In or der to have cavitation it would be necessary

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Inordertohavecavitation,itwould benecessaryonlytoovercomethesurfacetensionforcesofthesenucleiratherthantheintermolecular
(8bondingforcesoftheliquid.Thestrengthofanucleusdependsverymuchonitssize.Asaresult,thegascontentoftheliquidisanimportantvariableinthestudyofacousticcavitation.Otherparametersincludehydrostaticpressure,frequency,temperature,viscosityandotherliquidproperties.GaseousCavitationWithwaterandusingafocussingsystemat60kcps,Blake(7)hasshownthattherearethreedistincttypesofcavitation.Atverylowpressures,largevisiblebubbles,whichdonotproducesoundareformedingassywater(watermoreorlesssaturatedwithgas).Thisistermedquietdegassing.Athigherpressures,a moreorlesscontinuousstreamofsmallergasbubblesisobserved.Thesestablebubblesareoriginatedatthefocusofthesoundfieldandtheyareaccompanied by asofthissingsound.Blakehasobservedthatthereisacriticalpressure,orthreshold,fortheformationofthesestreamersandthattherateofformationisverysensitivetochangesinthesoundpressureamplitude.Thissecondtypeofcavitationiscalledgaseouscavitation.Likethequietdegassing,gaseouscavitationoulyoccursinwaterthatisalmostsaturatedwithgas.Fora soundfieldof60kcps,Blakereporteda soundpressureamplitudeofabout0.25atm.tobethethresholdvalueforquietdegassingandabout1.25atm.forgaseouscavitation.The boundary wherethetwotypesofcavitationare(8910)dividedisnottooweIldefined.Manyinvestigators"havecom-binedthetwo and termed themasgaseouscavitation.
Asmentionedabove,cavitationnueleiaresmallair-orvapor-filledcavitiesstabilizedincracksonthesurfaceofsmallpar-ticlessuspendedinliquid.Onthebasisofamicroscopieexaminationofwater,RoSenberg(ll)reportedthattheseparticlesaremainlydust9particlesrangingfromO.~to10;Uindiameter.Itappearsthatsomemechanism mustexistbywhichanucleuscanincreaseitsaircontentandgrowtoalargevisiblebubblewhenitissituatedinanacousticpres-surefield.InarecentstudybyBarger(8)ontheeffectofsuspendedparticlesizeoncavitationthreshold,thewaterusedwasfilteredthrougha0.45~filter.Itwaa assumedthatthediameterofthepar-ticlesremaininginthewatermusthavebeenlessthanabout0.5~andthattheradiusofthesmallbubbleathatweredrawn fromthecrackswaslessthanabout0.2~.Witha soundpressureof0.45atm.andafre-quencyof27kcps,theresultingcavitatingbubbleshada meanradiusofabout40~.AtheorycalledrectifieddiffusionwasfirstproposedbyBlake(7)andlaterelaboratedbyHsiehandPlesset(12)toexplainthegrowthofacavitybyanetinfluxofgasasitoscillatesintheacous-ticpressurefield.Accordingtothistheory,asmallbubblefilledwithgasisassumedtopulsateinphasewiththepressureamplitudeoftheacousticfield.Forthisphaserelationshiptooccur,thebubble

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