Monday, 19 January 2015

The Surface Tension


In the field to the liquid state each molecule is surrounded by other molecules: the attractive forces between molecules, for the fact that each of them is completely surrounded by other, are balanced allowing each molecule moves freely (there is no prevalence of forces in some direction).

Just below the surface of the liquid, we can consider a layer as thick as the diameter of the molecules forming the liquid: this layer is called the boundary layer and is that on which the transition from liquid to gas, a solid or another liquid. A molecule that is located in this area has other molecules of the same type above it. If a molecule that is located Nallo boundary layer is raised, the links between it and the adjacent molecules are stretched, creating a force which tends to draw the molecule to the surface. Similarly, by placing a tiny body on the surface of a liquid, the surface molecules latter is pushed downwards by generating a return force directed upwards.

The surface of a liquid then behaves like a stretched membrane.

The surface tension of a liquid is effort that must be done to bring a ample number of molecules from the inside of the liquid to the surface to form a new unit area of said surface. This work numerically coincides with the contraction force exerted on a hypothetical line laid on the surface.
It defines the coefficient of surface tension measured in N / m or J / m2

The surface tension is measured directly by measuring the force required to break the surface of the liquid raising a thin wire from the surface itself.
Needle up from the surface

It is found that this force is directly proportional to twice the length of the wire (which isi explains what thinking that there is the surface of liquid on both sides of the wire).
The force required to pull a thread mass m and length L is:

F = 2 tau L + m g