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