This preview shows pages 1–2. Sign up to view the full content.
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
Highlevel 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
Hardsphere approximation
Liquids
Shortrange 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 smalldiameter 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
This preview has intentionally blurred sections. Sign up to view the full version.
View Full Document
This is the end of the preview. Sign up
to
access the rest of the document.
 Fall '10
 Staff
 Physical chemistry, pH

Click to edit the document details