Interfacial energy explained

# Interfacial energy explained

Figure 1: Using interfacial energy, it is possible to make statements about wetting properties.

To enlarge an interface, work must be done to move the atoms or molecules to the interface. This work remains stored as energy in the atoms or molecules at the interface and is called the interfacial energy.

## What is the interfacial energy?

The atoms or molecules of a solid or liquid are held together by interactions. Inside the phase, the atoms or molecules are in equilibrium of forces. At the interface, the forces from outside are much weaker, which is why there is a force into the interior of the phase (see Figure 2).

To bring an atom or molecule to the interface of a phase against this force, work must be done. This work remains stored in the atom or molecule at the interface as potential energy. The stored energy is called interfacial energy.

In other words, the interfacial energy is the energy needed to enlarge the interface. In physical terms, the energy ΔWInt is required to enlarge the interface by the amount ΔA. The interfacial energy ε is given in J/m2 and defined as:

Figure 2: At the interface, a resulting force is directed into the interior of the phase.

## What effects does the interfacial energy have?

The higher the interfacial energy of a solid against air (also called surface energy), the lower the contact angle. The contact angle is the angle a liquid drop forms on this surface. If the contact angle is low, wetting is greater. Generally, this means that the adhesion between the liquid and the solid is greater. In industry - for example, in bonding and printing processes - a high surface energy of the solid is therefore desirable.

## What is the difference between interfacial energy, surface energy, interfacial tension, and surface tension?

Interfacial energy is the generic term that can be used for all phase boundaries. However, separate terms have become established for particularly common interfaces. For a solid-gas phase boundary, one often uses the term surface energy.

The interfacial tension is another way of describing an interface. While the interfacial energy describes the potential energy that would be required to enlarge an interface, the interfacial tension describes the tensile force that counteracts an enlargement of the interface. While interfacial energy is measured in the unit of joules per square meter, interfacial tension has the unit of Newtons per meter. However, the two concepts are very closely related. In fact, for liquid-liquid and liquid-gas phase boundaries, the interfacial or surface energy is the same as the interfacial or surface tension. For the sake of simplicity, the same formula symbol σ is therefore often used in the literature for the interfacial and surface energy as well as the interfacial and surface tension.