Post on 13-Jan-2016
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Soil Physics 2010
Outline
• Reminder
• Where were we?
• Capillarity
Soil Physics 2010
Reminder
Homework 1 is due
Soil Physics 2010
Uh-oh
Quiz
Solutes lower the water’s energy
Soil Physics 2010
Fresh water
Salt water
Water moves from higher (pure) to lower (salty) energy state
Where were we?
How do we know it’s energy?
Soil Physics 2010
Fresh water
Salt water
At equilibrium, the higher pressure balances the energy-lowering effect of the salt.h
This is the osmotic pressure,
Soil Physics 2010
0
500
1000
1500
2000
2500
CH4 NH3 H2O HF Ne H2S
Latent heat of fusion
latent heat of vaporization
Water and heat
Water is resistant to temperature change, including phase change
Heat capacity in J/kg,liquid:Methane 4022Ammonia 4708Water 4186HF 2580Neon 2009H2S 2509
Soil Physics 2010
Capillarity
demoroom.physics.ncsu.edu
Soil Physics 2010
Cohesion
Soil Physics 2010
Cohesion
If each water molecule has the same cohesion, then molecules on an
interface cohere preferentially inward and along the surface.
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.
From John Bush, MIT
A soap boat
Soil Physics 2010
Soil Physics 2010
Surface tension Interfacial tensionSurface energy
One effect of surface energy is to minimize the surface area
Soil Physics 2010
Surface tension
One effect of surface energy is to minimize the surface area
Soil Physics 2010
Adhesion
But also:Water “sticks” to other
materials, not just to itself
Soil Physics 2010
Capillarity
Bloody2.com
demoroom.physics.ncsu.edu
Soil Physics 2010
Capillarity
At equilibrium,Force up = Force down
Force = Mass * acceleration(Newton’s 2nd law)
Force down = mass * g
= w ( r2 h) * g
(or better) = (w – a) * ( r2 h) * g
r
h
Soil Physics 2010
Capillarity
At equilibrium,Force up = Force down
r
Force up = * 2 r
Force up is:
•proportional to surface tension •exerted around the inside perimeter of the tube
Soil Physics 2010
Capillary rise equation
At equilibrium,Force up = Force down
Force down = (w – a) * ( r2 h) * g
r
h
Force up = * 2 r
(w – a) * ( r2 h) * g = * 2 r
r g
2h
aw
Soil Physics 2010
Wetting angle www.alsapaint.com
Soil Physics 2010
“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
Changing the wetting angle