Outline
• Reminder
• Where were we?
• Capillarity
Soil Physics 2010
Reminder
Homework 1 is due
Soil Physics 2010
Uh-oh
Quiz
Soil Physics 2010
Where were we?
Solutes lower the water’s energy
Fresh
water
Salt
water
Water moves from higher (pure) to lower (salty)
energy state
Soil Physics 2010
How do we know it’s energy?
Dh
Fresh
water
At equilibrium, the
higher pressure balances
the energy-lowering
effect of the salt.
Salt
water
This is the osmotic pressure, P
Soil Physics 2010
2500heat
Water and
Latent heat of fusion
latent heat of vaporization
2000
Heat capacity in J/kg,
liquid:
Methane 4022
1500
Ammonia 4708
Water 4186
HF 2580 1000
Neon 2009
H2S 2509
500
0
CH4
NH3
H2O
HF
Ne
H2S
Water is resistant to temperature
change, including phase change
Soil Physics 2010
Capillarity
Soil Physics 2010
demoroom.physics.ncsu.edu
Cohesion
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Cohesion
If each water molecule
has the same cohesion,
then molecules on an
interface cohere
preferentially inward
and along the surface.
Soil Physics 2010
A soap boat
A floating body (length 2.5cm) contains a small volume of
soap, which serves as its fuel in propelling it across the free
surface. The soap exits the rear of the boat, decreasing the
local surface tension. The resulting fore-to-aft surface
tension gradient propels the boat forward. The water
surface is covered with Thymol blue, which parts owing to
the presence of soap, visible as a whitish streak.
Soil Physics 2010
From John Bush, MIT
Surface tension
Interfacial tension
Surface energy
One effect of surface energy is to
minimize the surface area
Soil Physics 2010
Surface tension
One effect of surface energy is to
Soil Physics 2010
minimize the surface area
Adhesion
Soil Physics 2010
But also:
Water “sticks” to other
materials, not just to itself
Capillarity
Bloody2.com
Soil Physics 2010
demoroom.physics.ncsu.edu
Capillarity
At equilibrium,
Force up = Force down
Force = Mass * acceleration
(Newton’s 2nd law)
Force down = mass * g
r
= rw (p r2 h) * g
(or better)
Soil Physics 2010
= (rw – ra) * (p r2 h) * g
h
Capillarity
At equilibrium,
Force up = Force down
Force up is:
• proportional to surface tension g
• exerted around the inside perimeter of the tube
r
Force up = g * 2 p r
Soil Physics 2010
Capillary rise equation
Force down = (rw – ra) * (p r2 h) * g
Force up
=g*2pr
At equilibrium,
Force up = Force down
r
(rw – ra) * (p r2 h) * g = g * 2 p r
h
2g
h
r w  r a  g r
Soil Physics 2010
Wetting angle
Soil Physics 2010
www.alsapaint.com
Changing the wetting angle
“A raindrop is extended like a
film by water processing on
the surface of a mirror, and
the visibility in case of rainy
weather is raised.”
Honda ad, honda-car.jp
www.croda.com
Soil Physics 2010