Nor is it unique to employ scales based upon comparison. Hardness scales are such. Although these are based on surface energy, there are other factors in play so that SE alone does not determine hardness.

This is new information for me. (Not the comparison aspect, but the surface energy...) I know several hardness scales are based primarily on measuring the elastic limit of the material. If I conclude the elastic limit is governed in large part by surface energy, would I be far from the mark?

 

Hi,
We investigated controlling parameters of hardness in brittle materials by exploring the correlation between hardness and shear mode cracking. Density functional theory was used to calculate the unstable stacking energies (shear resistance to irreversible deformation) and the surface energies (tension resistance to fracture) for comparison.

 

The following qualitative theory works for crystalline solids, not jellies.

Surface energy arises because all the atoms or molecules at an external boundary have unsatisfied bonds and are thus in a higher energy state. The sum of all these higher energies is called the surface energy.

Each time new surface is created energy must therefore be added. This is the basis of dislocation theory and fracture mechanics.

When we scratch a rock with a penknife to measure the Mohs hardness we are opening new surface and adding new energy.
Similarly, engineering indenters have to add energy to enlarge the surface and the effort is proportional to the energy needed and thus provides a measure of the hardness.