Lecture3 - Origins of viscosity Fluids try to achieve...

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1 Physical Chemistry Lecture 3 Viscosity and sedimentation Origins of viscosity Fluids try to achieve uniform flow across any region (i.e. a constant speed independent of position) Fluids resist a gradient of speeds Described by a drag force that slows fast molecules and speeds up slow molecules Produces a flux of linear momentum The viscous force is proportional to the gradient of the speed Proportionality defines the coefficient of viscosity, Measured in poise (10 -1 kg m -1 s -1 ) dx dv J dx dv A F momentum viscous Viscosity Gases Momentum transfer between “fast” and “slow” molecules High-level kinetic theory predicts (slightly different from what is quoted in your text) Importance of prediction is Increase with square root of temperature Increase with square root of mass Considers only repulsive interactions during collisions Hard-sphere approximation Liquids Short-range attractive intermolecular forces dominate interactions Hard to model exactly Described empirically Viscosity usually decreases with increasing T 2 32 ~ 5 m v N ave Comparison of temperature- dependent viscosities Gas: oxygen Increase with temperature Liquid: aniline Decrease with temperature Viscosity of Oxygen 0 50 100 150 200 250 50 100 150 200 250 300 T (K) (micropoise) Viscosity of Aniline 0 0.04 0.08 0.12 0.16 250 300 350 400 T (K) (poise) Measurement of viscosity Measure flow in the presence of a gradient of speed Determine the viscosity by measuring flow of the material through a tube Poiseuille’s formula for flow through a cylindrical tube subject to a pressure drop that forces material through the tube May also find result for gravity as the force causing motion l gh r t V P l r t V gravity 8 8 4 4 Ostwald viscometry Need a small-diameter tube (capillary) Measure time of flow of a specific volume through the capillary May use Poiseuille’s equation to calculate viscosity Often do not know the radius well
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Lecture3 - Origins of viscosity Fluids try to achieve...

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