ELECTROOSMOTIC FLOW MIXING AND MASS TRANSFER IN MICRO AND NANO FLUIDIC SYSTEMS

Abstract

This thesisaddressesthefeasibilityofintroducinggeometricvariationsandsurfacepotential heterogeneityinmicroandnano uidicsystemstocreatee ectsotherthanpuremoleculardi u- sion toenhancemixinge ciency.Geometricmodulationsareincludedbyconsideringdi erent shapesanddesignsofmicrochannelwithembeddedobstaclesalongthewalls.Inperforming chemicalorbiologicalanalysis,samplesandreagentsneedtobemixedtogetherthoroughlyand this isanimportant owaspectinminiaturizedTotalAnalysisSystems( TAS),wheremixing playsavitalroleforsystemanalysis.Inscalingdowndimensionsofmicro-devices, owdriving through di usionprocessbecomesane cientmethodtoachievehomogenoussolutionsasthe characteristiclengthscalebecomessu cientlysmall. In thisthesis,passivemixing,usinggeometricmodulationswithsurfacepotentialnon- homogeneityinmicro/nanochannelsisstudiedduetoitsadvantageoveractivemixingin terms ofsimplicityandeaseoffabrication.Themathematicalmodelisbasedonthecoupling betweenMaxwell'sequationforelectricpotential,Nernst-Planckequationforiontransferand Navier-Stokesequationformomentumtransport.Acontrolvolumebasedalgorithmisusedfor the numericalsolutionofthe owgoverningequations.Chapter1containsthebasicde nitions and varioussolutionapproachesusedfortheelectrokinetic owgoverningequations. The Chapter2dealswithelectrokinetictransportandspeciesmixinganalysisinanano- channelunderanexternallyappliedsteadyelectric eldwhichareapplicableinmicroelectrical mechanicalsystems.Thechannelgeometryismodulatedbyintroducinganon-conductingob- stacle onthebottomwall.Inadditiontogeometricmodulation,surfacepotentialheterogeneity is createdbyputtinganoverpotentialpatchontheupperfaceoftherectangularobstacle.The e ect ofblockheight,blockposition,strengthofthepatchpotentialandexternalelectric eld are analyzedanditisfoundthatmixinglengthmaybesigni cantlyreducedbyintroducing the heterogeneityinthe owstructure.Chapter3isconcernedwiththecombinedtheoretical i ii and numericalstudyoftwocomparativenano-structureddomainswithsurfacepotentialhet- erogeneityandwallcorrugation.Thecomparativestudyofmixinganalysisisdemonstratedin terms ofpressuregradient,electricpotentialsandaverage owrateforawiderangeof ow controllingparameters.Irrespectiveofallthephysicalparametersuseditcanbeconcluded that mixinge ciencyalongthedownstreamisenhancedwiththedecrementinelectric eld strength andDebye-H uckelparameter. Electrokinetic owreversalanditsimpactonmixingenhancementinasymmetricwavymi- cro channelisconcernedinChapter4.Thecombinede ectofwaveamplitudeandelectric eld strength isstudiedtopredictthethresholdparametricrelationshipbetweenwaveamplitude, external electric eldstrength,andtheratioofDebyelengthandchannelheight.Inaddition to owreversal,asigni cantcontributionof owseparationonmixinge ciencyenhancement is analyzed.Chapter-5presentsthestudyofelectrokinetic owreversalanditsimpactonmix- ing withperiodicallydistributedzetapotentialinasymmetricallyarrangedcorrugateddomains with di erentphaseshiftswhichisanextensionofchapter-4.Choosingthebestpossiblephase shift forNewtonian uidmodelintermsofmaximummixinge ciencytogetherwithminimum pressure drop,themodelisextendedforpower-law uid. Subsequently,acomparativestudyonnon-Newtonian owmixingandpressuredropincir- cular micro uidicdomainswithsuddenconstriction/expansionisstudiedinChapter6.The impact ofsurfaceroughness,potentialheterogeneityandpower-lawindexisdiscussedfortwo di erentcon gurations.Asuitablearrangementof owparametersareconsideredtoestimatea suitable balancebetweenthemixinge ciencyandpressuredropforboththecon gurationsto proposeane cientande ectivecylindricalmicromixerwhichcanproducemaximumpossible mixing e ciencywithminimumpressuredrop.