NNIN/LNF Microfluidic Workshop

NNIN/LNF Microfluidic Workshop March 6, 2014 Pilar Herrera-Fierro Behrouz Shiari What do we mean by Micro and Nanofluidics? Microfluidics The science and engineering of systems in which fluid behavior differs from conventional flow theory primarily due to the small length scale of the system. Nanofluidics The science of building microminiaturized devices with chambers and tunnels for the containment and flow of fluids measured at the nanometer level. Low flow (nanoliter, picoliter and femtoliter) fluid delivery systems. Flow in Micro/Nanochannels • Continuum model fails when the gradients of macroscopic variables become so steep that the length scale is of the order of average distance traveled by the molecules between collision. • Knudsen number ( Kn / L ) is typical parameter used to classify the length scale and flow regimes: Kn < 0.01: Continuum approach with traditional Navier-Stokes and no-slip boundary conditions are valid. 0.01<Kn<0.1: Slip flow regime and Navier-Stokes with slip boundary conditions are applicable 0.1<Kn<10: Transition regime – Continuum approach completely breaks – Molecular Dynamic Simulation Kn > 10 : Free molecular regime – The collisionless Boltzman equation is applicable. Continuum Slip Transition Free-molecular 0 0.01 0.1 10 Kn Microfluidic Channels Microfluidics is fluid mechanics in extremely confined space. What do we gain? • Minimize physical size and space • Low power and low production cost per device • Efficient use of reagents and reactants H. G. Craighead, Science, 290, 1532 (2000). • Fast response time • Precise volumetric control • Utilize microfluidic phenomena Micro/Nanofluids Modeling Family Tree Simulation Models Molecular Models Continuum Models Deterministic Statistical Navier-Stoks MD DSMC LBM Stokes Navier–Stokes Equations, Coupled with other Equations ANSYS CFD (ANSYS Fluent and ANSYS CFX), , COMSOL, CFD- ACE+, Coventor are Structure some of commercially available software packages for simulating some of multiphysics models. Model Geometry Inlets (p=p0) Modeling Process To characterize the fluid flow’s turbulent behavior scientists generally use the Reynolds number where ρ is the fluid density, u is flow velocity, L is a characteristic length, and η is the fluid’s dynamic viscosity. Turbulent flow takes place when the Reynolds number is high, typically when Re > 2000. In the device microchannels , for water-like substances Re is less than unity. The fluid flow is thus clearly laminar, so effective mixing of fluids in the devices requires other means. Modeling: Coupled Physics We build the model with two interfaces: Laminar Flow interface solves for the fluid flow (incompressible Navier-Stokes equations) : where ρ is fluid density, u = (u, v, w) is the flow-velocity field, p is fluid pressure, I is the unit diagonal matrix, η is the fluid’s dynamic viscosity, and F = (fx, fy, fz) is a volume force affecting the fluid. F = 0 because there are no volume forces. Transport of Diluted Species interface solves for the concentration field in the fluid (convection-diffusion equation): where c is the concentration, D is the diffusion coefficient, and R=0 is the reaction rate. Modeling The selection for the Transport of Diluted Species interface ensures that no artificial accumulation of concentration arises when solving the model. Because the concentration does not affect the fluid flow, it is not necessary to solve the interfaces simultaneously. By solving them sequentially, starting with the Navier-Stokes equations, we improve the solution’s convergence and reduce the solution time. Using this technique it takes only a few minutes to solve this model. On the microchannel and mixing-chamber walls, the no-slip boundary condition applies. However, in the vertical direction, due to the geometry, we can use a symmetry boundary condition. Modeling Steps Fluid Velocity m/s Fluid Streamline Vorticity Magnitude .

Load more