Today we continue with Physics, and more specifically the branch of "Classical Mechanics", in order to talk about Surface Tension.
So, without further ado, let's get straight into it!
Surface Tension
In the previous post about Buoyancy, while relating the concept to density we found out that only objects with a density less than that of the fluid can float. But, there are still cases where denser objects float. For example, a needle which is much denser than water still floats on top of it. This is caused by a phenomenon known as surface tension.
Responsible for surface tension are the cohesive (intermolecular attractive) forces between the molecules in a liquid. All molecules within a liquid interact with other molecules in all directions. But, in the case of the surface of a liquid, only inward cohesive forces are present, which makes those cohere more strongly to molecules on the surface.
Adhesive Forces
The forces of attraction between a liquid and a solid are called adhesive forces. Adhesion is basically the attractive force between unlike molecules. The difference between those and the cohesive forces determine the behavior the liquid shows when in contact with a solid surface.
S.I. Unit
Surface tension is calculated in N/m, but it's quite common to use cgs or dynes/cm, which equals 10-3N/m = 1 mN/m.
The surface tension of water at 25°C is 72 dynes/cm. It decreases significantly as temperature increases.
Mathematical Definition
Surface tension is denoted by γ and is defined as:
where d is the total length of contact between the liquid.
Thus, if an object has length l, the total "distance" of contact would be d = 2l, as it would interact with the liquid on two sides or surfaces. Each side receives an equal force of:
Capillarity
Surface tension also explains capillary action (or capillarity), which gives some liquids a concave and others a convex shape when confined in a tube. The meniscus is caused by the adhesive forces between the liquid and the glass material of the tube.
Water rises at the side as the adhesive forces between it and the glass tube are stronger (water "wets" glass), whilst a fluid like mercury shows stronger cohesive forces, causing it to rise upwards in the middle.
