A drop of liquid placed on a solid surface forms the contact angle Θ. The contact angle depends on the surface energy of the solid, the solid-liquid interfacial energy, and the surface tension of the liquid. This relation can be expressed by the Young equation. Contact angle measurements provide information about the wetting behaviour of a liquid on a solid.

The contact angle at the three-phase contact line

To describe the situation at the interface between different materials in more detail, the contact angle Θ is one of the most important measures. The contact angle can be measured by depositing a drop of liquid on a solid surface. Now three different phases - solid, liquid and gas - meet at the three-phase contact line.

If the system is at rest, there is an equilibrium of tangential forces at the three-phase contact line. In this state, the static contact angle Θ_C is formed at the three-phase contact line. If the system is in motion, for example when the solid surface is tilted, the dynamic contact angles Θ_Adv and Θ_Rec can be determined.

Measurements of the contact angle can be made with contact angle meters, such as those in the OCA series from the manufacturer DataPhysics Instruments. Worth mentioning here is the large number of additional modules that enable investigations for a wide variety of applications. These include nanolitre and picolitre dosing systems for particularly small droplets, temperature and environmental control systems for investigations in the temperature range between −30 °C and 1800 °C as well as humidity generators for controlling the air humidity in the measuring chamber.

The Young equation: relationship between material properties and contact angle

In the static case with contact angle Θ_C, there is a vectorial equilibrium between surface energy of the solid σ_S, surface tension of the liquid σ_L and the solid-liquid interfacial energy σ_SL (see Figure 2). This equilibrium is expressed in the Young equation:

Where:

• σ_L: surface tension of the liquid
• θ_C: equilibrium contact angle
• σ_S: surface energy of the solid
• σ_SL: interfacial energy between liquid and solid

The surface energy of the solid and the interfacial energy between solid and liquid are usually unknown. However, if the contact angle of several test liquids on a solid surface is measured, the surface energy of the solid can be determined with the help of interaction models (see: Determining surface energy). In addition, further investigations of the work of adhesion are possible.

Contact angles provide information about wetting properties of the solid surface

Measuring the contact angle of a liquid on a solid provides information about the wetting behaviour of this combination. The desired wetting behaviour depends on the particular application.

With a contact angle of 0°, the droplet is completely spread on the solid surface. The drop forms a thin, sometimes monomolecular liquid film on the solid surface. This case is called complete wetting. With a contact angle of 180°, the drop lies spherically contracted on the surface and touches the solid only at one point. This case is called complete dewetting.

In practice, the contact angle usually lies between these two extreme values. Solid surfaces on which a contact angle Θ_C of less than 90° is formed are generally described as having good wettability. Higher contact angles indicate that a surface is hardly wettable or not wettable at all.

One of the most vivid examples of particularly high contact angles can be found in nature: water droplets roll off the leaves of the lotus plant. No wetting takes place and the contact angle is correspondingly high. Because the beading drops transport dirt particles away, the leaves possess a self-cleaning effect. Today, attempts are being made to emulate this lotus effect in many technical areas: self-cleaning facades, ceramics and window glass are examples of such technical developments. Contact angle measurement is indispensable in the development of such products.

Contact angle measurements are the basis of many standards and norms that describe wetting characteristics. A substantial list of standards can be found here.

Application examples for contact angle measurements

Contact angle measurements are always useful when solids and liquids meet and when wetting and adhesion properties need to be controlled or modified. The fields of application are accordingly diverse. Important examples are:

• Surface coatings: Measurement of the contact angle on coated materials to evaluate the quality and uniformity of the coating.
• Painting and printing industry: Evaluation of the wettability of paints, varnishes and printing inks on various substrates.
• Textile industry: Determination of the wettability of textile fibres to check the quality of textile treatments, such as impregnation or dewatering.
• Medical devices: Characterisation of the wettability of medical implants to assess their biocompatibility.
• Food industry: Evaluation of the wettability of packaging materials to ensure printability and improve storage properties.
• Semiconductor industry: control of wettability on surfaces of semiconductor chips and substrates for microchip manufacturing.
• Environmental science: measuring the contact angle on leaf surfaces of plants to study self-cleaning effects and water distribution.
• Nanotechnology: Characterising the wettability of nanomaterials and nanoparticles to optimise their properties in various applications.
• Oil and gas industry: Assessing the wettability of rock surfaces to better understand the behaviour of oil and gas reservoirs.
• Cleaning and degreasing processes: Evaluating the efficiency of cleaning and degreasing agents on different surfaces to optimise effectiveness.