Highly energetic phenomena in water electrolysis

1.4 MB·

by  A. V. Postnikov

Highly energetic phenomena in water electrolysis
A. V. Postnikov1, I. V. Uvarov1, M. V. Lokhanin2, and V. B. Svetovoy1,3,*
1YaroslavlBranchoftheInstituteofPhysicsandTechnology,RussianAcademyofSciencies,Universitetskaya21,
Yaroslavl,150007,Russia
2P.G.DemidovYaroslavlStateUniversity,Sovetskaya14,Yaroslavl,150000,Russia
3MESA+ InstituteforNanotechnology,UniversityofTwente,P.O.Box217,Enschede,7500AE,TheNetherlands
*[email protected]
7
1 ABSTRACT
0
2
Waterelectrolysisperformedinmicrosystemswithafastchangeofvoltagepolarityproducesopticallyinvisiblenanobubbles
n
containingH andO gases. Inthisformthegasesareabletothereversereactionofwaterformation. Herewereportextreme
a 2 2
J phenomenaobservedinamillimeter-sizedopensystem. Underafrequencyofdrivingpulsesabove100kHztheprocessis
accompaniedbyclickingsoundsrepeatedevery50msorso. Fastvideorevealsthatsynchronouslywiththeclickabubbleis
4
growingbetweentheelectrodeswhichreachesasizeof300µmin50µs. Detaileddynamicsofthesystemismonitoredby
1
meansofavibrometerbyobservingapieceofsiliconfloatingabovetheelectrodes. Theenergyofasingleeventisestimated

] as0.3µJandasignificantpartofthisenergyistransformedintomechanicalworkmovingthepiece. Theobservationsare
ft explainedbythecombustionofhydrogenandoxygenmixtureintheinitialbubblewithadiameterofabout40µm. Unusual
o combustionmechanismsupportingspontaneousignitionatroomtemperatureisresponsiblefortheprocess. Theobserved
s effectdemonstratesaprincipalpossibilitytobuildamicroscopicinternalcombustionengine.
.
t
a
m
- Introduction
d
n Standardcombustionprocessissupportedbytheheatproducedinthecourseofcombustionreaction1,2. Whenavolumewhere
o
theprocessproceedsbecomessmallthereactionstopsbecauseoffastheatescapeviathevolumeboundaries3,4. Forthisreason
c
[ aminimalsizeofmicrocombustorscannotbemuchsmallerthan1mm5,6andtheirvolumeisatleastafewcm3. Inspiteof
thesefactsspontaneouscombustionofhydrogenandoxygenwasobservedinmicrosystemsinnano7andmicrobubbles8(see
1
alsoarecentreview9). Mechanismofthecombustioninsuchsmallvolumesisnotclear,butobservationssuggestthatthe
v
7 bubblesurfaceplaysarolesimilartoacatalyst.
2 Nanobubbles(NBs)containingstoichiometricmixtureofH andO gaseswereproducedinmicrosystemsusingvoltage
2 2
9 pulsesofalternatingpolarity(AP)9.UndersufficientlylargeamplitudeU>4−5Vandpulsesrepetitionfrequency f ∼100kHz
3
alocalconcentrationofbothgasesabovethesameelectrodereachesveryhighvalues. Itcanbesolargethatthebubbles
0
. containingmixtureofgasesarenucleatedhomogeneously. Duetothehomogeneousformationalargenumberofsmallbubbles
1 emergesinsteadofasmallnumberoflargebubblesasithappensinthenormalelectrolysis10. Thesesmallbubblesdonotgrow
0
largerthan200nminsizebecausethereactionbetweengasesisinitiatedspontaneously7. Althoughthecombustionwasnot
7
1 observeddirectlyduetoashortlifetimeandasmallsizeoftheobjects,aseriesofsignaturesstronglysuggestthatthereaction
: occurs.
v
i NotallthegasproducedelectrochemicallyisburnedinNBs. Somebubblescontainonlyoxygenoronlyhydrogen. These
X NBssurviveresultinginagradualpressureincreaseinaclosedmicrochamber11. Whenthepulsesareswitchedoffthepressure
r relaxesin100µsi.e. muchfasterthanthetimeneededtodissolvethegasesintheliquid. Fastrelaxationofthepressurewas
a
explainedbymergingofH andO nanobubbleswithformationofabubblecontainingastoichiometricmixtureofgases. The
2 2
latterrapidlydisappearsduetothecombustionreaction.
AnewphenomenonemergeswhenconcentrationoftheH andO nanobubblesbecomessolargethatthebubblestouch
2 2
eachotherandmergeduringtheprocess8. Inthiscasemicrobubles(MBs)withatypicalsizeof10µmappearinthechamber
ascanbeseeninstroboscopesnapshots. Dynamicsofthebubbleswastoofasttoobserveitoptically. AppearanceofaMBis
accompaniedbyasignificantpressuresurgeintheclosedchamberwhichlastsforafewmicrosecondsaswasmeasuredbya
vibrometer. TheeffectwasexplainedbymergingofmanyNBswiththesubsequentcombustionofgasesintheformedMB.
Theenergyproducedinoneeventwasestimatedas3nJ.
Observationofthecombustioninnanoandmicrobubblesshowsthatitispossibleinprincipletoovercomethefundamental
limit for scaling down the internal combustion engines9, which can be used to power different kind of microdevices12–15.
However,theenergyproducedbythecombustioninNBsturnsmostlyintoheat11. Only5%ofthecombustionenergyinMBs
istransformedintomechanicalworkdonebyaflexiblemembranecoveringthemicrochamber8.
Inthispaperwereportformationofexplodingbubblesandtheirdynamicsinamillimeter-sizedsystem. Thecombustion
reactionbetweenhydrogenandoxygeninthesebubblesisignitedspontaneouslyatroomtemperature. Formationofthebubbles
isaccompaniedbyaudiblesounds,theenergyreleasedintheexplosionistwoordersofmagnitudelargerthanwasobserved
previously,andasignificantpartofthisenergyistransformedintousefulmechanicalwork. Ontheotherhand,thereactionis
notinitiatedinthebubblesgeneratedfromanexternalsourceofthegasmixture.
Results
Figure1showsadeviceusedtogenerategasinthesystem. Acircularshapeoftheelectrodeshelpstobetterlocalizethe
producedgas. WhenaDCpotentialisappliedtotheelectrodesweobserveintenseformationofwellvisiblebubblesonboth
electrodes. ThegasesarehydrogenandoxygensinceasmallconcentrationofdissolvedCu2+ionsdoesnotplayasignificant
role. WhentheAPpulsesareappliedatafrequencyof100kHzormorenogasisvisible. However,theFaradaycomponentof
thecurrentincreasesincomparisonwiththatintheDCregime. Thiscomponentcanbeseparatedbyfittingeachpulsewiththe
functionoftimeI(t)=I +I e−t/τ (see10fordetail),whereI istheFaradaycurrentandthesecondtermisresponsibleforthe
F 1 F
charging-dischargingeffects. FordrivingpulseswiththeamplitudeU =6.75Vandthefrequency f =200kHzwehavefound
I ≈120mAandτ ≈1µs.
F
Figure1. (a)SchematicrepresentationofthePCB.(b)Ready-to-usedeviceinaPetridish. Apieceofsiliconisfloatingabove
oneofthestructures. (c)Scanningelectronmicroscopeimageoftheelectrodes. (d)Profilometerimageoftheelectrodes.
Colorsfrombluetoredcorrespondtoincreaseoftheheight.
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Figure2. SoundandcurrentatU =6.75V, f =200kHz. (a)Sound(red,rightaxis)fromeightclicksandtotalcurrent(blue,
leftaxis)asrecordedsynchronouslybythePicoScope. Onlyenvelopinglinesforthecurrentareshown. Aregionaroundthe
redarrowiszoomedin(b),wherethesoundsignalisaveragedover50µs. Panel(c)showsthepowerspectrumasafunctionof
soundfrequency f .
s
Soundandcurrent
AlthoughnovisiblebubblesareproducedintheAPregime,theprocessisaccompaniedbyclickingsounds,whicharerepeated
every50−100ms. ThesoundgeneratedbytheprocessandthetotalcurrentinthesystemareshowninFig.2. Theamplitude
ofthedrivingvoltageanditsfrequencywereU =6.75Vand f =200kHz,respectively. Onlyenvelopinglinesforthecurrent
areshownsincefastoscillationscannotberesolvedonthetimescaleofthefigure. Asonecanseeinpanels(a)and(b)each
clickisrelatedtoacurrentdroplastingforabout500µs. Anexpecteddelayof0.4msisobservedbetweenthebeginningofthe
currentdropandthestartingmomentofthesoundbecausethemicrophonewaspositionedatadistanceof12−15cmfrom
theelectrodes. Theclicksarenearlybutnotexactlyperiodic. Theiramplitudeandtheintervalbetweentheclickscorrelate
withthedepthofthecurrentdrop: thedeeperthecurrentdropisthehigheristheamplitudeandthelongeristheintervalto
thenextclick. Frequencyspectrumofthesoundisshowninpanel(c). Itvarieswiththedishsizeandshape,dependsonthe
proximityoftheobjectiveandothergeometricalcharacteristicsofthesetup. AsdemonstratedinSupplementaryInformation
(Fig.1S)separatelinesoftheclickareessentiallydefinedbytheeigenmodesofthesystem. Afterthedropthecurrentslowly
returnstothevalueithadbeforethedrop. Itisbecausetheliquidnearthestructureisheatedbyafewdegreesbythecurrent
butthebubbleformationandterminationbringcolderliquidtothestructure(seeFig.2SandexplanationsinSupplementary
Information).
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Figure3. ThecurrentdropatU =6.75V, f =150kHzandthesynchronizedpositionoftheframes(dots)inthefastvideo. A
fewframesindicatedbythearrowsareshownasinsets. Thecentralelectrodeandtheemergingbubbleareinthefieldofview.
Inframe4theinitialbubbleiscircled.
Fastvideo
Fastvideo(seeSupplementaryinformation,fileV1)revealsthereasonforthecurrentdrop. Abubblegrowingattheperiphery
ofthecentralelectrodeblocksthecurrent. Figure3showsthecurrentasafunctionoftime. Reddotsindicatepositionofthe
framesinthevideoandafewframesareshownasinsets. Theinitialsizeofthebubblecanberoughlyestimatedfromthe
frame4wherethebubbleappearsforthefirsttime(surroundedbyacircleofdiameter100µm). Thebubblediameterisin
therangeof30−50µm. Largeuncertaintyisduetodifficultytodefinethebubbleedgeinthisframe. Lowqualityofoptical
imagesismostlyduetopoorreflectivityofthestructure. Notethatwhentheinitialbubbleappearedthecurrenthasnotyet
changed. Theinitialinflationrateisestimatedfromframes4and5as6m/sbutitslowsdownwithtime. Thebubblereachesa
sizeof300µmin50µs(frame9). Atthispointtheinflationbecomesslowandafterframe23thebubblegetsoutoffocus. Itis
notpossibletoseeonthisvideodisappearanceofthebubblebutforsmallerbubbles,whichstayinfocus,onecanseethatthe
bubbleshrinksanddisappears(seeSupplementaryInformation,fileV2).
Vibrometermeasurement
Thesoundproducedbytheprocessisasignthathighly-energeticeventsarehappeninginthesystem. Tocharacterizethese
eventsweusethevibrometer. Apieceofsiliconwithdimensionsof17×8×0.5mm3floatsinthePetridishwithitscenter
justabovetheelectrodes. Thelaserbeamispositionedonthecenterofthepiece. Thevibrometerregistersvelocityofthe
Sisampleinitscenter. TheresultispresentedinFig.4(a)synchronouslywiththecurrentinthesystem. Thevelocitysignal
isinagreementwiththefastcameraobservations. Growingbubblemovestheplateupbutshrinkingbubblepullsitdown.
However,whatisstrikingisthemagnitudeoftheeffect. Anon-zerosignalappearsabout10µsbeforethecurrentstartsto
decrease. Theinitialaccelerationisestimatedas710m2/s. Velocityofthesamplereachesamaximalvalueofv≈6cm/sat
themomentt=146µs. Thesignaliszeroagaininthemiddleofthecurrentdrop,whereinflationofthebubblechangesto
deflation. ThelattercorrespondstonegativevaluesoftheSi-samplevelocity. Whenthebubbledisappearsthesignalchangesits
behaviorandoscillateswithafrequencyofabout10kHzasonecanseeinpanel(b). Theoscillationsarerelatedtothelowest
flexuralmodeoftheSi-sample. Forafreesampleitsfrequencyisestimatedas15kHzbutthefloatingsampleisnotactually
freeanditsfrequencyisreduced. Sincethemainwavelengthλ =52mmofthesoundismuchlargerthantheliquidthickness
H betweentheelectrodesandfloatingsilicon(estimatedasH=2−0.5=1.5mm),theliquidmovestogetherwiththesample
andthefrequencyreductionfactorcanbeexpressedviatheaddedmassas[ρ h/(ρH+ρ h)]1/2≈0.66,whereh=0.5mmis
s l s
thethicknessofthepiece,ρ =1g/cm3 andρ =2.33g/cm3 arethedensitiesoftheliquidandsolid,respectively.Thus,the
l s
flexuralfrequencyoftheSisampleisestimatedas9.9kHz.
Figure4(b)showsthevelocitysignalanddisplacementoftheSi-sampleonalargertimescale. Thebulkypieceofsiliconis
movedupto9µmbytheprocess. Thevelocityofthesamplemeasuredbythevibrometergivesallthemechanicalinformation
onitsmovement. Usingthework-energyprinciplewecanestimatetheworkW donebytheinflatingbubbleonthepieceasits
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Figure4. MovementofthefloatingSi-sampleatU =6.75V, f =200kHz. (a)Current(leftaxis)andthevibrometersignal
(rightaxis)recordedsynchronouslyasfunctionsoftime. Thearrowindicatesthepointwherethesignalisthreetimeslarger
thanthenoiselevel. Thispointistakenast=0. (b)Thesamevibrometersignal(velocity)onalargertimescale(rightaxis).
Thegreencurve(leftaxis)showsthedisplacementofthesiliconpieceinitscenter,whichisjustthesignalintegratedovertime.
maximalkineticenergyW =mv2 /2≈0.28µJ,wherem=156mgisthesamplemass. Thisworkisalowerlimitforthe
max
energyoftheeventE becausesomeenergydissipatesandsomeescapesviatheliquidduetothelongitudinalmovement.
ev
Nevertheless, we can take E ≈0.3µJ as a good estimate since the quality factor is not small as oscillations in Fig.4(b)
ev
demonstrateandonlyasmallpartoftheenergyescapesinthelongitudinaldirectionbecausethesamplesizeismuchlarger
thantheliquidlayerunderneathit.
Microfluidicgeneration
WedidaspecialinvestigationtocompareMBsproducedbytheelectrochemicalprocessandMBsgeneratedfromtheexternal
sourceofstoichiometricmixtureofgases. Inthelattercasethebubbleswereproducedbyamicrofluidicbubblegenerator23.
Thestoichiometricgasmixturewasfedintoonechannelofthegeneratorwhiletheelectrolyteorcleanwaterwasfedintothe
otherchannel. Thebubblesproducedbythegeneratorhaveasizeof10−12µm(seeFig.5). Thesebubblessurviveatleast
900msbeforegoingoutofthefieldofview.
Discussion
AsfollowsfromFig.4(a)thevelocityoftheSi-samplebuildsupinlessthan10µs. Sincetheinitialaccelerationislarge∼70g,
wheregisthefree-fallacceleration,weconcludethattheprocesseshaveanexplosivecharacter. Presenceofthesoundisan
additionalcharacteristicfeatureoftheexplosion. ThesesignaturescanbeproducedbythecombustionreactionbetweenH and
2
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Figure5. GenerationofMBsfromanexternalsourceofgasmixture. (a)Supplyingchannelsforgasandliquidandthe
diffuser,wherethebubblesaregeneratedfrombreakingthemeniscus. (b)TrainofMBscontainingH andO mixtureinthe
2 2
electrolyteproducedbythebubblegenerator.
O gasesintheinitialbubble. Ifthebubbleisfilledwithastoichiometricmixtureofgases,thecombustionenergyisestimated
2
asE =2N|∆H|/3,whereN isthenumberofgasmoleculesinthebubbleand∆H≈−242kJ/molistheenthalpyofwater
com
formation. ThenumberN canbeexpressedusingtheequationofstateatroomtemperaturebeforethecombustion. Thus,we
findthatE ≈0.3µJfortheinitialradiusofthebubbleR ≈22µm. Thissizeisinagreementwiththeroughestimatefrom
com in
thevideo. TheratiooftheusefulworktotheexplosionenergyW/E ∼1canbecomparedtoasimilarvalueW/E (cid:39)0.05
com com
achievedin8,wheretheworkwasdonebytheflexiblemembraneclosingthemicrochamber.
Underwater combustion and explosions were investigated in many papers (see, for example,16–20) but it is difficult to
compare the results with our case due to much larger scale of the events and different geometrical configuration of the
experimentsandmodelling. Combustioninbubblesassmallas10µmindiameterwasconsideredin21butitwassupportedby
thehightemperatureinsidethebubblerapidlycollapsinginanacousticfield.
Directcomparisoncanbedonewiththeexperiment22wherecombustionwasobservedinabubble(inwater)of2mmin
diametercontainingacetylene-oxygenmixture. Asignificantenergyof20mJwassuppliedbyasparktoignitethenormal
combustioninthebubble. Inourcasethereactionisinitiatedspontaneouslyinasmallerbubblebuttheratesofinflationare
comparable: 6m/sinourcasevs8m/sin22. Anadditionalrelevantquantityistheenergydensity. Inourcaseitcanbeestimated
directlyfromtheexperimentasE /V =5−21MJ/m3,whereV =4πR3/3istheinitialvolumeofthebubble. Wideinterval
ev in in in
hereisduetouncertaintyoftheinitialbubblesize. IfweuseE insteadoftheexperimentalvalueE thevolumeV fallsout
com ev in
andwefindthetheoreticalvalue6.5MJ/m3forthehydrogen-oxygencombustion. Thisenergydensitycanbecomparedwith
15MJ/m3fortheacetylene-oxygenmixture. Itdiffersfromthatforhydrogen-oxygenbecausetheenthalpyoftheacetylene
combustionisratherlarge,∆H =−1318kJ/mol. Fromthecomparisononecanconcludethatwereallyobservecombustion
C2H2
ofhydrogeninmicrobubbles.
TheinitialbubblecontainingstoichiometricmixtureofgasesisformedbymergingoftheNBsproducedbythealternating
polarityelectrochemicalprocess8. Forthis,thedensityofNBsmustbesohighthatthebubblesarenearlytouching. Thisis
whytheeventsareseparatedbyatimeofabout50msascanbeseeninFig.2. AfterexplosionthesystemwaitswhilenewNBs
willbecollectednearthestructure. Fluctuationsofthelocalconcentrationofnanobubblesexplainthattheclicksoccurrednot
completelyregularly.
Areasonforthespontaneousinitiationofthereactioninarelativelylargemicrobubbleisnotclear. InNBscontaining
mixtureofgasesthereactioncouldbeinitiatedasasurface-assistedprocess9,butforlargerbubblesthismechanismhasto
besuppressedduetoasmallersurface-to-volumeratio. Thestoichiometricbubblesproducedbythemicrofluidicgenerator
surviveatleast900ms. Ontheotherhand,inthebubblesformedbycoalescenceofNBsthereactionisinitiatedspontaneously
inlessthan10µs. Wedonotknowthereasonforthisdifferencebutitcouldberelatedtonanodropletsleftinthebubbleafter
mergingofmanyNBs. DenselypackedNBsfillonly74%ofvolume,therestistheliquidtrappedinbetweenthebubbles.
AftercoalescenceofthebubblesthetrappedliquidwillformnanodropletsdistributedinsideoftheMB.InthisMBanygas
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moleculeisseparatedfromtheliquidbyananoscopicdistance. Possiblyitcanplayarolefortheinitiationofthereaction.
WhentheexplosionhappensintheinitialMB,thepressureandtemperatureinthebubblerise. Ifheatexchangewiththe
wallsisslowincomparisonwiththereaction,thecombustionenergyE isspentonlyonheatingofthewatervaporbeing
com
formedinthereaction. Assumingthatthebubblesizechangesinsignificantlyduringthecombustionphasewecanfindthe
pressureinthebubblefromtheequationofstateasP/P ≈2|∆H|/3T =64.7,whereP ≈1baristheambientpressureandT
0 0 0 0
istheroomtemperature. Themostefficientchannelfortheheatexchangeiswatervaporizationfromthewallsbyenergetic
molecules. Forsuchmoleculesatimeneededtoreachthewallisestimatedasτ ∼R2/D ,whereD istheself-diffusion
h in gg gg
coefficientforwatervapor. UndernormalconditionsD ≈2.8×10−5 m2/sandwefindτ ∼10µs;thistimeisscaledwith
gg h
temperatureas(T/T )1/2. Spontaneouscombustioninmicroandnanobubbleshappensinafewmicroseconds9. Sinceboth
0
thetimefortheheatexchangeandthetimeforcombustionarecomparablethepressureandtemperatureintheinitialbubble
immediatelyafterthecombustionisdefinedbythedynamicsbutanywayitisexpectedtobehigh. Thesourceofthesoundis
thepressuresurge. Duringtheinflationphasethepressuredecreasesandgoingbelowtheatmosphericpressurebyinertiaasit
happens,forexample,incavitation. Atthefinalstagethebubbleshrinksanddisappears. Thetimescaleforheatexchange
explainswhytheusefulworkobservedinthisstudyismuchlargerthanthatinthemicrosystem8. Inthelattercasetheinitial
bubblehadaradiusofR ≈5µm. Duetosmallersizetheheatexchangehappensmuchfasterandthepressureandtemperature
in
reachsmallervalues.
Alternatively,onecouldtrytoexplaintheobservedphenomenabyheatingoftheelectrolytebytheFaradaycurrent(Joule
heating). Avaporbubblecouldbeformedduetotheheating. However,theheatingoftheelectrolytehastoresultinthecurrent
increase. Thiseffectcanbeusedasabuild-inthermometer11. Whatisobservedisonlyasmalltemperatureincreasebetween
the clicks as explained in Supplementary Information, Fig. 2S. In this scenario there is no a driving force for the bubble
inflationsincepressureinthebubblecannotreachahighvalue. Asanadditionalscenarioonecouldimaginethatthecurrent
passingthroughasmallliquidvolumevaporizesthisvolumeproducingtheinitialbubblewithahighpressureinside. The
numberofmoleculesinthebubbleisestimatedasN <E /∆H ≈4.4×1012,whereE ≈0.3µJistheobservedenergyand
v ev v ev
∆H =41kJ/molistheenthalpyofwatervaporization. Wetookonlytheupperlimitbecausetheheatingofthemoleculeswas
v
neglected. ForthisN theradiusofvaporizedliquidspheremustbeR <3.2µm. TheexperimentalvalueoftheFaradaycurrent
v l
densityisestimatedas j ≈200A/cm2. Itissimilartothatusedfornormallyfunctioningmicrodevices10,11. Thiscurrentcan
F
transfertheenergyE ≈0.3µJtoN moleculesforthetimeτ ,whichcanbefoundfromtherelationπR2j Uτ =E ,where
ev v E l F E ev
U =6.75V.Itgivesτ >700µsthatisconsiderablylargerthanthetimeneededtoproducetheinitialbubble,whichissmaller
E
than10µs.
For application in microcombustors it is not efficient to produce exploding gas electrochemically. Instead, one has to
provideappropriateconditionsinamicrovolumewithpremixedgases. Ifthevolumeisclosedbyaflexiblemembrane,the
explosionwillmovethemembraneup. Therefore,acriticalstepistounderstandwhythegasisignitedspontaneouslyinMBs
producedelectrochemicallyandensuretherightconditionsinthemicrovolume.
Inconclusion,weobservedhighlyenergeticeventsinwaterelectrolysisproducedbyshortvoltagepulsesofalternating
polarity. Theprocessisaccompaniedbythesoundclicks,whichoccursynchronouslywiththecurrentdropsinthesystem. The
observationswereexplainedbyspontaneouscombustionofH andO gasesintheinitialbubblewithadiameterofabout
2 2
40µm. Thecombustionproducesthepressurejump,whichgeneratessoundanddrivesthebubbleinflation. Unusualreaction
mechanismhastodrivetheprocesstoprovidespontaneousignitionatroomtemperature. Significantpartofthecombustion
energywastransformedintomechanicalworkinsteadofheat. Itopensapracticalwaytobuildatrulymicroscopicinternal
combustionengine.
Methods
Standardprintedcircuitboards(PCB)withCufoilisusedforbubblegenerationintheAPregime. OnePCBcontainsfour
pairsofelectrodesandcontactlinesasshowninFig.1. Typicalsizeofthecircularstructureis1mm,thelinewidthis150µm,
andthelinethicknessis50µm. Onlythecircularstructuresareintheelectricalcontactwiththeelectrolyte,therestiscovered
withastandardinsulatingcoatingusedforPCB.
ThePCBdeviceisplacedinaPetridishandcoveredby2-3mmoftheelectrolyte(1mol/lsolutionofNa SO inwater).
2 4
One electrode is kept grounded but the other one is at the alternating potential with an amplitude of 6−7V. A relatively
highpotentialisusedtoproduceasignificantsupersaturationwithH andO gases. Thevoltagepulsesweregeneratedbya
2 2
PicoScope5000andamplified15times. Thecurrentinthesystemandtheproducedsoundswererecordedusingdifferent
channelsofthePicoScope. Onalongtimescale(≥1s)WindowsSoundRecorderwasused. Thesoundwasrecordedinair
withamicrophoneinstalled10−12cmfromthePetridish. OneofthesoundfilesisprovidedinSupplementaryInformation.
VisualdynamicsofthesystemwasanalysedwithafastcameraPhotronFastcamSA1.1attheframerateupto100000fps. A
detailedanalysisoftheprocesswasperformedwithavibrometerPolytecMSA-400.
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Acknowledgements
WethanksD.Lohse,A.Prosperetti,S.WildemanforhelpfuldiscussionsandG.Lajoinie,M.vanLimbeek,R.Sandersfor
technicalassistance. ThisworkissupportedbytheRussianScienceFoundation,grant15-19-20003. V.B.S.acknowledges
supportfromtheNetherlandsCenterforMultiscaleCatalyticEnergyConversion(MCEC),anNWOGravitationprogramme
fundedbytheMinistryofEducation,CultureandScienceofthegovernmentoftheNetherlands.
Author contributions statement
V.B.S.conceivedtheexperiments,A.V.P.andI.V.U.conductedtheexperiments,thedatawasanalysedbyM.V.L.andV.B.S.
Allauthorsreviewedthemanuscript.
Additional information
Supplementaryinformationaccompaniesthispaperathttp://www.nature.com/articles/srep39381
Competingfinancialinterests: Theauthorsdeclarenocompetingfinancialinterests.
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