主要区别-表面张力与表面能

表面张力和表面能是组成材料的分子间作用力的测量值。由于这些分子间的作用力，液体表面总是被向内拉。如果要拉伸表面，就必须克服分子间的作用力。液体表面的张力和拉伸该表面所需的功可以测量：这些测量值对应于表面张力和表面能。表面张力和表面能之间的主要区别在于，表面张力测量的是每单位长度表面的力，而表面能测量的是每单位面积拉伸表面所需做的功。

什么是表面张力(surface tension)？

考虑容器中的液体样本。液体是由组成液体的分子之间的内聚分子间作用力结合在一起的。容器内的一个分子被周围的其他分子拉向各个方向。然而，如果你考虑上表面的分子，它们下面的分子仍然在把它们拉下来，但是上面没有液体分子可以把它们拉上来。这意味着在这些分子上有一个净向下的力。结果，液体的整个表面被向内拉。当液体自由时，正是这种作用在表面上的内向力使它们形成大致呈球形的液滴。

Molecules on the surface of a liquid are exposed to cohesive forces from below them. This makes liquid surfaces to be pulled inward.

Surface tension causes water to form roughly spherical droplets.

The surface of the liquid acts like a membrane under tension. Surface tension  can be quantified as the force acting per unit length  of the surface:

Here, the force acts parallel to the liquid surface and  is the length over which the force acts. For instance, imagine pulling a film of liquid and stretching it. This is shown in the diagram below:

Surface tension on a film of liquid

Here, the force  pulls on the membrane shown in dark blue, stretching the liquid film. Since there are two surfaces in the film (top and bottom surfaces), the total length is  . This means that for this film,

什么是表面能(surface energy)？

Due to the intermolecular forces between molecules, work needs to be done on a surface in order to stretch it. The surface gains an amount of energy equal to the work that was done on the surface. Surface energy refers to the amount of energy required per unit area to stretch it. For the film of liquid above, suppose the membrane is pulled through a distance  . Then, the work done is  . The increase in the surface’s area is given by  . So, the surface energy is given by:

表面张力(surface tension)和表面能(surface energy)的区别

它测量什么

表面张力测量平行于每单位长度表面施加的力。

表面能测量每单位面积创建新表面所需的能量。

Image Courtesy

“Wasser in Tropfen und an der Phasengrenze” by User:Booyabazooka (Own work) [Public Domain], via Wikimedia Comm***

“此图说明了增加表面积所需的力……”作者：Rudolf.hellmuth（自己的工作）[CC by-SA 3.0]，通过Wikimedia Comm***（修改）

“荷叶上的水滴”作者：aotaro（自己的作品）[CC by 2.0]，通过flickr