Megan Kramer JaunichDecember 12, 2006.ppt

Slip in Microchannel Flow Megan Kramer Jaunich December 12, 2006 Introduction Slip in Gas Flow Slip in Liquid Flow Wall Condition Hydrophilic vs. Hydrophobic Reflectivity Surface Roughness Molecular Level Liquid-Solid Interaction Presence of Vapor Layer (Nano-bubbles) Type of Fluid Liquid Flow Velocity Profile with Slip Slip in Gas Flow Flow defined by Knudsen Number Four regions Very Low – No-slip boundary condition Low – Slip boundary condition…………… Large – Boltzmann Equation Very Large – Solid-Fluid Molecular Interactions Slip in Liquid Flow Depends on channel configuration and experimental conditions Empirically developed equations Wall Condition Hydrophilic vs. hydrophobic Slip velocity ~ proportional to square root of shear rate [4,5] Slip length increases ~ linearly with shear rate Reflectivity Surface Roughness Tetradecane and DI water – rougher surface meant better agreement with no-slip condition Minimum surface roughness for no-slip condition Molecular Level Fluid-Solid Interaction Negligible importance on macro-scale Molecular Dynamics simulations Koplik, et al. Double Lennard-Jones potential, or remove it Zero LJ – small slip length on micro-scale Double LJ – no-slip condition held Cieplak, et al. Modeled as LJ potential Controlled velocity through wall parameter, A Slip length, shear stress varied with slip velocity Type of Fluid Water – No-slip holds to 40 nm [11] With controlled channel configurations, height 40 nm, the fluid was varied Slip length varied with size of fluid molecule, larger molecules had larger slip lengths Hexadecane – β ~ 25-30 nm Decane – β ~ 14 nm Hexane – β ~ 9 nm Presence of Gas Layer (Nanobubbles) at Channel Surface Some experiments indicate fluid slip due to a gas layer forming at the solid surface Depleted water region develops when water flows across a hydrophobic surface Assuming no-slip at solid-gas and gas-liquid interfaces [9]: Presence of Gas Layer (Nanobubbles) at Channel Surface Better assumption for gas l