Basic Principles in Microfluidics 1
Newton s Second Law for Fluidics Newton s 2 nd Law (F= ma) : Time rate of change of momentum of a system equal to net force acting on system!f = dp dt Sum of forces acting on control volume = Rate of momentum efflux from control volume + Rate of accumulation of momentum in control volume 2
Navier - Stokes Equation Navier-Stokes equation applies when: (1) There are more than one million molecules in smallest volume that a macroscopic change takes place. (2) The flow is not too far from thermodynamic equilibrium. 3
Navier - Stokes Equation! du dt = "#P +!g + $# 2 U + $ 3 #(#iu) For noncompressible Fluid!iU = 0! du dt = "#P +!g + $# 2 U du dt =! "P # + g + $ # "2 U 4
Navier - Stokes in Microfluidics Terms become dominant based on physics of scale In microfluidics inertial forces dominate due to small dimensions, even though velocity can be high du dt du dt =! "P # + g + $ # "2 U =! 1 " #P 5
VISCOSITY 6
Viscosity Viscosity is a measure of resistance (friction) of the fluid to the flow This determines flow rate Symbols: η and in some books µ Units: Poise (gram/sec * Cm) 7
Viscosity Viscosity is a measure of resistance (friction) of the fluid to the flow. This determines flow rate. Units: Poise (gram/sec Cm) 8
Basic Properties - Viscosity Fluids and gases are very different Fluids become less viscous as temperature increases Gases become more viscous at temperature increases 9
Viscosity in Gases and Fluids Gases! =! 0 (T 0 - constant) (T 0 - constant) " # $ T T 0 % & ' 3 2 Fluids η η 0 e (Τ Τ 0 ) 10
Interfaces and Surface Tension 11
Interfaces Interface: Geometric Surface that delimits 2 fluids Separation depends on molecular interactions and Brownian diffusion 12
Interfaces Interface: Geometric Surface that delimits 2 fluids Simplified view: At interface: different energies Interaction between molecules 13
Interfaces If U is the total cohesive energy per molecule and d is a characteristic molecular dimension, d 2 is its surface, then the energy loss (surface tension) is given by:! = U 2d 2 14
Laplace s Law Minimization of surface energy, create curvature of fluids on other surfaces (fluids) Curvature 1/R Laplace s Law, the change in pressure is related to the curvature of the surface. For a sphere: For a cylinder: P = 2 (γ/r) P = γ/r 15
Droplet on a Surface of Two Properties Simulations 16
Coarsening Two Droplets linked by a precursor film 17
Coarsening Two Droplets linked by a precursor film 18
Contact Angle Surface tension (force per length) Angle is determined by the balance of forces at the point of interface Hydrophilic Hydrophobic 19
Contact Angle Surface tension (force per length) Angle is determined by the balance of forces at the point of interface Oil on Water 20
Hydrophilic - Hydrophobic 21
Surface Tension Droplet on a surface Forces on cross section of drop Surface tension along periphery Pressure on section area Pressure difference outside/inside drop Force =!PA = "r2!p Surface Tension=2!r"! = r 2 "P 22
Forces - Capillary Effects A wetting fluid will rise in a capillary tube Equilibrium: pressure drop across meniscus Surface tension Viscosity h = 2! Cos(") #gr 23
Capillary Force 24
Capillary Forces 25
Capillary Forces Small Channel (capillary) - Surface tension draws fluid of density ρ into the channel of radius ( r) F = 2!r" Cos(#) θ = contact angle γ = surface tension (N/m) Height of Fluid in a tube in the presence of gravity h = 2! Cos(") #gr 26
Forces - Capillary Effects 27
28
Capillary Forces 29
Droplet on Surfaces 30
Droplet on Irregular Surfaces r: roughness f: ratio of contact angle to the total horizon surface Young s critical angle cos(θ) = (f-1) / (r-f) 31
Wettability and Roughness 32
Reynolds Number 33
Fluids - Types of Flow Laminar Flow (Steady) Energy losses are dominated by viscosity effects Fluid particles move along smooth paths in laminas or layers Turbulent Most flow in nature are turbulent! Fluid particles move in irregular paths, somewhat similar to the molecular momentum transfer but on a much larger scale Reynolds Number Re is a measure of turbulence 34
Reynolds Number Reynolds number (Re) = inertial forces / viscous forces Re = Kinetic energy / energy dissipated by shear Implies inertia relatively important Re = 1 2 mv 2 D 1 Re = 2!V A D (!AL)V D "A Re =!V D L " V D = Drag velocity, L = characteristic length, η= viscosity, ρ = density Re < 2100 : laminar (Stokes) flow regime slow fluid flow, no inertial effects laminar flow in microfluidics slow time constants, heavy damping Re > 4000 : unstable laminar flow - turbulent flow regime 35
High and Low Reynolds number fluidics When the Reynolds number is low, viscous interaction between the wall and the fluid is strong, and there is no turbulences or vortices 36
37
Is this Flow Turbulent? Channel Geometry - Use a characteristic length : D h Re =! " VD h D h is a geometric constant 38
Is this Flow Turbulent? 39
Mixing Re = 12 and Re = 70 Cycle 1 Cycle 2 Cycle 3 40
Microchannels Cross Sections 41
Re and Size 42
Re - Some examples Re Friction factor ~ 1/ Re 43
Human Circulatory System 44
Flow associated with Skin 45
Knudsen Number Knudsen number assumes that we can treat the material as a continuum Continuum hypothesis holds better for liquids than gases also, K n =! mfp D h K n =!" 2 ( M Re ) λ mfp = mean free path of molecules, D h = hydraulic diameter K n measures deviation of the state of the material continuum K n < 0.01 continuum 0.01 < K n < 0.1 slip flow 0.1 < K n < 10 transition region 10 < K n molecular flow 46
The Smallest Length Scale of a Continuum High Re Low Re K n = M Re!" 2 47
Stokes - Einstein Diffusion Stokes - Einstein Equation Diffusion of a particle (gas, fluid) η Translational Diffusivity Rotational Diffusivity D t = K BT 6!"a D r = K BT 8!"a 3 48
Diffusion in Fluids Very short diffusion times x = 2D!! = 1 2 D = diffusion constant X = diffusion length τ = diffusion rate x 2 D Laminar flow limits benefits for fluid mixing. Highly predictable diffusion has enabled a new class of microfluidic diffusion mixers 49
Fluid Squeeze 50
Squeezed film damping Squeeze a film by pushing on the plates (one is not moving) Viscous drag is opposing the motion of the fluid Beam displacement Flow of fluid (Reynolds equation) Knudsen number, K, is the ratio of the mean free path to gap Squeeze number: relative importance of viscous to spring forces! "2 U "t 2 + EI "4 U "u 4 = P + F L 12! d(ph) dt P = b du dt = "{(1+ 6k)h 3 P"P} b = 96!W 3 " 4 h 3 L 51
Concluding Remarks 52
Summary Re = turbulent / viscous stresses Re < 2100 : laminar (Stokes) flow regime, slow fluid flow, no inertial effects laminar flow in microfluidics slow time constants, heavy damping Re > 4000 : turbulent flow regime 53
Fluid Behavioral What happens when the fluid is on the micro - nano scale? We discussed scaling - this is a review Quantities proportional L 3 Inertia, buoyancy, etc. Quantities proportional L 2 Drag, surface charge, etc. Quantities proportional L 1 Surface tension 54
Who Rules η 55