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FromThe69thMeetingofTheAmericanPhysicalSociety–DivisionofFluidDynamics
Archimedes'LawofBuoyancyTurnedUpsideDown
Counterintuitive to Archimedes’ famous law of buoyancy, researchers create a scenario in which bubbles sink while dense
steel spheres rise within a complex fluid
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Washington,D.C.,November21,2016--AteamattheUniversityofIllinoisatUrbanaChampaign,exploringhowairbubbles
risewithinacomplexfluid,likethosefoundwhileprocessingwetconcrete,wonderediftheycouldactuallygetthemtosink
insteadbyshakingthemixtureintherightway.
Duringthe69thAnnualMeetingoftheAmericanPhysicalSociety’sDivisionofFluidDynamics(DFD),beingheldNovember2022,inPortland,Oregon,RandyEwoldt,anassistantprofessorwhorunstheEwoldtResearchGroupintheDepartmentof
MechanicalScienceandEngineering,andJeremyKoch,aPh.D.candidate,willpresenttheirworkstudyingbubbleswithin
complexfluids.
“Certaincomplexfluidsare‘solid-like’whenyoudon’tpushonthemveryhard,”Kochexplained.“Oneconsequenceofthisis
thattheycantrapairbubblesandholdtheminplaceindefinitely.”
Thisphenomenonbegsthequestion:Howdoyougettheairbubblestomove?“Wecanstirthefluidandmovethebubbles
aroundmanually,orwecanalsoputthefluid’scontaineronacentrifugeandforcethebubblesout,”Kochsaid.“Howdoesthe
centrifugemakethishappen,andwhydothebubblesmoveinthedirectionthattheydo?Byunderstandingthosequestions,
we’llbeabletodescribehowtomovethebubblesinotherdirections.”
Intermsofthebasicconceptsbehindthegroup’swork,oneisacceleration,whichrequiresforce.“Shaking,a.k.a.‘accelerating,’
acontainerofliquidcreatesapressureforcethroughtheliquid,”Ewoldtsaid.“Theairbubblesinsidearealongfortherideand
feelthesameforceastheliquid,butatalowerdensitysotheforcetheyfeelis‘larger’thanneededtomatchtheliquid
acceleration.Whenthisforcecounteractsbuoyancy,itcanpotentiallymoveairbubblesdownwardwithinthesurrounding
fluid.”
Toputtheirtheoriestothetest,thegroupintroducedbubblesandsolidsphereswithdiametersontheorderofafew
millimetersintofluidsofvariousthicknesses.Theyusedarigidcontainertocontrolthemovementoftheimmersedparticles.
Theirresultsrevealedthenecessaryconditionsandfluidpropertiestopreventorproducethesinkingmotionofthebubbles
andheavyparticles–bothwithandagainstgravitationalforces.
“Complexfluidsareactuallyquitecommonineverydaylife,fromwhippedcreamtocustardtopumpkinpie–andfresh
concrete,”Ewoldtsaid.“Allofthesematerialscanbesculptedintoashape(inaliquidstate)andholdtheirshapeevenunder
gravity(asasolid).”
Inotherwords,theirworkshowsthatrigid-bodyaccelerationsaffectbuoyancyandweightinthesamewaygravitydoes.“But
youcan’tsimplyaccelerateacontainerindefinitelyinonedirection,sowefocusedonperiodicmotion:thecontainerreturns
toitsinitialpositionandspeedafteracertaintimeinterval,”Kochsaid.
Theydemonstratedascenarioinwhichbubblessinkwhiledensesteelspheresrise,whichiscounterintuitivetocommon
expectationsofhowbuoyancyworks.Intuitionsaysbubbleswillriseandsteelobjectswillsinkwithintheliquids,although
theyfoundtheexactoppositewastrue.“Archimedes’famouslawofbuoyancedoesn’tconsideraccelerationsor‘complex’
fluids,”Ewoldtsaid.
Thismadethegroupwonder:Hasitreallytakenhumansthislongtorevealthisbasicbehavior?And,ifso,whatelsearewe
missing?
Therearecountlessapplicationsforsuspensionswithinreal-worldfluidsand,thankstothiswork,moreinformationisnow
availabletohelpmakebetterdecisions.Whenthemotioninvolvedisperiodic,doesitmeantheeffectiscancelledout?“If
viscosityisconstant,theanswerisyes,”Ewoldtsaid.“Butifviscosityisn’tconstant--ifitchangesthemoreyouflowthe
fluid—theanswerisno.”
Timeseriesofanairbubbleforcedtosinkwithinacomplexfluid.(Credit:UniversityofIllinoisatUrbanaChampaign.)
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Abstract:A21.00002:"SinkingBubbles,"byJeremyKochandRandyEwoldtisat8:13-8:26amPST,November20,2016in
RoomD139-140
FormoreinformationabouttheAPSDFD2016meeting,visit:http://apsdfd2016pdx.org/
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MOREMEETINGINFORMATION
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USEFULLINKS
Mainmeetingwebsite:http://apsdfd2016pdx.org/
Technicalprogram:http://meetings.aps.org/Meeting/DFD16/Content/3199
Meeting/Hotelsite:http://apsdfd2016pdx.org/?page_id=30
PressRoom:http://www.aps.org/newsroom/index.cfm
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Wewillgrantfreeregistrationtocredentialedjournalistsandprofessionalfreelancejournalists.Ifyouareareporterand
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LIVEMEDIAWEBCAST
ApressbriefingfeaturingaselectionofnewsworthyresearchwillbewebcastlivefromtheconferenceonMonday,November
21st.Thefirstbriefingat2:00pm(EST)isabouttheforensicanalysisofbloodspatterandhowchangingthepositionofyour
fingerscanhelpyouswimfaster.Thesecondoneat4:00pm(EST)isaboutcat'sVelcro-liketonguesandbubbles.More
informationcanbefoundatthefollowinglink:https://www.aps.org/units/dfd/pressroom/
ABOUTTheDIVISIONOFFLUIDDYNAMICSOFTHEAMERICANPHYSICALSOCIETY
TheDivisionofFluidDynamicsoftheAmericanPhysicalSocietyexistsfortheadvancementanddiffusionofknowledgeofthe
physicsoffluidswithspecialemphasisonthedynamicaltheoriesoftheliquid,plasticandgaseousstatesofmatterunderall
conditionsoftemperatureandpressure.https://www.aps.org/units/dfd/
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