Bubble Detection

Acoustic bubble detection The detection of transient

- II:

• • cavItatIon by Dr T C Leighton Institute of Soand and Vibration Research University of SOlllhampton, Highfield, SOIll/wllI/JlOIl S017 1 HJ

This paper follows Part I, which described primarily acoustic techniques for the detection of stable bubbles (gas bodies). Other techniques, based on optical, chemical, and erosive effects, were discussed. This paper descrihes techniques in the same four categories, though again concentrating primarily on acoustic methods, for the detection of transient cavitation, which is characterised by the sudden growth of a bubble from a small seed nucleus, followed by an energetic collapse and rebound.

I n Pan I the range of techniques available for the detection of bubbles was suhgrouped into three cat­egories: acoustic, optica l, and those associated with

damage or chem ical e ffects. OpLicaltcchn iqucs (and those acollstic techn iques involving shadowgra ph y or the bubble resonance) were shown to be particularl y appropriate for the detection and sizing of stable gas bodies. Th is paper concen trates primarily on the detection of less stable en­lities involved in more e nergetic processes, typified by sign ificant bubble expansion and subseque lll colla pse. Such cavities might be formed by the sudden dumping of e nergy into a liquid , for examp le through the actio n of lasers, sparks, or ionising radiation. Alternativel y small bubbles in high-amplitude pressu rc penu rbations (usually tensions, either acoustic o r hyd rodynamic in origin ) may grow to attain a radius several times the 'equilibrium' one; if, during this process, there is insu fficient diffusion or dissolved gas from the liqu id into the body, then the permanent gas pressure inside the 'cavity' when it is so expanded will be in sul1icient to prevent subsequent rapid collapse. This behaviour is termed "transient cavitation" or "unstable cavitation". Duringcollapse, the gas compressed within the bubble may reach temperatures of several thousand degrees [1-4] , and gas shocks may propagate, re fl ectinga nel focus ing within the bubble [4§4.4.8;4§5.2. 1 b; 5-7]. Bot.h these processes can in princi ple generate rree radica ls and electronica lly-excited chem ical species. Elec­trical elh:cts may a lso occu r [4§5.2.1c; 8,9]. The collapsing bubble may rebound, e mitting a pressure pu lse into the liquid. The rebound may leave the bubble in tact, or cause [i'agmentation. AJternatively, if the bubble environment contained directional innuences (for exa mple th rough the presence ofanoLher bubble or a boundary, o r the passage ora shock wave), the bubble may involute on collapsc and form a liquid jet [10, 11]. T hese processes are illu stratcd in Fig 2 of Part l.

Detection through optical, erosive and chemical effects \,Vh ilst optical studies of ' transient cavitation ' are not un­known [12], their im plementation can sometim es be dif­ficul t. SOllle drawbacks, such as opacity of media o r

16

containers, are outlin ed in P~ll'l l. Others are specific to the nature o f transient cavitation. An optical image of a size large enough to encompass the full y-ex panded bubble will rarely be able to resolve the initial pre-existing gas pocket which seeds this growth. O ften the precise location of the event is d ifficu lt to determine in advance, depending on the spatial and temporal coincidence of a suitable seed and an app ropriate excitation (though focllsed acoustics and the use of spa rks and lasers can in flu ence the siting).

In contrast techniques which exploit chem ical a nd ero­sive efTens of transient cavitation by definition can inter­rogate a ll of the sample which contains c~lvila tion sullicient to ca use thai e necl. However continuous real-Lime moni­to rin g by these techniques can sometimes be difficul t, measurements often requiring a cessation of cavitation ; and in terpretation of such effects in terms o f cavitation characteristics may be limited. Such techniques usuall y in volve the detection of effects associated with physical changes (eg erosion) due to jet impacL [13] a nd rebound pressures (either from single or cloud cavitation events [14]); anel resulting from aeliabatic heating of gas com­pressed within the bubbles, or Lhe chem ical and biological effects of the radical species wh ich may be associated with both this heating and the propagation of gas shocks. These cffens are d iscussed in chapter 5 o r reference [4]. The ran ge of eITects available can be illustrated through the hots pot resulting [rom the heating of the bubble gas. This can be detected through its abi lity to ignite explosives [15]. Additionally it call be monitored through efTects resulting from the generation wit hin the bubble gas of unstable chemical species, someofwhich maydifTuse into the liq uid. T hese include: (i) sonolum inescence (Fig I ), a faim light emission [3] which resu lts fi ·olll the radiative recom bina­tion of free radicals; and (i i) specific sonochc mical reac­tions [16], includ in g techniques such as spi n trapping which are designed to identify the nature of the short-lived free radicals by allowing them to react wilh diamagnelic nitroso or nit-rone species to produce a lon ger-l ived rad ical which can be ident.ified through electron spin resonance (ES R) spectroscopy [17].

Chem il um inescence may be produced through the radiati ve reaction o f free rad ical species generated within

Bubble Detection

Fig / A j}/w/ogra/Jh. laH.f'u through Ihe image intensifier, oj sOlloillmille,\Cf!II(f! 1/1 a 90O/C g(ltrerl1lellOo/i.. water mixtllre, occurrmg within a C)'linlirtCllliy-s)"nmetrir .\olllld field of 2 alm./m!SSll1"(' am/J/i. llide (see [4} for delatts). Each /)oinl of light reprf.\fl1ls the delec/ion oJa single IJll%. The bright SOU1U,~ ofligh' ari.\l' from SOllOl'lminesceIlCl'

from bubbles, originflting/rom lwo regions. COllllfr/;ug these iJ a Jlri1lg oj bubbles, which may be luminescing or simply reJ1fcling lighlfrom the main sources o!:i01lofumillfSCellu. The [ramI' measures J J. 75111111 x 17 111m.

lhe colbpsillg bubble with a chemica l dopant (called luminol, a lso triaminoplllhalic hyd raz inc) which is dis­solved into the liquid . PhOlOlllu llip licalion of this has demonstrated that ulLrasonic pulses containing essentia lly one cycle al I ~I Hz and a I: 10 dUly cycle could generale cavitation with a 20 atm. threshold acoustic pressure a mplitude [18]. Roy and Fowlkes [19] have demonstrated that the sensiti vity of the chc miluminesce lll tech nique is lower than that o f passive acoustic detectors of the type discussed later. Luminolluminescence persists for about 1/20th second <lfter the caviliHional processes which gen­erate the exci ted specics has ceased [20]. The othe r disad­vantage of the che milumi nescen l system is that it necessitatcs conside rable interference with the liq uid, making it less suilable for usc wi lh . sa). biological syslems [4§5.2.2a(iil].

Biological effects of cavitation may be related to either the sonochemical [2 1] or physical [22] mecha nisms. Cavi­t.ation can produce a great man y efTecLs which have prove n to be so complicated in mechanism as to make them as ye t unsuitable for use in measure men t beyond the most rudi­mentary indication of the type of cavit.aLion occurring. Exa mplesofthese include the bioeffecLSassociated with gas dep lelion when bubbles grow by recl ified dilTusion [23]. microstrea ming [24] a nd with the highspeed translalion of bubbles under radiation forces [25].

As discussed in Pan I, whilst suITicien Ll y energetic cavi­tation may be deteCled through the chemica l effects and damage it wi ll generate, the process of re lating sllch obser­vations to ga in informa tion about the bubble Geld can be difficulL, as the mechan isms by which these effeCls can arise arc complicated (pa rticula rly, as is often the case, when many bubbles ;.:lre in volved ). However the volume changes associated wilh transient cavitation may be coupled La acoustic fields, and provide a mOre read ily interpretable signal.

Detection through acoustic emissions Bubble volume cha nges can cause the e missio n of acoustic waves [26,27]. J n Part I some of the em issions associated with the nonlinear oscillaLions of stable bubbles were dis­cllssed . Transient cavitation G IO also gene rate acoustic e mi ssio n . Dege ne racy d oes ex ist : line spectra and

18

broadband signals, for example, have been detected from both transient and stable cavitat ion. In certain circum­SLa nces some signals prove to be ullsuitable for bubble d etection since it is not a lways possible to determine the type of bubble activit) responsible for its generation.

In 1980 Ncppiras [1] summarised the available experi­melllal data as a progression of emissions. I f a liquid comain ing a bubble population is insonated al low power levels, continuous-wave at the fundamental frequenc), v, the d etected acoustic e missions are at v onl y. At higher inte nsities, but belm\ the threshold level required to gen­erate transient ca\'italion, harmonics are emitted at integer multiples ofv up to high order. The 2v emission is promi­nent, its amplitude being proponionalto the square of the fundam enta l. Low-level broadband continu um noise is present, which becomes vcr)' sLrong as the transient cavi­tation threshold is approached. T he v/2 subharmonic appears interm ittentl)" the duration of the emission being much shoneI' than the 'off-times'. Other subharmonics and ultraharmonics can be detected.

The increases in the broadband continuum , harmonic, ultraharmonic and subharmonic e missions above that encoulll ered in stable cavitation, h;:lve the potential for indicating the onset of violent cavitation. The increase in the sub harmonic prompted some workers in the past to use it as a n indicator of the onset of transient cavitation . However this is not recornrn end cd, as it does not provide a unique and uneq uivocal indicator [3,4]. If conditions allow Lhe assumption that only transient cavitation is oc­CUlTin g, Lhen subharmonic generation may occur through the prolonged expansion phase and delared collapse which can occur during transient cavitation , the bubbles su rviv­in g for one, two or three aco ustic cycles before collapse [2]. Noppiras [I] also suggesls lhaL a form of periodic unslable oscillation of a bubble driven at twice its resonance near threshold , might em it at subharmonic freque ncies. Il ow­ever, as Va ughan and Leeman [5] stated in 1986, " the generation o f fractional harmonics, in panicular the first and third half-harmonic, is a genertl l characterisLic of non ­linear bubble pul s<:ltion and does not speci fica lly indicate lhe occurrence of transient cavitation". The exciuuion of harmonics, subharmonics, and ultraharmonics th rough the non linearity inherent in the pulsation of individual bubbles was described in Part I.

Th ree o th e r theories for the gene ra ti on of the sub harmonic in stablecavit.ation were sUlllmarised in 1980 by Neppiras [I ]. The firsl is para mel ric amplification b) lhe liquid: if propagation through the medium itse lf is non linear. parametric excitation can occur in the absence o f bubbles. T he seco nd is that lhe lower frequency e mis­sions might origin ate from surface waves on lh e bubble wall. In 1969 Neppiras [28] wrole "Allhough il has never been found possible to relate subharmonic signa ls with surface-wave aClivity on the bubbles, surface waves would be expected to contribute to lhe v/2 signals -lhe possibi lity is not ret ruled Olll. ·' The third theory suggests that such emission might be the resu lt o f the acoustic fi eld acting on a population of bubbles containing, either wholly or in part, bubbles with an equilibrium radius lwice the siLe of the radius wh ich would be the pulsation resonance of Lhe acoustic riekl [29]. Sim ilar mechanisms involving progres­sively larger bubbles to account for the lowersubhannon ics were proposed. T his mechanism does nOL explain t.he subharmon ic em issions from single bubbles.

\Vhen one has a poorly-characterised sou nd field aCling upon a n unknown bubble populalion, the fan that a signal may arise by more than one mechanism makes it unsuit­able as a sizin g tool. For a large bubble population in a

Bubble Detection

powerful acollstic field , cOllllllonly some bubbles will be undergoin g stable, and some transie nt, cavilauon . All the above mechanisms may operate. I n addition broad ba nd contributions may a rise. Stable bubbles Illay contribute LO this no ise through the e mission of microbubbles from large-a mplitude surface waves [30], o r to reversion of the oscillations of shocked bubbl es Ollt of the steady-sta le::, so thallhe ir own natural frequencies a ppear in the spcClrulll [ I] . Such 'no ise' would lhe n be d ependenL on the bubble population , and have st ructure. Random frequ encies. unconnected with v, were thouglu to be the result of the shock excitation of large bubbles which would th en osc il ­late at their own natural pulsatio n resona nces [28,31 ]. Broad band contributions from tra nsie nt cavitation are discussed in the next seClion .

Attempts to correla te acoustic with o the r indicators of transie nt cavitation ra nge fro m inves tigations on the re­latio nship between acoustic emissio n and cavita tion ero­sion [32]. to a ttempts to monito r the e ITect o f ultr~l sound o n bio logica l ma terials. Examples of the Ialter include th e explo ita tion of subha rmonic acoustic emissio ns [33J, and addi tionall y broad band acoustic signals [34] , to rela te the acoustic e mission to the bi logical effects and damage induced by rhe ul trasound in cells suspended in vitro . Eastwood a nd \Vatmough [35] measured the sound pro­du ce d wh e n hum a n blood pl as m a was m a d e to sonolumincsce. Atte mpts to correlate the presence o f th e half-ha rmo nic with the onset o f violent cavita tion and sonolumincsccnce are inconclusive [4§4.4. 7]. Expe rime nts have shown that the appearance of t he half-harmonic is not con elated to theonset o f so no luminescent acti vity [36· 39].

The most effective methods [o r acoustic detection of transie nt cavita tion usually employ acou stic signals that ca n be more readily interpreted in te rlllS of cavita tion c.haracterislics. T hese are d iscussed in the followi ng sec­lion.

The acoustic detection of transient cavitation The concept o f a stead y-state resona nt respo nse, as dis­cussed in Part 1, is in appropriate fo r the d ynamics of transient cavitation, the de tection of which foll owing insonation by short, microsecond pulses of ullrasound being of panicular impa rlance. This is because of the clinical releva nce of such pulses, and the need to assess th e likelihood of cavita tio n-induced effects during ex posure. By th e mid 1980's calculatio ns suggested that transient cavita tion could occur in liquids in response to such ultra­sonic pulses [40-44]. Free I"adical production in response to the high temperatures gene rated within the collapsin g bubble had also been indicated fo r microsecond pulses in experime nts [18, 45·48]. Of pa rticular impo rLa nce l'or clinical applications are techniques which a re minimall y invasive bC)lond the efTect cau sed by the primary cavitating bea m. These fa ll into t\\'o broad types: those which d etect the pressure wave e mitted in to the liquid on rebound , and those which ex plo it the enhanced reverberation of the prim ary bea m which the bubbles ca use.

Detection through rebound pressure pulses In 1968 IYesl and IIm,'lell [49] sel up a 20.25 kH z (con­tinuous-wave) standing-wave condition ill a cylindrica l transducer fill ed with degassed tectrachlo roetll yle ne. At certa in times, related to the phase of the continuous-wave fi eld , they nucleated the medium using a pulsed neutron source. T he shock waves emitted by the collapsin g bubbles were used to count the number of cavita tion events (up to

1\ larch 199.5

25 bubbles pe r seco nd , compared to about one per minute when no neutron source was used ).

Th e shon duration of rebound shocks indicates a broad freque ncy co llI.e n t. Negishi [50] f'ound a continuum in the aco u s ti c sp ec t r um occur red wh e n h e d e tected sonolulllinescence. Kuurufr [51 ] confirrned these results by examinin g th e ci rcular shock waves produced by cavi­ties undergoi ng tra nsie nt collapse using schlieren optics.

Roy et al. [52] subjected liquid to a continuous-wave spherically-symmetric sta tionar y acoustic fi e ld ge nerated at 6 1.725 kH z within a spherical resonator. Th e acoustic p ressure amplitude was automaticall y ramped until cavi­tation sufficie nt fo r the d eteClion of sonolumincscence through photoll1ultiplication just OCCUlTed . In additio n the operator could liste n in on the liquid contained within the spherical o l\' ita tion cell via headphones connected LO

a microphone. Aud ible 'clicks' o r ' pops' were ta ke n to indicate tra nsient cavita tion. T hey found th at th e pressure threshold fo r audible sound emission was always less tha n o r equal to that lo r sonoluminescence. T hey deduced that "sound a nd light e mission indicate thresho lds fo r two diffe re nt types of phe nornena associa ted with transie nt cavitatio n", a nd concluded that "if one d esires a thresho ld fo r 'violent' cavita tio n, then sonolul1Iinescence is a fittin g crite rion", a nd that " light emission may scrve as an ideal indicaLOr of whaL Apfel [40] ca lls the 'Lhresho ld fo r tran­sient-violent cavitation '."

The resonalll bubble d elecLO r (RBD) d escribed in ran I has been e mployed in a ttempts 1.0 d etect the 'stabl e' bubble products following shock-induced tra nsie nt cavita­tion downstream from the ex posure site o f a Dornicr Syste m lithOiripte r [53 ]. Extraco rpo rea l shock wave lilhOlrips)' (ESIVL) is a lechnique by which sha n pulses o f high pressure are focused into the bod y in order to bl"eak kidney or gall SLOnes [54]. The 1.65 ~1 Hzco ntinuous-wave RBD was sensiti ve to bubbles ofraciius Ro =2± 0.5 11111 at t he detection site. Th ough bubbles were d etected i ll vit ro in water and blood , and in blood pumped by th e heart thro ugh a plastic ~Inerio- venous shunt, cavit,-Ition was no t detected ill villo in the canine abd ominal ao rta. The RBD effectively detects through the stable oscillations it excitcs in relali vely long-Iasling bubbles.

Since litho tripter cavita tion is characterised by transien t cavitation and rapid changes in bubble size, the sL.,1.ble bubbles being the remna nts from collapse, a more direct rorm of detection would e mploy Ihe e nergetic e missions associated with rapid cha nges in bubble size. Cole ma n et a l. [55, 56] d eleCleci cav ila lion pro du ce d b y a n electrohydraulic lithOLripte r using. as a pass ive re mote hydrophone, a focused bowl lead zirconate tita nate (PZT ) piezoceramic transducer , o f 100 111m diameter a nd 120 Illm focal length in wate r. the focus measuring 5 mm (on axis) by 3 mm wide. This transducer had a I M li z reso­nance: th e de l.ection process would th erefo re correspond to its response to the broad band signal cha racte ristic of the rebound pressure pulse. It is inte res tin g to no te that a skilled operato r ca n, by liste ning to the secondary acoustic e missions durin g clinical lithou'ipsy, determine whether or nOl the shock has hil the slone [57].

Passive acoustic detection through reverberation If an ultrasonic pulse ca uses transie nt cavita tion , the sud­den exte nsive bubble growth associated with Lheeve nllllay c.ause enh anced reverbe ratio n of the ultrasonic pulse, bo th through the presence of the expanded bubbles,and through t he increase in gas bodies present after the collapse (the net volume of gas in the fragme nts may be greate r than that

19

Bubble Detection

cOntained within the seeds owing to the exsolution of previously-dissolved gas during the expansion). Atch ley et al. [58] lIsed this enhanced scattering La Gnd acoustic pressure th resholds for the cavi taLion as ~I fUIlClion of pulse duration t'p and pulse repetiLion frequency vrcp m insonauon frequencies 0[0.98 and 2.30 ~I Hz, lor distilled, degassed, dcionil.cd water. filtered to 0.2 Jllll and seeded with hydro­phobic carboxyl latex parlicles (I JlITI diameter). Similar ex periments [52, 59, 60] used ShOrlLOne bursts of ultra­sOll nd to rind the threshold acoustic pressure am p litude for transient cavitaLion in a fluid which was carefully prepared to remove uncontro lled seed nuclei (cg solid panicles) before the introduction of a known nuclei popu­lation (including AJbunex TN spheres, and hydrophobic polys tyrene spheres of nom inal radius 0.5 J.lm).

Fig 2 shows the output of the passive detector of Roy et <:I I. [61 ] with and without cavitation . The tOP trace (a) illustra tes the primary pulse fro m the 757 kll z transducer , followed by a stable low-amp litude background resulting fi'om multiple-path scallering and reverberation in the f1uid-filled test chamber. Its stability is testament to the stationary nature of the scattering surfaces. I n the lower trace, (b), this scallered background contains a perturba­tion indicative of a time-\'ar ying scatterer, which Roy et a!. showed to be at the focal region of the primal') transducer. Such signals indicate cavitation . The apparatus used to obtain this result in addit ion contained an acti ve detection syste m, and is described in the next section.

Active acoustic detect,ioll A system which is more sensitive to bubbles in the micron size range is the active deteclOr described by Ro) e t "I. [61 ). Subsequent to their production by the pulse from the first trtlnsducer (v=757 kH z; vr("P= I kHz; t l) = 10 J.lS), the cavitation bubbles the n backscatter high frequency pulses (v=30 MH z; t p= 10 J.lS) frolll a second tran sduce r. Roy et a l. [61 ) in fact deployed both active a nd passive acoustic detectors in their syste m , which is illustrated in Fig. 3. The cavitation cell and the electronics associated with the trans-

Possive acoustic delection 1757kHz primary; lMHz receiver}

a ~ , • 0

.. A 11\/\ 1 /l1\~, /1/111 .n III n A A AI III n • .Q

~ I "V II VII ;/II v V 11111 'V IV V v I v • VIV V v "0_ • Time (5~/divl ~~ I, " ..... "> 2::.. I I

b ., % 0

,A A A A 11111 ~IIII\' ,. IA. Q ~ , V V V II IV I V VV 'jV v Q;,

lUI ~~ I' . « . '-"·1 -

~~. ,-

~::.. I \

Fig 2. nil' Olllplit oj 'he pa.Hillf' rlp/erlor wilh (lnd wit liolll cavitation. (a) The primal)) pulse frolllllll' 757 kNzlr{lllSrillcer,[olioU'ed b)' (f stable low-a III/)it! ude backgroulI(/ /'e~III'U/gfrolll IIIIl/ti/Jle-lm/1i .\callering a nd rf1lerbfmtio1l ;11 the chamber. SCll/": J I'/div. (b) The scoltered back­grolllu/ (ontains a /Jl!rtllrbalioll mdiralive oj (l time-vll1)'illg scallerer. Srale J A I 'lei;'). Nole 'he (llf/erma in ve11ical scales bftWPI'11 Ihe two lm(e.~ . • Ifie,. (with jJermiHioll) Ho)' ('I al. (6 J).

22

ducers arc shown in (a), whilst in (b) the fluid manageme nt appara llls (a closed-now system) is illustrated.

From the reservoir , where lhe fluid is degassed lIsing a vacuum pump, it is lhen sequentiall y deioniled, cleared of organics. and filtered down to 0.2 11m. The nuid is then passed into the cell , which can be flu shed of impurities ll sin g flow rates up to 5 li tres/minutc. ACO LI stic stream ing, ge neratin g nows of up to 3 cm/s, could be used LO convect nuclei into the foca l region. The cavitation ce ll is subSl.an­tiall)' simi lar to those e mplo)/ed in the passive detection studies described above. Separated from the tes t chamber by a 9 I.un thick stain less steel acoustic window is a second chamber containing an identical Ouid and an absorber of rho-c n' rubber (so-called as it is impedance matched to water).

This is done to minimise spurious reflections and inhibit the introd uction of standing-waves (the steel window pre­vents contamination o f the test liquid by dirt or sllla ll particles from the rubber). The 757 kHz transd ucer, which generates the cavita tion and is therefore labelled the 'pri­mary', is mou nted on a two-dimensional translating stage toenable its focu s LO coincide with that of the 30 Mill pulse­echo detector. The I MH l unfocussed transducer is cou-

b

CtN.lUhor'l ee!l

Circuloloon

""'"

DetOnrl(l lrOfl olld orgonic removol

,,;'"''''

Fig 3. A s),slem emjlloying bOlh (lctille alul passiveacolulir deLl'Clion.(a) The ravilalion cell and (lssociall'll ell'rtronics. 71le jlrimary bf(l1ll, which causes ((wila/io ll , is gellerated b)' lhe focused 757 kl h lraw,miller. The I Mllz rfceiver is llsed /or j)(Jssive detectioll, and Ihe 30 Mllz lransmilter/receiver lIsed /01' acllve deleclion.(b) The closedflow circlI­lalioll ,))'$Iem /01' cLeausillg aud degassing the sam/)Ie liquid. 71le 'cavitalion all'shown is dellllied ;11 (a). From [-Ij, after (w;lh permis­SlOII) Ro)' e/ at. /6 1j.

Bubble Detection

"'

~,

1--+---+'~IIilI--t--t--+- ~:<x. ~t/clov Ilobn ~oee 1000v/div

,<,

T1mebo"" ll~/d;"'1

Fig -I. nil' act 1111' acoustic delee/iou oj bubbles. (a) The electrical signal which drivfs Ihe 757 kHz primary transducer (.\((I/e: 50 fI/t/iv). (b) Till' signal/rom (Iil' active detertor sy.<;tem ill lhl' absence of (fIvilalion, fhe 1IIaill pulse repreunlillg 'he interrogating 30 M l iz signal (scale: 100 m I'/div). (rJ 711l' sigllal from 'he activl' detector ill the Im'set/a of the ClIvi/ation gel/Prated by the 757 kl-lz lraJlSdltCf'f (scale: 100 m Vldiv). The reflected signal from lhe bubbles is clearly evident in (e). 711e osrilloscopedigilizillg rate was J 00 Alsllmples/s . Afler(wilh permission) Roy ,/ al. (6/ J.

pled with gel to the cell , opposite the 30 M I-I z active delecwr.

Fig.4 shows (a) the electrica l signal which drives the 757 kHz primary transducer, which in LUrn gene rates the GI\itaLion. Trace (b) shows the signal [i·oln the active detecLOr system in the absence of cavitation, the main pulse representing the interrogating 30 M Hz signal. Trace (c) shows the signal from the active detecLOr in the presence ofthecavitation generated by the 757 kHz transducer. The reflected signal from the bubbles is clearly evident.

Holland and Apfel [59] comment theil, when the thresh­olds measured by the aClive and passive systems for vel") similar fluid systems (eg having the same polystyrene sp here and gas concen l rations) were compared, they were found LO be very close, suggesting that the acoustic pres­sure threshold to ca usc one bubble LO go transient in this systcm is thc sa me as that rcquired to make a cloud of bubbles go transienl.

Holla nd et a l. [62] used the active deteClion system alone to investigate in vitro transien t cavitation from shon-p ulse diagnostic ultrasound, both imagin g M-mode and Dop­pler. under cond itions comparable to the clinical situation, with v=2.5 or 5 r..IHz. Cavitation was detected in water seeded with 0.125 ~"" mean radius hydrop hobic polysty­rene spheres at 2.5M ll z, wi th a threshold peak negative pressure of 1.1 ~I Pa. in both ~I-mode a nd Doppler insonations. No cavitation was detected at 5 MH z even at peak negative pressures as high as I . I M Pa . No cavitation was detected in water seeded with Albunex TN at e ither frequency.

Madanshc lty el al. [63] discuss the e ffect of the sensor syste m o n the cavitation threshold for water containing a micropanicie suspension . \·Vith the aClive systcm turned 01T the passive system detected a cavitation threshold at around 15 bar; with the active syste m on the latter detected a threshold atonly 7 bar. This implies that either the active system is more sensiti ve, or that it encourages cavita tion

March 1995

itself, or both. The f~ICl thaI. the passive system also detccts a rcduClion in thres hold (to around 8 bar) suggests that bOlh might bc true, and that the GI\'it3tional erTect of the active system might be considerable. To investigate this Madanshett),ctal. [63] reduced the peak negative pressure from the active transducer to onl y 0.5 bar, so that its in fluence on transient cavitation shou ld be negligible, and found that the aclivc detector a lone at th is inlensit)' could not generate cavitation events. They found that the active detector could elTect c~lvitati(}n through the streaming it develops (which is promoted by its high frequency and focused nature). This flow COlwects nuclei into the cavita­tion lone. A second mechanism by which lhe active detec­tor can anect cavitaLion is through the accelerations it imparts to particles. ·Ihe combination of these accelerations with thedcnsit),conl.rastgenerates kinetic buoyancy forces. These cause liny gas pockets on the surface of solid par­ticles to aggregate into gas patches ofa size su itable to act as nuclei for cavitation. Another possibilit) suggested by Madanshclty et al. [63] is that potential nuclei. which would normally be driven b) the 0.75 MI17 transducer, would be det::.ined in the region where the fields are strongest by cross-streaming with the flo w generated by the active transducer, so enh ancin g the likelihood of a cavitation event at lower insonating pressures. Citing simu­lation results of Church [64), Madanshetty et a l. [63) rule out the possibility of the active detector influencing cavi­tation through rectified difTusion.

In comparison with the passive detector, the active detector seems to be more sensitive . may affect caviullion itself, and is sensitive predominantly in its focal region , so may more readily be used to give a degree of spatial information. On the o ther hand, the passive detector is sensitive to a larger region of space, and rernains continu · ally alen for cavitation: for the active detector to operate its interrogating pulse must arrive at its focus at I he instant when the transient bubbles are present. There are impli­ca tions regarding the strain placed o n the user of these systems inherent in the nature and display of the detected signals [63).

Conclusions Transient cavitation can produce a number of elTects which may be used to detect and, to some extent , qualllify it. Chemic~d , biological, and erosive elTects may be diOicult to interpret in order to obtain information about the cavitationa l fi e ld and bubble dynamics. Acoustic e missio ns, based on lhe rebound shock, and the excess scalLering by the cavitation bubbles either of the field thai induced the growth, or of a second field app lied specificall) for detec­tion purposes, may be particularly useful in detecting the transient growth and collapse phases. Ilo\\'e\er it shou ld be noted that transient cavitation often is seeded by a pre­existing nucieus, which ma)" ilSelfbc Slable [4§2. 1; 65, 66). After th e eve nt there wi ll be gas bod}' remnants, which again might be stabilised or not. Fragmentation wi ll pro­duce daughters, increasing the number of such bodies. Therefore the detection of stable bubbles. as discussed in Part I, lTIay be employed to characterise the origins and products of transient cavitalion.

Acknowledgements The author wishes to thank NERC (51 DAL 00670) '\!ld SERe for funding. This article conta ins material from The Acoustic Bubble (T G Leighton), Academic Press, London, 1994.

Referenc~ on pagE 25

Bubble Detection

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NOISE CONTROL - Sourcebook 9

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