbines and.were, thus, obliged to take into account cavitations produced by therapid rotation.Cavitation can occur as soon as the hydrodynamical pressure in aliquid is reduced to the vapour pressure. Where this is the case the liquidmay disrupt and so be transformed into a two-phase system, containinga fluid and a gaseous phase. This disruption is somewhat analogous tothe disruption of stretched liquids in the experiments of M. Berthelot,Worthington and J . Meyer.8 These will always be referred to as the experi-ment of Berthelot. Now in these cases the liquid does no t disrupt underwell-defined conditions ; the phenomenon is largely dependent uponfortuitous circumstances. It is commonly known, for instance, thatstretched fluids are very sensitive to feeble shocks and furthermore thatit is absolutely essential to remove all nuclei of gases. Impurities likewisehave a marked influence on the tensile strength of liquids. The theoreticalvalues, as calculated from van der Waals equation, have never beenrealised, the maximum stretches found experimentally amounting only toa few per cent. of those theoretically possible. From this fac t it is to beconcluded tha t there are always oose-spaces ( Lockerstellen ) presentin a liquid. It is probable that dissolved gases are instrumental in theformation of such loose-spaces. If a liquid, saturated with gas, isstretched, a gas nucleus may be formed which can act as a oose-space and so lead to the disruption of the fluid, provided that it is stretched toa sufficient extent. described a very impressive experiment demon-strating the formation of cavities in a streaming liquid. Water, allowedto stream through a con-vergent-divergent tube (seeFig. I), turns opaque a t thenarrowest constriction (of the

FIG.I. tube), if the rate of flowexceeds a certain minimumvalue. The zone of opacity extends fo r some millimetres down stream.At the same time a loud hissing noise is heard ; Osborne Reynolds callsthis phenomenon the boiling of water in an open tube a t ordinary tem-perature and points ou t that the hissing sound arises from the samecause (i.e. he collapse of cavities) as the singing of a kettle shortly beforethe water boils. In both cases, the hissing is caused by the condensationof steam bubbles passing into regions of higher pressure or lower tempera-ture respectively.This experiment of Reynolds can readily be repeated by connectingto the water mains a not too narrow glass tube drawn down in the middleto about I mm. inside diameter.

Osborne Reynolds

The possibility of th e formation of cavities was discussed by L. Euler asfar back as 1754 n his Theor ie plus compl2te des m achine s , qui s o n t m i s e s e n m o u v e -m e n t 9 a r l a d a c t i o n d e l ea u. A general account of cavi tation is given in H y -drpul ische Probleme, V. D. I. Verlag, Berlin, 1926, nd H. Mueller, Naturwis sen-schaf ten, 1928,16,423.It is not known, as yet, whether this is also true for absolutely pure and gas-free liquids.M. Berthelot, Ann. P h y s iq u e e t C h i m . (3) ,1850,30,232.Proc . Roy. SOC.,1892,J. Meyer, Zur K e n n t n i s d e s n e g a t i v e n D r u c k s an Fl i i s s igke i t en , Ab-Osborne Reynolds, Papers on Mechanical and Phys ical Subjec ts , Cambridge,

8A. M. Worthington, P h i l . T r a n s . , 1892, 183A, 355 :50, 423.handlungen d . Deutsch. Bunsengesel lschaj t , Nr. 6 (191 ).1901, 2, 578.

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C. BONDY AND K . SOLLNER 837The cavitation in such a tube is easily understood. According to

( P being the hydrodynamical pressure, p the density and 3 the velocity),the sum of pressure and kinetic energy in a flowing liquid is a constant.Thus the velocity of a liquid passing through a restriction may becomeso high that the hydrodynamical pressure is reduced to the vapour tension.According to Reynolds the presence of dissolved air exerts a distinctinfluence. In liquids which, like tap water, contain a sufficient amount ofdissolved air, cavitation is always accompanied by partial de-gassing. @This may be explained by the fac t th at minute gas bubbles are formedwhere the liquid disrupts. These bubbles quickly unite to larger ones,which are not so readily redissolved in regions of higher pressure.

The Collapse of Cavities.One effect brought about by the collapse of cavities has already beenmentioned, viz. the singing of a kettle. Cavities collapse, as soon as theconditions which have led to their formation cease to exist. In most casesdecavitation, so to speak, is due to rising pressure or decreasing temperaturein the surrounding liquid.The investigations of Reynolds inspired the la te Lord Rayleigh tocalculate the pressure developed during the collapse of a spherical cavity."The result of his derivation is given by the equation :

P'2= - ( RoS-I).2B 3 R( P being the pressure a t infinity external atmospheric pressure ;Ro the initial radius of the cavity ; /Ihe coefficient of compressibility ;

P' and I2 being the correlated pressure and radius of the cavity duringthe collapse )Calculation shows that pressures of thousands of atmospheres may bedeveloped at the moment when the cavity collapses to a small fraction ofits original diameter. Obviously such collapses may cause enormousmechanical effects, high kinetic energies being concentrated a t very smallspots. In fact, mechanical engineering and, specially, naval constructionare quite familiar with effects of this kind. The mechanical impact dueto decavitation produces extremely heavy erosion, which can be distin-guished from ordinary corrosion by i ts different appearances and by theplaces where it occurs. These places moreover may be predicted fromhydrodynamical reasoning. Fottinger,12 who advanced this theory oferosion by cavitation, was able to prove his views by showing that glass,which is chemically inert, is also attacked under corresponding conditions.To give some impression of the amazing effects of cavitation, it may bementioned th at after a destroyer had rushed for several hours a t maximumspeed, the armour plates above the propeller were pierced by a hole ofthe dimensions of about one square foot. Likewise turbines may be stronglyattacked during short runs.13lo0.Reynolds describes this phenomenon as follows : " When the hiss is on,th e water in,the tube will be somewhat opaque-rather foggy-which fog dis-appears after th e hiss is stopped. This fog is caused by th e separation of the airoccluded in the water and corresponds exactly to t he separation of t he a ir, aswhen the temperature of t he water in th e kettle is above 174"F. In the case ofthe ube th e bubbles of air, which separate out, are very much smaller tha n thosein th e ket tle on account of t he greate r violence of t he action."l1Lord Rayleigh, Phil. Mag. (6), 19x7, 4,94.l2H. Fottinger, Hydraulische Probleme, Zoc.l3Apart from these mechanical effects, chemical effects of cavitation mustalso be assumed according to H. Fottinger, loc. it.,^ who points out, th at owing

p. 14.

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838 EMULSIFICATION BY ULTRASONIC WAVESIt will be apparent therefore t ha t strong mechanical action may arisefrom cavitation. Now, we hope to show that intense ultrasonic wavesare able to produce this phenomenon, and furthermore tha t emulsificationunder their influence only takes place when there is a chance for t he for-mation, as well as for the collapse, of cavities in the radiated system.

Emulsification Caused by the Collapse of Steam Bubbles.We will first prove that the collapse of cavities does indeed representa most efficient method for dispersing systems of water and oil.Probably the simplest way of producing cavities is to let steam con-

dense in its own liquid phase, as in the case of the singing kettle. Iffor instance steam from boiling water-or even better, superheated steam-is brought through a nozzle of about I mm. diameter into the inter-face of water and oil, i t condenses with the well-known rat tling noise,a highly dispersed emulsion of the O/W type being formed a t the sametime. When using a soap solution instead of pure water, concentratedemulsions may be produced, provided the experiment is continued longenough. It is evident that this experiment is fundamentally differentfrom all those where an emulsion is obtained when the steam of t hedisperse phase is introduced in to the cool medium of dispersion. Thefact that emulsification ceases as soon as the water becomes too warmto allow of sufficiently rapid condensation proves that in our case thecollapse of cavities is essential. If air, instead of steam, was blown' in tothe system, there was no effect whatsoever, in agreement with what wasto be expected. On the contrary the addition of air to the steam reducesit s efficiency, since the gas buffers the impact of the collapsing steambubbles. The method of dispersing organic liquids in water by meansof steam turned ou t to be of quite general value when producing allsorts of emulsions.

Cavitation Caused by Acoustic Waves.How far may ultrasonic and also acoustic waves give rise to similareffects ? Since sound waves consist of periodical compressions and

expansions, it seems quite legitimate to suppose that, provided theenergy is sufficiently high, cavities may be formed in a liquid during theexpansion phase. Some six ty years ago it was shown that acousticwaves are able to disrupt a fluid. Kund t and Lehmann l4 in their well-known paper on dust figures and velocities of sound in liquids describean interesting experiment as follows :" If a tube was completely filled with water which had not beende-gassed, but from which all-even the smallest-bubbles had beenremoved, and was set into vibration by strongly rubbing the glass rod,air bubbles were formed which grew markedly when the vibrationcontinued. This is able to drive the dissolved air out of the water.The air bubbles disappear again, i.e., the air is absorbed, if the experi-ments are stopped for 'some time."Even more conclusive is the following observation of Kundt andLehmann, which was made with a carefully de-gassed fluid : " Whilethe whole system was vibrating vehemently we several times noticedto locally developed high pressures and temperatures and to the possibility ofballo-electric phenomena the occurrence of oxidations, dissociations, etc., is tobe expected.

1*-4.undt-and0. Lehmann, Ann. Physik, (Pogg . ) , 1874,153, I.

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C. BONDY AND K. SOLLNER 839that the water close to the end of the vibrating rod turned turbid. Sinceit was entirely free from air, these small bubbles causing the turbiditycould only be due to the disruption of the water (Zerreissen in kleinePartien) under the influence of these intense vibrations.This is evidently the same phenomenon as the formation of cavitiesin Reynolds experiment mentioned above.Kundt and Lehmanns experiments definitely show that acousticwaves may cause cavitation in de-gassed liquids and the expulsion ofdissolved gas in gas-containing liquids.

Cavitation Caused by Ultrasonic Waves.In these circumstances one need not hesitate to assume that intenseultrasonic vibrations may also cause cavitation. The following facts

confirm this assumption :I . Hopwood showed that liquids having a low boiling-point maybe made to distil a t room temperature when radiated by ultrasonics.15This agrees with the fact described by Richards and Loomis l6 that ultra-sonics lower the boiling-point.2 . The ability of ultrasonics to evolve gas from gas-containing

liquids. 73. The fact l8 that liquids stretched according to Berthelots method(ie., iquids cooled down slowly after having been sealed into a tube athigher temperatures) are disrupted at a higher temperature, i . e . , a t afeebler static stress, when exposed to ultrasonics. Thismay be correlated to the fact, already emphasised by Wood and Loomis,that ultrasonics are specially active at interfaces (heat effects, etc.).I t can be seen from an experiment of Freundlich and Lindau th at cavitiestend to appear a t the interface of two liquids. If water, in a Berthelotexperiment, is stretched in a tube which also contains mercury, bubblesappear notably a t the interface water/mercury, when the liquid disrupts.When an oil-wetted tube is used, many small bubbles are formed at thewall on disrupting, whereas only one bubble appears, when the wallsof the tube have been wetted with water. The most direct evidenceis that, when radiating a two-phase system with ultrasonics the forma-tion of gas bubbles is seen to be most pronounced a t the interface.

The formation of cavities seems to be favoured a t interfaces.

Emulsification by Ultrasonics.When applying these results to the question of emulsification thefollowing assertion has to be proved : emuIsions are only obtainedwhen the conditions are such that cavities are not only produced but alsocollapse. Their formation alone does nok give rise to any remarkable15F.L. Hopwood, Nature, 1931, 128, 748. This reminds of the remark of

0.Reynolds quoted above : The boiling of water in an open tube a t ordinarytemperature.l6 Richards and Loomis, Zoc. it.^l7 It is important to remark tha t de-gassing does not implicitly requirecavitation. Expansions, many times smaller than those necessary for cavitation,are sufficient to cause bubbling in gas-containing fluids. This was first em-phasised by R. W. Boyle and G. B. Taylor, P h y s i c . R e v . (z),1926, 27, 518.This typicalexample may be mentioned: de-gassed water, sealed into a tube a t 43 C. dis-rupts a t 31 c., f cooled down in the usual way, i t already disrupts at 36 c. ifradiated by ultrasonics.laUnpublished experiments of H. Freundlich and G. Lindau.

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840

Pressure inHg,

0060I00150200300600760I 0 0

I50023004000

EMULSIFICATION BY ULTRASONIC WAVES

studied a t different air pressures, the liquidsbeing always in equilibrium with the gas.in Per Cent. The experiments with pressures smallerthan atmospheric were done in sealed tubes,

the others in open tubes of exactly theO d same shape, the latter being connected to0.3 a compressed air cylinder. When compar-1'0 ing the resulting emulsions of toluene in

water, it was evident that a high concen--6 tration was not reached until the pressure'24'14'4 exceeded a value of about 100 mm. Hg;5'0 then, there is a broad range where the5'7 Concentration increases with increasingpressure. At still higher pressures, above

about 2 atmospheres, e-mulsification de-

~i sp en e hase(after 3O sec-)

0 '0

4 ' 11 '2

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C. BONDY AND K. SdLLNERacts again as a buffer, as was the case in the experiments done withsteam.21There is yet another most important similarity to the dispersing actionof steam : emulsification by ultrasonics is much feebler when hot liquidsare used. This is quite contrary to the usual experience in the techniqueof emulsification. It is hard to imagine any other mechanism in whichemulsification would be decreased by a rise in temperature. All kindsof stirring effects, as assumed by some previous authors, should beimproved a t higher temperatures.Some details as to the formation of cavities by ultrasonic wavesalso deserve further consideration. When discussing the experimentsof Berthelot and Osborne Reynolds, i t was emphasised that the dis-ruption of liquids largely depends upon a ready production of nucleifrom dissolved gases. As was to be expected, a similar influence is foundwith ultrasonics : pairs of liquids such as waterlbenzene or water/nitro-benzene-saturated with gas a t atmospheric pressure-were radiateda t higher hydrostatic pressures.22 The liquids were contained in longtubes (about 25 cm.) of 10 mm. diameter. The amount of the heavierliquid was so small that its height was not more than 2.5 cm., while thelighter liquid practically filled the rest of the tube. Pressure was pro-vided by a compressed-air cylinder. As the pressure increased emulsi-fication rapidly decreased, and no emulsion was obtained when thepressure exceeded a certain value. This value depends on the ultra-sonic energy, the temperature and the liquids used. A hydrostaticpressure of several hundred mm. Hg exerted a decisive influence.23This behaviour was not changed by stabilisers such as soaps or gelatin.Control experiments a t normal pressure always showed strong emul-sification.This result may be explained as follows : when hydrostatic pressureis applied, the system is no longer saturated as to the gas, the highcolumn of liquid preventing saturation. Nuclei, which might have beenpresent, are dissolved and conditions are unfavourable for the produc-tion of new ones. Consequently no cavities are formed and emulsi-fication is prevented. It is evident that for this reason also, the liquidsare not de-gassed under these conditions.The stretching of a liquid in theexperiments of Berthelot is different from that caused by ultrasonicwaves, in so far as, in the former case, the liquid only undergoes a stat icstress, whereas ultrasonic cause a stress varying with the period of thevibration, thus' causing strong movement. The lat ter circumstanceevidently favours disruption. This is also borne out by the fact thatin Berthelot's experiment the stretched liquid is sensitive towards gentleshocks.We leave undecided the exact mechanism of this process, there beinginvolved the extremely difficult problem of the formation of primarynuclei and their size.

Another point needs discussion.

21 This buffering action is perhaps also the reason why a certain time of initi-ation must elapse before strong emulsification begins.22 Experiments, mainly biological, with pressures both lower and higher thanatmospheric have already been carried out by several authors : F. 0. Schmitt,C . H. Johnson and A. R. Olson, C. H. Johnson, E. N. Harvey, Zoc. ciL423 It is well known th at cavitation can be prevented by an external pressure.Engineers make use of this fact by applying hydrostatic pressures to pumps andturbines and by designing propellers of ships to operate at the lowest possibleposition below the water level.

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842 EMULSIFICATION BY ULTRASONIC WAVESThe influence of gases is so important and so complicated that it

I. Dissolved gases may favour emulsification, because they favourthe formation of nuclei and thus of cavities :At higher concentrations they may be unfavourable to the productionof emulsification, because they have a buffering effect upon thecollapse of cavities.11. An (external) hydrostatic pressure may be exerted by gases,and thus they may exert an influence.Pressure may favour emulsification, because it is necessary for causingA higher pressure may be unfavourable to emulsification :-

seems advisable to summarise all effects discussed here.

an efficient collapse.I . If equilibrium is attained, because the energy of the ultra-sonics applied can only overcome a certain pressure when formingcavities ;2. If there is an over-pressure, no t in equilibrium with the gasin solution :-( a ) Because over-pressure disfavours the formation of nucleia t the interface and therefore the production of cavities ;( b ) for the same reason as mentioned under 11, I .

Ultrasonics do not reveal any peculiarities as to the ease of emulsi-fication, apart from the fact that emulsification is favoured at lowtemperature ; the properties of the emulsions formed are also the sameas those of emulsions produced by the usual methods. Ultrasonics,nevertheless represent a very convenient, and efficient, method of pre-paring protected and unprotected emulsions under clean and repro-ducible conditions.We are inclined to believe that many of the destructive effects ofultrasonic vibrations upon living cells as described by biologists 24 arebased on the same mechanism as emulsification, i .e . , on the collapse ofcavities.25

Summary.I. The emulsifying action of ultrasonic waves in oil/water systems isdue to cavitztion.2. It is shown that cavitation can be caused by sound waves andultrasonics.3. The influence of gases upon cavitation is complex. Dissolvedgases as nuclei favour the formation of cavities. The hydrostatic pressureexerted by gases is necessary for the collapse of cavities.4. A new and general method for the preparation of emulsions basedon the collapse of cavities was found : the sudden condensation of vapours,such as steam, at the interface of two immiscible liquids rapidly causesstrong emulsification.

Emulsification occurs when cavities collapse.

24 R. W. Wood and A. L. Loomis, Zoc. tit.', C. H. Johnson, doc. it.,^ E. K .Harvey, Zoc. cit.' (which see for further references).25 It does not seem improbable tha t th e chemical effects of ultrasonic wavescan be explained i n t he same way, as was assumed by H. Fattinger, Zoc. cit.l*, forchemical effects of cavitation in general.

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C. BONDY AND K. SOLLNER 843Our heartiest thanks are due to Professor H. Freundlich for his veryhelpful criticism and advice during this work. We are also greatlyindebted to Professor F. G. Donnan, F.R.S., for his generous hospitality

and his interest.F r o m the S i r W i l l i a m RamsayLaboratories of Inorganic andPhysical Chemis try ,University College, London.

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bondy - 1935 - on the mechanism of emulsification by ultrasonic waves

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