文档介绍：第二章表面热力学性质
单位体积表面积
A0 = A/V
m-1
单位质量表面积
A0 = A/m
m2kg-1
立方体:
A0 = 6l2/l3
= 6/l
球形:
A0 = 4r2/ (4/3r3)
= 3/r
例 1g水分散成半径为10nm水珠时,
A0 = 3/r = 3×108m-1
= 300m2/g
Surface tension
1) Some interesting phenomenon
Nickel -floating on Shuyu Spring
A nickel floating on the surface of water
Once immersed in water, the nickel would sink at once.
Buoyancy
Gravity
?
A water strider is Surfing the water in Baimai Spring Park (Zhangqiu, China)
水黾 water strider Aquarlus elongatus
Damo crossed the Yangzi river by floating on a piece of reed. Could it be true?
Ice column: suspending of water drop
?
Dewdrops on spider thread
Free water droplets are always in ball shape.
Slide l
Soap film
The soap film contract spontaneously.
To prevent contraction, a force f must be applied. If f decreased slightly, contraction occur, and if f is increased, the film extends.
This suggested that there existed a force f’ that makes the film shrink.
f
f ’, tension
Experimental study
In 1805, Thomas Young pointed out that the mechanical properties of a surface could be related to those of a hypothetical membrane under tension stretched over the surface.
波
The skin of water?
2) Micro-mechanism of surface tension:
Unbalanced force and surface tension
attractive force exists for molecules at the surface.
The energy of a molecule at the surface is higher than that in the interior
Surface area enlargement
To move molecules toward the surface, energy must be expended.