Fluid ®lling into micro-fabricated reservoirs F.-G.Tseng a,* ,I.-D.Yang a ,K.-H.Lin b ,K.-T.Ma a , M.-C. Lu b ,Y.-T.Tseng a ,C.-C.Chieng a a Department of Engineering and System Science, National Tsing Hua University, Hsinchu 30043, Taiwan, ROC b Department of Power Mechanical Engineering, National Tsing Hua University, Hsinchu 30043, Taiwan, ROC Received11June2001;receivedinrevisedform5November2001;accepted5November2001 Abstract Thisstudyreportsthatthesuccessofreservoir-®llingstronglydependsonthedesignsofthehydrophilicwallsurfaceandthewellshape/ sizeofthe¯ownetwork.Theideaisillustratedbothbyexperimentsandnumericalsimulations:micro-particle-image-velocimetry(m-PIV) systemissetuptomonitortheprocessofaliquidslugmovinginandoutofthemicro-reservoirandnumericalcomputationsareperformedby solving®rstprincipleequationstoprovidethedetailsofthe¯owprocess.Thecross-checkbetweenmeasurementsandcomputationsvalidate the computations. Numerical computations solve conservation equations similar to homogenous ¯ow model used in two phase ¯ow calculation in co- operationwithvolume-of-¯uid(VOF)interfacetrackingmethodologyandcontinuumsurfaceforce(CSF)model.Thesimulationsshowthat wall surface property as hydrophilic/hydrophobic is a dominating factor in ®lling processes of reservoirs of various shapes. A¯owsystemconsistingofmicro-channelsandmicro-wellsisfabricatedusingMEMStechnologytodemonstratethe®llingprocessand validatenumericalsimulation.Theagreementbetweenmeasurementandcomputationhelpstofullyunderstandtheprocess. # 2002Elsevier ScienceB.V.Allrightsreserved. Keywords: Hydrophilic; Surface tension; Micro-reservoir; Computational model 1. Introduction Micro-¯uidic devices for bio-medical applications usually are combinations of passive and active parts, net- workofthemicro-channelsandmicro-wellsarethemajor passive components of the device. Micro-channels direct anddistributethebio-¯uidstomicro-wellsofwhichspaces for bio-reactions and processes. Successful ®lling of the micro-well is not as trivial as the ®lling in macro-scale system,whichisresultedfromthelargeratioofsurfacearea and volume. Surface properties have signi®cant effects on micro-scale ¯ow system (<1mm) and have been demon- strated in many studies. Controlling liquid slug motion insidemicro-channelsisaninterestingissueandthemanip- ulationofthe¯owbasedonpatterningsurfacefreeenergies is an example [1]. The fabrication of the ¯ow system is complicated,time-consumingandexpensive.Itisinterested tohaveareliabledesigntooltoforeseethe®llingprocess. In the present study, ®lling processes of liquid slugs through three con®gurations of the ¯ow network are demonstrated either by experiments or numerical simula- tions,thewallsurfacepropertyofhydrophilic/hydrophobic istheemphasis.Bothapproachesofexperimentsandnumer- icalsimulationsshouldbeabletorevealdesignfeaturesfor complete®lling,partial®lling,orzero®llingofthemicro- reservoir. 2. Numerical simulation approach The approaches of numerical simulations are performed for the movement of a liquid mass inside ¯ow network. Sincesurfacetensionforceisthemajoreffectandthetotal surfaceforcedependsontheorientationandsurfaceareaof theliquid±gasinterface,precisedeterminationofthemov- ing location and shape of the interface is the key issue of accuratecomputation.Asaresult,thenumericalschemefor interface tracking must be robust with high resolution. Present study chooses volume-of-¯uid (VOF) method [2,3] for two phase homogenous ¯ow model [4] and the interface tracking technique in co-operation with CSF sur- facetensionmodel[5],the¯ow-®eldofgasphaseaswellas theliquid¯ow-®eldmustbecalculatedbecausethebound- aryoftheliquidphase(liquid±gasinterface)ispartofthe SensorsandActuatorsA97±98(2002)131±138 * Correspondingauthor.Tel.: 886-3-715131x4270; fax: 886-3-5720724. E-mail address: fangang@ess.nthu.edu.tw(F.-G.Tseng). 0924-4247/02/$ ± see front matter # 2002ElsevierScienceB.V.Allrightsreserved. PII:S0924-4247(01)00826-3