orrection which increases as D
p
decreases.
Brownian motion: random movement of particles in air

Difficulties in accurately modeling the billions of individual molecules results in using
diffusion
to describe the
macroscopic
scale movements

Mean velocity of molecules of a pure gas B
=
cB
8RTπMB
=
/
cair
468 m s
Viscosity: a measure of the resistance of a fluid under shear stress. For laminar flow (smooth,
uniform) the sheer stress is:
=
τ
μdudy
Knudsen Number: defining the nature of air relative to the particle
=
Kn
2λDp
Three regimes of particleair interactions
a.
Continuum regime (Kn approaches 0)
b.
Free molecule, kinetic regime (Kn approaches infinity)
c.
Transition regime (Kn ~ 1)
Diffusion of Gasses
Fick’s First Law: the diffusive flux goes from high to low concentrations and is proportional to
the concentration gradient
= 
J
DdCdx
Where J is the flux, D is the diffusion coefficient, C is the concentration, and x is distance. The
() is used to denote the face that diffusion always occurs from high concentration to low.
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 Winter '10
 Li
 Thermodynamics, Entropy, Vapor pressure, Diffusion Diffusion

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