Interfaces form where two phases meet. Interfaces have various properties, such as their interfacial tension or zeta potential, which can be quantified with the right measurement methods. The study of these properties is important when an interface is modified, for example to produce stronger or weaker wetting or adhesion.

What is the difference between an interface and a surface?

An interface is the contact plane between two phases which do not mix. A phase is an area in which physical quantities do not change abruptly. Phases can be solid, liquid, or gaseous. There are five types of interfaces:

• solid-solid
• solid-liquid
• solid-gaseous
• liquid-liquid
• liquid-gaseous

Since gases mix with each other, there are no gaseous-gaseous interfaces. The term surface is commonly used for solid-gaseous and liquid-gaseous interfaces. Thus, interface is the generic term for all contact areas. A surface refers to two specific forms of interfaces.

What does an interface look like in detail?

Interfaces are not sharp, two-dimensional boundaries. Especially in the case of the liquid-gaseous interface, there is a three-dimensional, extended area where the two phases transition gradually into each other (see Figure 1). However, for macroscopic observations, the simplified assumption of a thin interface with an abrupt transition is sufficient.

What properties can interfaces and surfaces have?

Interfaces can have properties that differ from the rest of the phase. In recent decades, therefore, a separate scientific branch has emerged that deals exclusively with the study and modification of interfaces and surfaces.

The precise characterisation of interfaces is of great importance not only in research but also in many industries. The properties of interfaces are extremely versatile. The measuring instrument manufacturer DataPhysics Instruments has therefore specialised in the investigation of the most important parameters.

The investigation of the wetting behaviour of a liquid on a solid is relevant in many industries. Important parameters for the wetting behaviour are static and dynamic contact angles. Contact angles can be measured, for example, when a drop sits on a solid surface. Other parameters, such as the surface energy of solids with their polar and dispersion components, can also be determined from the contact angle.

The adhesion between two phases results from the interfacial energy and other properties of the interface, such as its structure and roughness. The adhesion can be calculated as the work of adhesion or measured directly as an adhesive force.

To investigate the charge situation near the surface, the measurement of the so-called zeta potential is useful. DataPhysics Instruments offers a measuring device which can investigate the zeta potential of powders, fibres, and plate-shaped solids.

Other parameters include the surface and interfacial tension of liquids. The surface tension that a liquid develops in the surrounding air influences the wetting behaviour of the liquid on a solid. The interfacial tension between two liquids can provide information about how these two liquids behave in regard to each other.

To analyse the stability of liquid dispersions over time, an optical stability and ageing investigation has proven useful. Liquid multiphase mixtures should thus be examined regarding their destabilisation processes, such as sedimentation and creaming.

Environmental influences can change the properties of interfaces. These include, for example, the concentration of surfactants, air humidity, or temperature. These environmental influences should be monitored and, if necessary, controlled in surface science experiments.