Have you ever overfilled your glass of water so that the water level is higher than the glass and still the water remains in the glass? This seemingly gravity-defying observation indicates there is a force that wants to keep the liquid together. This force is called the surface tension.
Surface tension explained
Imagine the interface between a liquid and a gas. Most of the molecules in both phases are in the bulk but some of the molecules are at the interface facing the other phase. The molecules in the bulk interact similarly with all the molecules surrounding them – there is a similar pull to all directions resulting in zero net force.
On the contrary, the molecules at the interface experience a stronger pull towards the bulk of their own phase. The net force, which effectively aims to keep the liquid together, is called surface tension.
What is the unit of surface tension?
Mathematically, surface tension is the force exhibited by the intermolecular forces divided by the length of the contact line between the phases. Therefore, the unit of surface tension is N/m. The forces are generally so small that the unit of mN/m has established itself as the standard unit being used. Water, for example, has a relatively high surface tension of 72 mN/m in room temperature due to its strong hydrogen bonds.
Many of the measurement methods for surface tension are also based on directly measuring the force exhibited by the surface tension per length of a measurement probe.
To learn more about what is surface tension and how it can be measured, please download the white paper below.