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1、Microfluidics and“Lab-on-a-Chip”Modules,Societal and economic trends likely to affect your career.What is a“Lab-on-a-Chip”?-Examples of the technology-Rationale for using it-What is the role for Chem Es,NY Times,March 4,2004Thomas FriedmanBANGALORE,India Jerry Rao wants to do your taxes.Ah,you say,y
2、ouve never heard of Jerry Rao,but the name sounds vaguely Indian.Anyway,you already have an accountant.Well,Jerry is Indian.He lives in Bangalore.And,you may not know it,but he may already be your accountant.,Societal and economic trends,Societal and economic trends,Societal and economic trends,Soci
3、etal and economic trends-Questions to think aboutWhat advantages/disadvantages do U.S.educated have vis-a-vis various internationally-educated Chem Es?What aspects of Chem E are easiest to“outsource”?Which are the hardest?What Chem E employment sectors are likely to stay in USA?What are their distin
4、guishing traits?,What is a“Lab-on-a-Chip”?,Images from http:/,Why“Lab-on-a-Chip”instead of regular analysis?,Data from http:/,Another“Lab-on-a-Chip”example,Images from http:/,http:/,Another“Lab-on-a-Chip”,Images from http:/,Sip reagents into storage wells,Another“Lab-on-a-Chip”,Images from http:/,Mi
5、x and react sample with reagent,ATP-dependent kinetics at 37 C,Another“Lab-on-a-Chip”,Images from http:/,Flow products to separation column,Run electrophoretic separation,Electrophoretic separation,Time(s),Do you have what it takes to design a“Lab-on-a-Chip”?Key Elements(all done at the micro-scale)
6、:Flow of fluids in channelsAutomated control of thermal and fluid stystemsChemical ReactionsMass Transfer/SeparationsAs Chem Es you have the technical foundation needed,but now need to learn SPECIFIC information,FAST!,Specific information will come fromTaking a“short-course”Talking to expertsWorking
7、 on prototype problems(Tuesday,Wednesday)Doing simulation-based research(Tuesday,WednesdayLife-long learning-do it or stagnate as a professional.,Physics of Microfluidics(a.k.a.Flows for L 1 mm)Some important length-scales for the physics of fluids LCharacteristic geometry for the flow domain L 103
8、m in microfluidics dMean free distance molecules travel prior to molecule-molecule collisions d=kT/(2Ps2)for ideal gases;d=6.5 x 108 m for air at STP d O(s)for liquidswhere k is Boltzmanns constant,T is absolute temp,P is pressure,and the symbol O(x)can be though to mean“has an order of magnitude of
9、 x”s Molecular diameter s O(5x1010 m),L,Physics of MicrofluidicsFlow traits are dictated by comparison of d and s to LAn important ratio is Kn=d/L,the Knudsen Number,When L s,one often uses molecular dynamics approaches,Physics of MicrofluidicsLength-scale ratios dictate approach for understanding f
10、lowAn important ratio is Kn=d/L,the Knudsen Number,Relevant Region,Continuum flow region is traditional Chem E fluid mechanics,Physics of MicrofluidicsHow does a small L influence things in the continuum flow region?,Physics of MicrofluidicsHow does a small L influence things in the continuum flow r
11、egion?Viscous Forces tend to dominate Inertial ForcesRe=VL/n 1 Reynolds Numberwhere V is characteristic fluid velocity,and n is kinematic viscosityExamples Ant Brain vs.Human Brain Streamlines at a T-junction(w/and w/o inertia),Physics of MicrofluidicsHow else does a small L influence things?Viscous
12、 Forces tend to dominate Body Forces(e.g.due to gravity)Gr=gL3r/(rn2)1Grashof Numberwhere g=10 m/s2,r is density difference,and r is mean densityTemperature and concentration gradients dont tend to produce strong natural convectionin microscopic systems,Physics of MicrofluidicsIn short,the flows you
13、 will be working with here obey the same basic physics taught in Chem E fluids,soLook at dimensionless groups from your Fluids course to see how L changes their magnitudeUsually means viscous drag is a major factor in microfluidicsBut,some additional continuum forces that were ignored by the macro-f
14、ocus of Traditional Fluids also need to be considered.,Physics of MicrofluidicsSurface Forces are important in microsystems(Surface-to-Volume ratio is proportional to L1)Surface Forces can rival Viscous ForcesViscousCapillaryAnother way to think about the Capillary numberCa=(Characteristic Viscous P
15、)/(Capillary pressure difference),=V/gCapillary Number(Ca)where g is surface tension(dyne/cm),is viscosity(g/cm-s),PG PL=2 g cosq/L,q,Physics of MicrofluidicsSurface Forces are important in microsystemsTypical values for Surface Tension g(dyne/cm or mN/m)near 25 CLiquid-Vapor SystemsWater 72Propylen
16、e carbonate 41Ethanol 22Perfluorpentane 10Hg486Liquid-Liquid SystemsWater/n-Butyl Alcohol 2Water/Benzaldehyde16Water/Benzene35Water/n-Heptane50Water/Flourcarbon polymer57,Physics of Microfluidics,Physics of Microfluidics,Physics of Microfluidics,Physics of MicrofluidicsOther Surface-related dimensio
17、nless groupsBond Number(Bo)Gravitational Forces/Surface Forces Bo=rgL2/gRise of a liquid in a capillary is evidence that surface forces are bigcompared to gravity as dimensions shrink.,Physics of MicrofluidicsOther Surface-related dimensionless groupsBond Number(Bo)Gravitational Forces/Surface Force
18、s Bo=rgL2/gRise of a liquid in a capillary is evidence that surface forces are bigcompared to gravity as dimensions shrink.,Physics of MicrofluidicsAn additional“surface”driven flow in microfluidic devices is called electroosmosis.Electroosmosis is actually a flow driven by a body force that is impo
19、rtant only very near charged surfaces(usually within nanometers of a surface).,-,+,+,+-+-+-+-+-+-+-+,+-+-+-+-+-+-+-+-,+-+-+-+-+-+-+-+-,neutral,net+,negatively charged surface,solution with cations and anions,Physics of Microfluidics,-,+,+,+-+-+-+-+-+-+-+,+-+-+-+-+-+-+-+-,+-+-+-+-+-+-+-+-,negatively
20、charged surface,+,-,Veo,Veo=zDE/4 for a capillaryz is the zeta potential(a measure of surface charge),D is the dielectric constant of the medium,E is the applied electric field,Physics of MicrofluidicsSo,how do we incorporate these various physics into models?We need governing equations and boundary
21、 conditions.For a Newtonian,incompressible fluid start with Navier-Stokes Equations:,What forces are represented by these vector equations?,Physics of MicrofluidicsNondimensionalize the Navier-Stokes Equations using Vas characteristic VelocityL/Vas characteristic Time(alternative,L2/n)L as character
22、istic LengthmV/Las characteristic Pressurewhere Re=VL/n,If Re 0,then forces on left hand side become less important,Physics of MicrofluidicsThe remaining forces show up in the Boundary Conditions applied at surfaces between two phases:Some conventional boundary conditions:No slip at solids vt=0No pe
23、netration of fluids at impermeable boundariesvn=0No gradients at symmetry linesnv=0For gas-liquid and immiscible liquid-liquid interfaces well need to talk,but the basic idea is generally:Normal interfacial stress balance:P2 P1=2Hg where H is the interface mean curvatureTangential interfacial stress
24、 balance:Electroosmosis can look like a“slip”velocity at the charged surface:vt=Veo,Physics of MicrofluidicsHighlighted some similarities and differences from traditional FluidsReduced importance of inertia simplifies Navier-Stokes EquationsDiscussed role of surfaces and dimensionless numbers that describe surfaces forces Introduces some strategies where surface forces enter models Accurately modelling free surface flows at finite Ca is an area of active research.Electroosmosis was introduced as a body force that happens very near surfaces,soit can look like an interfacial slip velocity.,
