Minimising microbubble size through oscillation frequency control

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control

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Brittle, S., Desai, P. orcid.org/0000-0001-5266-0359, Ng, W.C. et al. (4 more authors)

(2015) Minimising microbubble size through oscillation frequency control. Chemical

Engineering Research and Design, 104. pp. 357-366. ISSN 0263-8762

https://doi.org/10.1016/j.cherd.2015.08.002

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chemicalengineeringresearchanddesign 104 (2015)357–366

ContentslistsavailableatScienceDirect

Chemical

Engineering

Research

and

Design

jo u r n al ho m e p a g e :w w w . e l s e v i e r . c o m / l o c a t e / c h e r d

Minimising

microbubble

size

through

oscillation

frequency

control

Stuart

Brittle

a,∗

,

Pratik

Desai

a

,

Woon

Choon

Ng

b

,

Alan

Dunbar

a

,

Robert

Howell

b

_,

_Vaclav

_Tesaˇr

c

_,

_William

_B.

_Zimmerman

a

a_Department_of_Chemical_&_Biological_Engineering,_University_of_Sheffield,_Sheffield,_United_Kingdom

b_Department_of_Mechanical_Engineering,_University_of_Sheffield,_Sheffield,_United_Kingdom

c_Institute_of_{Thermomechanics}_AS_CR,_v.v.i.,_Czech_Academy_of_Sciences,_Prague,_Prague,_Czech_Republic

a

r

t

i

c

l

e

i

n

f

o

Articlehistory: Received15April2015

Receivedinrevisedform10July2015 Accepted2August2015

Availableonline8August2015

Keywords: Microbubbles

Processintensification Transportphenomena Systemdesign Fluidicoscillator Processoptimisation

a

b

s

t

r

a

c

t

Microbubblesarebubblesbelow1mminsizeandhavebeenextensivelydeployedin indus-trialsettingstoimprovegaseousexchangebetweengasandliquidphases.Thehighsurface tovolumeratioofferedbymicrobubblesenablesthemtoenhancetransportphenomena and thereforecanbe usedto reduce energydemands inmany applications including, wastewateraeration,frothflotation,oilemulsionseparationsandevaporationdynamics. Microbubblescanbeproducedbypassingagasstreamthroughamicro-porousdiffuser placedatthegas–liquidinterface.Previousworkhasshownthatoscillatingthisgassteam canreducethebubblesizeandthereforeincreaseenergysavings.Inthisworkweshowthat itispossibletofurtherreducemicrobubblesize(andconsequentlymaximisethenumber ofbubbles)byvaryingthefrequencyoftheoscillatinggassupply.Threedifferent microbub-blegenerationsystemshavebeeninvestigated;anacousticoscillationsystemandamesh membrane,afluidicoscillatorcoupledtoasingleorificemembraneandafluidicoscillator coupledtoacommerciallyavailableceramicdiffuser.Inallthreebubblegeneration meth-odsthereisanoptimumoscillationfrequencyatwhichthebubblesizeisminimisedand thenumberofmicrobubblesmaximised.Insomecasesareductioninbubblesizeofup to73%wasachievedcomparedwithnon-optimaloperatingfrequencies.Thefrequencyat whichthisoptimumoccursisdependentonthebubblegenerationsystem;more specifi-callythegeometryofthesystem,thetypemicro-porousdiffuserandthegasflowrate.This workprovesthatbytuningindustrialmicrobubblegeneratorstotheiroptimaloscillation frequencywillresultinareductionofmicrobubblesizeandincreasetheirnumberdensity. Thiswillfurtherimprovegaseousexchangeratesandthereforeimprovetheefficiencyof theindustrialprocesseswheretheyarebeingemployedtoproducebubbles,leadingtoa reductioninassociatedenergycostsandanincreaseintheoveralleconomicandenergetic feasibilityoftheseprocesses.

1. Introduction

Bubbling systems have regularly been employed in indus-trialprocessesinordertoachievegaseousexchangeofboth massandheatfromgaseousphasestotheliquidphaseand

∗ _{Corresponding}_author_._Tel.:₊₄₄_7888705187. E-mailaddress:s.brittle@sheffield.ac.uk(S.Brittle).

viceversa.Morerecentlymicrobubbleshavebeenshownto improvetheefficiencyofthesegaseousexchangeprocesses duetotheirhighersurfaceareatovolumeratio(Zimmerman

etal.,2011a,b).Microbubblescanbegeneratedbypassinggas

throughamicroporousdiffuseratthegas–liquidinterfaceor

http://dx.doi.org/10.1016/j.cherd.2015.08.002

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through other methods discussed in Zimmerman et al.

(2011a).Ithasbeenshownthatthegaspressure(andtherefore

energyinput)requiredtodothiscanbesignificantlyreduced ifanoscillationisappliedtotheflowinggasstreampriorto passingthroughthediffuser.Previousstudiesusingoscillatory flowhaveshownimprovementintopicssuchas

microflota-tion (Hanotu et al., 2012), algal growth (Ying et al., 2013),

wastewateraerationandtreatment(Rehmanetal.,2015)and oil-emulsionseparations(Hanotuetal.,2013).Hanotuetal., usedoscillatedandnon-oscillatedairwhichshoweda signif-icantsizereductionusingoscillatedair.Thisstudyreported reductioninbubblesizefrom1059␮m,usingasteadyairflow system, to84␮m, with anoscillated airmechanism, using adiffuserwithanaverageporesizeof38␮m(Hanotuetal.,

2012).

Surfaceareatovolumeratiohasbeenlongunderstoodto beextremelyrelevantinprocessesinvolvingheatandmass transfer(Birdetal.,2007).Thehighertheratio,thebetterthe performanceofthesystem.Iftheradiusofabubbleishalved bubblevolumewillbereduced to1/8itsoriginalvalueand thesurfaceareareducedto1/4itsoriginalvalue.Therefore thetransfercoefficientswhichareproportionaltothesurface areatovolumeratiowillbeincreasedbyafactorof2.Therefore ifbubblesizesarereduced,inturntheprocessefficiencyis improvedduetobetterheatormasstransfer(Zimmerman

etal.,2008).

Microbubbles provide unique opportunities due totheir abilitytobe manipulated photo-acousticallytherefore pro-vidingmanoeuvrability(Ashkin,1997;LauterbornandKurz, 2010), lower rise velocity meaning greater residence time

(Zimmerman et al., 2013) and have their ability to be

used as sensors (Darveau, 2011). Small (<8␮m) microbub-bles have other potential applications in medicine such as theranostics (Liu et al., 2006). The reduced buoyancy and size of microbubbles <8␮m means that they will not cause blockages in capillaries associated with larger bub-bles.For mostofthese applications it isdesirable to have a narrow size distribution. For example when applying photo-acoustic tweezing the microbubble manoeuvrability is size dependent, so having a narrow size distribution is hugely beneficial. For medical applications if a wider sizedistributionisgeneratedthe bubblesmustbe differen-tially centrifuged to select the desired size. This adds an additional process thereby increasing costs (Brodkey, 2004;

Feshitanet al.,2009).Therefore thereisconsiderable

inter-est in being able to control and therefore reduce the size ofmicrobubbles.Ingeneralbeingable toprovideanarrow distribution of very small bubbles will result in increased efficiencyand economicsofthe variousprocesses thatuse microbubbles.

Thefirstofthemicrobubblegeneratingmethods investi-gatedhereusesanacousticspeakertooscillatetheairstream beforeit flows through the diffuser.With this system it is veryeasytoexploreawiderangeofoscillationfrequencies asthefrequencyisdefinedbythewaveformplayedthrough thespeaker.Thetwoothermicrobubblegenerationsystems usemicrofluidicdevicesknownasaTesar–Zimmerman flui-dicoscillator(Jilek,2013;Tesaˇr,2012;TesaˇrandBandalusena,

2011;Zimmermanetal.,2011a,b,2010)togeneratethe

oscil-lation before the gas stream passes through two different diffusers,onewithasingleorificeand anotherwith multi-pleorifices(mesoporousdiffusers).Thelatterismosttypical of the large scale microbubble generators being used in industry.

TheTesar–Zimmermanfluidicoscillatorisamicrofluidic devicewithnomovingpartsthatcreatesadynamicjetwhich alternatesbetweentwoexitportsatafrequencydetermined bythe feedback characteristicsofthe oscillator.This oscil-latory flowisgenerateddue totheadherence ofthe jetto one wall,caused bytheCoandaeffect, and its subsequent detachment and adherence to the opposite wall due to a switchovercausedbypressurechangesinthefeedbackloop. Thegasstreamfromeitherorbothoftheexitscanbepassed through porous diffusers in order to engender microbub-bles in aneconomicalfashion. In this work acontrol loop has been usedso thatthe operating frequencyofthe flui-dicoscillatorcanbealteredusingthistechnique,whichhas beenadequatelydescribedinTesaˇrandBandalusena(2011). Clearlytheorificesizeinthemicroporousdiffuserwillplay asignificantroleindeterminingthesizeofthebubbles pro-duced(Cliftet al., 1978). However,reducing theorifice size increases the pressure required to push the gas through thediffuserandthereforeincreasestheenergyrequirements ofthe system.Thisshowsthat minimisingthe bubblesize fora fixeddiffusergeometryisofindustrialrelevance and highlybeneficialifthisresultsatnoadditionalexpenditurein energetics.

Themotivationofthisworkwastoinvestigatehow bub-ble size variesas afunction ofoscillationfrequency. From previous work (Zimmerman etal., 2011a,b,2010,2008)itis clearthatapplyinganoscillationtothegasstreamcanhelp reducebubble sizeand thiswasattributedtoanincreased rate ofbubble ‘pinch off’ dueto the oscillation. Three dif-ferent microbubble generators are studied, exploring the size of the microbubbles generated as a function of the oscillation frequencywhenproducing airbubbles inwater. Exploringthisrelationshipdeepenstheunderstandingofhow tocontrolmicrobubbleproductionandthereforeenablesthe requiredbubblesizesfortheirvariousindustrialapplications tobeeasilytargetedeconomically.Thisworkinvestigatesif improvementispossibleusingthreebubblegeneration sys-temsallutilisingoscillatedgasflowstreams.Itisimportant to know how the frequency of the oscillating gas stream affects thesizeofthebubbles producedinorder tofurther increase the impact ofthe oscillator on energy cost. This work investigates how frequencycontrolaffects microbub-ble generation using three different bubble generation systems.

2. Experimental

methods

Threetechniquestocreatebubblesusinganoscillating air-flowareusedinthisstudy.Theoscillationmechanismand thediffusertypei.e.mesh,singleorificemembraneor multi-orificediffuser(attheairwaterinterface)arevariedinthese 3methods.

I. Anacousticoscillationsystemandametalmesh mem-brane.

II. Thefluidicoscillatorcoupledtoasingleorificemembrane viaabespokevisualisationrig.

III. Thefluidicoscillatorandacommerciallyavailablediffuser withanaverageporesizeof20␮m.

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chemicalengineeringresearchanddesign 104 (2015) 357–366

359

Fig.1–Imageacquisitionsetup.

2.1. Imagingapparatus

Theimagingtechniquesusedineachofthethreesetupsis thesame.Thecamera,objectandlightsourceareplacedin anorientationsimilartothatdepictedinFig.1.

ThelightisgeneratedbyaThorLabsWhiteLEDArraylight source(LIU004)withanintensityof1700␮W/cm2andemitted atapeakof450nm.ThebrightLEDlightsourceisdiffused intoamoreuniformlightusingawhiteplastictranslucent opticaldiffuserlayer,beforeenteringthebubblevisualisation tankwherethebubblesareproducedandimaged.Thetank wasspecificallydesignedforusewiththehigh-speed cam-era,usingatransparentquartzglass.Thebubblesareimaged usingaPixelinkPL742camerainsetupIwithanadjustable magnificationlens.InsetupsII&IIIaPhotronFastCamSA3 wasusedwithaNikonAFNikkor(24–85mm1:2.8–4D)lens.

The bubbles were imaged and sized for each of the threemethodsatvariousoscillationfrequencies.Thebubbles imagedforeachofthethreemethods(I–III)varybothinthe numberofbubblesproducedandthesizeandshapedueto thenatureofthemesh,membraneordiffuserandtheextent

ofmagnification.Examplesoftheimagesproducedusingthe threemethodsaregiveninFig.2.

2.2. Imageanalysis

The imagesof the illuminated bubbles were captured and theseimagesanalysedusingbespokeimageanalysissoftware, inordertodeterminethemeanaveragebubblesizeusingthe equationshown:

D[1,0]=

n

1D

n

whereDisthediameterofanindividualbubbleandnisthe totalnumberofbubbles.Thepixelsizeforeach experimen-talsetupwascalibratedbyimaginganobjectofknownsize. TheD[1,0]methodofcalculatingaveragebubblediameteris chosenforsimplicity,andtoquicklyrepresentanychangesin bubblediameterthatmayoccurasaresultofsystem param-eteralterations.

Thebubbledensityinaliquidisanimportantparameter because variousapplicationsrequire differentbubble sizes. For examplebiomedicalimagingapplicationswillrequirea different bubble size to that of microflotation. This paper addressesageneraloptimisationforbubblegenerationunder oscillatoryflow.Comparisonstobubblesizewithandwithout oscillated flowhavebeendemonstrated previously(Hanotu

etal.,2012;Zimmermanetal.,2011a,b),alongwithassociated

bubble densityandsizedistribution. Itisalsoimportantto notethatnosurfactantwasusedinthisstudyasonly gener-atedbubblesareofinterest,notthoseretained.

TheceramicdiffuserusedinsetupIIIhasaporousstructure atwhichbubblesarecreatedandrequiresarelativelylargeair pressure.Theresultisthatthediffuserformslargecloudsof bubbles,similartothosereportedbyHanotuetal.(2012).The fluidic oscillator—singleorifice system requires alower air flowrateandproducesfewerbubbles,oneatatime,througha singleorificeandconsequentlythismethodproducesamuch

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Fig.3–Thethreeporousmediausedinthisstudy.Left,thenickelmesh.Centre,thesingleorifice.Right,theporousceramic diffuserimagedusingSEM.

lower number density of bubbles. The acoustic oscillation withmeshsystemisalsoarelativelylowairflowratesystem comparedwiththatoftheceramicdiffusersystemduetothe reduced numberofpores and producesconsiderably fewer bubbles asa result. However,the mesh isable to produce morethanonebubbleatatime,duetothenumerouspaths for air to flow through its grating and therefore produces an intermediate number density of bubbles compared to the other two setups. Images, acquired through differing techniques,oftheseporousmediacanbeseeninFig.3.

Sincethebubbledensityvariesbetweenthethreemethods itisalsonecessarytotakedifferentnumbersofimagestogain anappropriatelysimilarnumberofbubblestoanalyseforthe meanbubblediametercalculation.Thedegreeof magnifica-tionvariesfromoneset-uptothenextandthereforereference imageshavebeenusedtocorrectlysizebubblesfromthese images.

2.3. Bubblegenerationtechniques

2.3.1. Anacousticoscillationsystemandmesh

The acousticoscillation system is a bespoke device made toextensivelystudyawiderangeofoscillationfrequencies. The air oscillation is produced using an acoustic speaker and therefore the frequency ofoutput is easilycontrolled. Alarge rangeoffrequencieswith smallinterval step sizes can be studied easily compared to the intervals available whenusingtheinherentlymechanicalfluidicoscillator sys-temsdescribedbelow.Avirtualsignalgeneratorwasusedto producewaveformsusingbespokesoftwaredevelopedin Lab-View.Thewaveformswereusedtodrivetheacousticspeaker using a standard amplifier controlled by the computer’s soundcard.Variationinthefrequencyandwaveformshape couldthereforebeeasilycontrolledwithinthesoftware.The saw-toothwavewasobservedtoaidthebubblepinchoffbest,

ascomparedtosinusoidal,squareandtriangularwaveforms. ThesystemsetupisshowninFig.4.

Thecompressedairsupplypassesthroughacontrolvalve followed byapressureregulatortoensureaconstant pres-sure. Theflowcontroller (Bronkhurst,EL-FLOW),positioned beforetheacousticbubblegenerator(ABG),maintainsasteady influxofairintotheABGTheflowrateiskeptconstant dur-ing frequencysweepsoftheABGinordertoisolateeffects relating tofrequencyon thebubblesize.Furtherfrequency sweepsareperformedwhentheflowrateischanged.TheABG superimposesanoscillationontotheflowingairstreamviaa signalgeneratorandthenfunnelsthepulsedairtowardsthe meshmembraneattheairwaterinterface.Themeshused inthisexperimentisnickelwitha200grating,giving spac-ingofapproximately130␮m.Thedepthofthewatercolumn abovethemeshatthemesh/air/waterintersection,iskept suf-ficientlylowsuchthatthewaterpressuredoesnotovercome the surface tension insidethe porous mesh and therefore inhibitsthebackflowofwaterintotheABG,evenwithoutany airflow.

Themeanbubblesizeisplottedagainstthefrequencyof theoscillation(pulsedair),inFig.5fortheAcoustic Oscilla-tionandMeshsystem,inFig.7forthefluidicoscillator—single orificemethodandinFig.9forthefluidicoscillator—diffuser method.Thebubblessizeshavebeennormalised,withrespect tothelargestbubblesinadataset,simplytoallowfor eas-iercomparisonbetweenresults.Thenormalisationfactorsare giveninthefigurecaptions.

Theresultsfortheacousticoscillationandmeshsystem areshowninFig.5.EachgraphinFig.5representsadifferent type of mesh at the air water interface of the system or flow rate. All the plots illustratethat there isa distinctive minimum in each of the data sets, representing a ‘sweet spot’ofspeakeroscillationthatcreatesthesmallestaverage diameterbubbles(D[1,0]).ThedifferencebetweenMesh1and

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Fig.5–Datatakenusingtheacousticoscillation–meshtechnique.Mesh1withaflowrateof5ml/min(a),mesh1witha flowrateof25ml/min(b),mesh2withaflowrateof5ml/min(c),andmesh2withaflowrateof10ml/min(d).861␮m(a), 3190␮m(b),1141␮m(c)and2104␮m(d)arethenormalisingfactors.

Mesh2isthegratingsize.Mesh1hasa200gratingwhereas Mesh2is250,thusslightlyreducingtheporesizeinMesh2 butincreasingthenumberdensityofpores.

Itisobservedthatasflowrateincreasesinthissystem,the frequencythatresultsintheminimumbubblediameter,also increases.ThisisdeducedbycomparingFig.5(a)withFig.5(b), whereanincreaseinflowratefrom 5ml/minto25ml/min increasestheoptimumfrequencyfrom110Hzto180Hz.And alsobycomparingFig.5(c)withFig.5(d),whereanincreasein flowratefrom5ml/minto10ml/minincreasestheoptimum frequencyfrom250Hzto300Hz.Themaximumreductionin averagebubblediameterusingthisexperimentalmethodis ∼73%.

2.3.2. Fluidicoscillator—Singleorificemethod

Thesetupforthefluidicoscillator—singleorificemethodis showninFig.6.Theinletairflowentersthesystemthrougha shutoffvalve.Thepressureandglobalflowratearecontrolled byapressureregulatorandrotameterrespectively(Norgren, OmegaInstruments).Therotametermodulatesinletflowrate, preparingthe airflowpriortoentering thefluidicoscillator. Thefluidicoscillatorcreatespulsedairflowatboththeoutlets

(Tesaˇr,2007).Oneoutletisconnectedthroughasingleorifice

membraneatthebaseofthebubblevisualisationtankand theotherisventedtoatmospherethroughableedvalve(inan industrialsetting,thesecondoutletwouldfeedasecond bub-blegeneratingdiffuseroranarrayofdiffusers).Thiswillwork aslongasbothofthemare‘balanced’together.Itisimperative thatbothoutletsaresuitablybalancedtomaintainthe oscil-lationforthisno-movingpartfluidicswitchingdevicesinceit isabistablevalveandthereforebistabilityneedstobe main-tained.Thereisasecondbleedvalveonabranchoftheoutlet

beingfedtothesingleorificemembranewhichisusedto con-trolthetotalamountofgasbeingfedtothemembrane.Thisis necessarytoallowappropriateflowintothebubble visualisa-tiontankwhilstsimultaneouslyallowingthefluidicoscillator tooscillateattherequiredfrequencies.Apressuretransducer ImpressG1000(Range0–1bar(g))isfittedinthesupplylineto thediffuserinordertoaccuratelymeasurethefrequencyofair ofthesystem.Theinputflowrateusedasstandardthrough thefluidicoscillatoris65l/minwithover99.9%beingventedin ordertobeusedwiththesingleorificemembraneandtheinlet pressureismaintainedat0.5bar(g).Theactualflowrate enter-ingbubblevisualisationtankvariesslightlydependingonthe fluidicoscillatorfrequencyduetochangesinthefeedbackloop andrangesbetween0.2ml/mand0.5ml/m.Thefrequencyof thefluidicoscillatorisdependsupontheinputflowrateand thelengthofthefeedbackloop.Sincetheinputflowrateand thesystemiskeptconstant,thefeedbacklooplengthisused asthe parameterdeterminingthe frequencyofthe oscilla-tor.Themembraneusedinthisstudyisasingle30␮morifice membraneprocuredfromPotomacPhotonicsInc.

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Fig.6–Setupforthefluidicoscillator—singleorificemethod.

associated with bubble formation and dynamics using a reduced bubble sample density compared to cloud bubble generationassociatedwithothertechniques,yetstillallowing thevalidityofthehypothesistobeexplored.Inthiscase find-ingthe dependencyofbubblesizeon oscillationfrequency oughttobeeasilymeasurableusingthistechnique.Thelower bubbledensityalsosimplifiestheautomatedimageanalysis greatlysincefewbubblesoverlap,imagesareclearerasthe intendedfocalplaneismorepredictableandtherearefewer bubble–bubbleinteractions.

2.3.3. Fluidicoscillator—Applicationofthefluidic oscillatoronamesoporousdiffuser

The methodology for creating pulsed air in this setup is identicaltothatdescribedinPartIIfluidicoscillator—single orifice method above. However,different setupparameters areneededanddifferentfrequenciesaregeneratedbecause aceramicdiffuserisusedattheairwaterinterface (rather thanthesingleorificemembrane)andthereforethesystem dynamicshavechanged.

Theairtakesthesameroutetothefluidicoscillator,asthat describedabovebutuponexitingtheoscillatorbothoutlets arefedtoseparatediffusers.Duetothenatureoftheporous diffusermaterialthepressure requiredtopush airthrough itsporesishigher(approximately1bar(g)).Alargernumber ofbubblesareformedduetothescaledupprocess—(when comparedtoasingleorifice). Thisprovidesalargersample sizeforexperimentalsignificance.

The bleedvalves performthe same function asfor the fluidicoscillator—singleorifice methodwhichistobalance theoutletlegsoftheoscillator(itneedstoperformbi-stably) whilstfeedinganappropriateamountofflowintothediffuser. Thediffusersareplaceddirectlyinabubblevisualisationtank andthereforeitisnecessarytomeasurethefrequencyatone oftheoscillatoroutletsusinganaccelerometer(ADXL345)and calibratedusinganImpressG1000PressureTransducer.Four flow rates (0.1l/min, 0.15l/min,0.2l/min & 0.35l/min) were used with frequencysweeps. Both the diffusers were kept undersimilarconditionsandwerematchedforperformance andfoundtobeequi-responsivewithlessthan5%variation inperformance(Fig.8).

Fig.9showsthegraphsofthedatatakenusingthefluidic oscillator appliedonamesoporousdiffuseratvariousinlet flowrates.Againeachofthegraphsshowadistinctive min-imumaveragebubblesizethroughoutthefrequencysweep. Themeanbubblesizeminimumforthissystemappearsat muchloweroscillationfrequencies,comparedtothosetaken withthefluidicoscillator—singleorificemethod.Thiseffect is due tothe difference in the set up ofthe system, such astheChambrevolumeaswell asthechange indynamics i.e.singlebubblegenerationandbubblecloudformation.The ceramicdiffusersusedhereinpresentadifferentscenarioto thebubblesformedthroughthesingleorifice.Thepressureof thesystemishigher,thevolumeofthediffuserislargerand thesefactorsaffectbubbleformation.Fromthedatashownin

Fig.9wecanobservethatanincreasinginletflowratealso

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363

Fig.8– Setupforthefluidicoscillator—diffusermethod.

increasestheaveragebubblediameter.Occurringat122Hz, 124Hz,124Hzand125Hz,forflowrates0.1l/min,0.15l/min, 0.2l/minand0.35l/min,respectively.Theminimumaverage bubblediameter(D[1,0])hasbeenshowntoreducebyupto ∼47%forthisparticularexperimentalmethod.Allofthedata plottedinFig.9illustrateahighdependencyofaveragebubble sizeonoscillationfrequencyandthereforetheimportanceof knowingthewherethisoptimumfrequencyforminimising bubblesizeishighlighted.

Wehavebeenabletoshowthatforeachofthethree bub-blesystemsthereisanoptimumfrequencyatwhichthemean bubblesizeisminimised.Thisissignificantbecausecurrent

practicewhenemployingmicrobubblestoimprovethe effi-ciencyofgaseousexchangeinindustrialprocessesissimply touseanon-optimisedfluidicoscillatoroperatingatadefault frequency.Allsystemshaveusedvariousflowratesettings. Theuseofthesespecificflowratesisdependentonthe ori-ficeareaorthroughputofthebubblegenerators.Thisstudy showsthatthesetypeofsystemhavetheabilitytobetuned inordertoreducethemeanmicrobubblesize,byupto∼73%. Thereductioninbubblesizeuponoptimisingthefrequency wasmostpronouncedfortheacousticsystemrelativetothe bubblesizeformedi.e.percentagereduction.Thisisattributed tothemuchfinercontrolofspeakerfrequencythatispossible

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Fig.10–Thebubblenumberdensityisdisplayedalongsidethebubblediameterasafunctionoffrequency.Theacoustic oscillation–meshmembranetechnique(I)usingmesh2andaflowrateof25ml/min,thefluidicoscillator—singleorifice method(II)using2.3ml/minflowrate,andthefluidicoscillator—ceramicdiffusermethod(III)at0.15ml/min.

inthissetup.Itimpliesthatbyfurtheroptimisingthelength ofthe feedbackloopsused,intheother twofluidic oscilla-torsetups,greaterreductionsinbubblesizecouldberealised theretoo.

Fig. 10 is used as an example of bubble number den-sityfigures.Itshowsanexampleforeachmethodandplots both bubble size and densities with respect to frequency.

Fig.10clearlyshowsthattheoptimumfrequencyfor produc-ingsmallerbubblesalsocorrelatestoanincreaseinnumber ofbubbles.Thisisexpectedbecausethe flowrate, ortotal gasthroughput,remainsconstantyettheaveragebubblesize decreasesandlargernumbersofbubblesareproduced.

3. Discussion

Bubblesareformedbypassinggasthroughasmallorificeinto aliquid,whensufficientgashasenteredthebubbletoforce ittopinchofffromtheorifice.Bubblepinchoffandbubble generation underoscillatory flow hasbeen elucidated well inpublicationsbyTesaˇr(2014,2013b,2013c)andZimmerman

etal.(2011a,b).Thisisdifferentfromthebubblegeneration

viathirdharmonicexcitationexplainedbyTesaˇr(2013a)but thegeneralreasoningremainsthesame.Inordertoproduce verysmallbubblesitisbeneficialtoforcebubble‘pinchoff’to occurasfrequentlyaspossible.Whetherornotbubble‘pinch off’occursisdeterminedbyseveralcompetingfactors.The buoyancyofthe bubbleand themomentumofthegas fill-ingthebubblewillencourageittopinchoffwhilstthesurface tensionoftheair–waterinterfacewillhinderpinchoff,instead

promotingthebubbletogrowlargerminimisingthesurface curvature.Eventuallyasthebubblesizeincreasesthe curva-tureoftheinterfacewillchangefromadomeshapetoamore spherical shape. Thesphere willcontinue to grow becom-inglargerthantheorificeatwhichpointthecurvaturenear the orificewillbeginbendingoutwardsand eventuallywill becomesoextremepinchoffoccurs.Clearlytheflowrateof gaspassing throughthe orifice willdeterminethe amount ofgasenteringtheformingbubbleanditsvelocity.Pinchoff willoccurwhenthebuoyancyforcescanovercomethe sur-facewettingforce,whichactstohold backtheflowofgas. Thewettingcharacteristicsdeterminedbythe hydrophobic-ityoftheorificematerialwilldeterminewhetherornotthe surfacewettingforceisminimisedbylateralspreadingofthe forming bubble. Hydrophilic surfaceswill resultin smaller bubblesbeingproducedbecausethewaterwillpreferentially sitnexttotheorificematerialforcingthebubbletoadopta smallerradiusofcurvaturethereforeencouragingearlypinch

off(KukizakiandWada,2008).

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365

sufficienttocreateabubblethenpinchoffmayoccurduring everypulse.Theliftingforceassociatedwiththemomentum ofthegasdeliveredduringsuchapulsewilladdtothe buoy-ancyforceandthereforepermitsmallerbubblesthanwould otherwisebepermittedtobeformed.Thecombinationofthe bubblebuoyancyandgasmomentumcanovercomethe sur-facetensioneffects.Frequenciesjustabovetheoptimumwill deliversmallermomentumcontributionsperpulseand there-fore resultsin increasing bubble sizes above the optimum frequency.Atevenhigherfrequenciesabovethesweetspot theincreasedfrequencymeansthateachpulsecontainslower amountsorairandlessmomentumisdeliveredtothrustthe bubbleawayfromthemembraneandovercomethesurface tension.Thus,thesmallerbubblefailstodetachandcanonly dosowhenfurtherpulsesofairenterthebubble,increasing thebubblevolumeanditsbuoyancyforcebeforedetaching. Frequenciesjustbelowtheoptimumwilldelivermorethan enoughgasperpulseforpinchofftooccurandasaresult thebubbleswillgrowthroughoutthepulseresultinginlarger sizedbubbles.

An alternative way to think of the oscillatory effect is interms of pressure fluctuations.During the on pulse, air pressure is high and air willflow into the forming bubble pushingoutwardsagainstthesurfacetension.Ifthisbubble growssufficiently large tobeclose topinch off the reduc-tioninpressureduringtheoffpulsewillcauseaperturbation ofthe bubble sizewhich may inducepremature pinch off (comparedtocontinuousflowconditions)resultinginsmaller bubbles.

This work has shown that it is necessary to tailor the oscillationfrequencytothetypeofmicrobubblegeneration systembeingusedinordertominimisethebubblesize gener-ated.Thereforefluidicoscillatorsformicrobubbleproduction shouldbebuiltwithaparticularapplicationinmindand engi-neeredanddesignedtocreateafrequencyoptimisedforthat system.

Thispaperhasexploredthreedifferentoscillatorysystems andwehavedemonstratedthatallthesesystemshavea fre-quencysweetspot.Wastewateraerationand microflotation systemshavepreviouslybenefittedbyoscillatoryflow demon-stratedbyRehmanetal.(2015)andHanotuetal.(2012).Our reportedoscillation tuningcanfurther improvethe perfor-manceofthesesystems.

4. Conclusion

Thisworkhasprovedtheexistenceofafrequencyoptimum formicrobubblegenerationusingoscillatoryairflowthrougha diffuserwherethebubblesproducedaresignificantlysmaller insizeandcorrespondinglygreaterinnumberthanelsewhere inthefrequencyrange.Applicationoftheoptimumfrequency canreducebubblesizesbyupto∼73%.Theoccurrenceofan optimumfrequencytominimisebubblesizeisobservedinall ofthesystemsstudied.Itispredictedthatthesameshould applyforanybubblegenerationsystemundergoingoscillatory airflow.

Thisdiscoveryofbubblesizereductionattheoptimal oscil-lationfrequencyisimportantbecauseithasthepotentialto improveprocessefficiencies,involvingtheuseof microbub-blesasheatandormasstransfervehicles,simplybyaltering the oscillation frequency, thus requiring no further power inputorsystemmodifications,onlytotunethoseoscillators alreadyinplace.

Acknowledgements

Thisworkwascarriedout aspartofthe“4CU”programme grant,aimedatsustainableconversionofcarbondioxideinto fuels, ledbyTheUniversity ofSheffieldand carriedout in collaborationwithTheUniversityofManchester,Queens Uni-versityBelfast andUniversity CollegeLondon.The authors acknowledgegratefullytheEngineeringandPhysicalSciences Research Council (EPSRC) for supporting this work finan-cially(Grantno.EP/K001329/1).Theauthorswouldalsolike toacknowledgethesupportprovidedbyElliotGunnardand Andrew“Andy”Patrickfrom thetechnicalworkshopatthe DepartmentofChemical&BiologicalEngineering,University ofSheffield.

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