Wettability describes how a liquid spreads on a solid surface. Wettability properties can be characterized using contact angle measurements. The wettability of a surface with a liquid is influenced by factors such as surface roughness, pretreatment and chemical composition.

Wettability is a term used to describe the behavior of liquids on solid surfaces. Wettability indicates how well a liquid spreads on a surface.

How can wettability be characterized?

Wettability properties are the result of interactions between the atoms or molecules of the liquid and the surface. Wettability can be characterized by the contact angle. The contact angle is the angle between the liquid surface and the solid surface at the contact line (Figure 2).

With complete wettability, the contact angle is 0°. This means that the liquid spreads completely across the solid surface. With partial wettability, the contact angle is greater than 0° but less than 90°. In this case, the liquid does not fully wet the surface, instead forming a hemispherical drop. With poor wettability, the contact angle exceeds 90°. This means the liquid avoids the surface, and in extreme cases remains as an almost perfectly spherical drop.

Which properties of the liquid and the surface influence wettability?

Wettability is influenced by various factors that can be attributed to either the liquid or the solid surface. The surface tension of the liquid plays a crucial role in wettability on a solid surface. To achieve good wettability, the surface tension of the liquid should be as low as possible. Liquids with low surface tension spread more easily and form a thinner film on the solid surface.

The properties of the solid surface also affect wettability. Surfaces with higher surface energy can be wetted more easily by liquids than surfaces with lower surface energy. In practice, this means that many solid surfaces need to be pre-treated before further processing. For example, plasma pre-treatment can increase the polar component of polymer-based surfaces, leading to better wettability in subsequent printing or coating processes.

The roughness or structure of the surface also affects its wettability. Depending on its structure, a rough surface can improve wettability by providing a larger contact area and enabling the liquid's atoms or molecules to interlock more effectively. Conversely, other structures contribute to poorer wettability, which is desirable for self-cleaning surfaces.

How do environmental conditions influence wettability?

Actual wettability is also influenced by environmental conditions. For example, temperature can alter the behavior of liquids. As temperature increases, the surface tension of liquids usually decreases, improving wettability. Temperature also affects the viscosity of the liquid. Lower temperatures can lead to higher viscosity, making the liquid thicker. This can impair wettability.

Humidity also affects wettability, particularly with water-based liquids. High humidity can cause materials to absorb water from the air. These surfaces can then become saturated with water and be less likely to absorb water from other sources. However, high humidity can also cause condensation on cold surfaces. This can lead to spontaneous wettability, with water droplets appearing on the surface. In this case, therefore, humidity can promote wettability.

For this reason, it is advisable in sensitive applications to also consider environmental conditions such as temperature and humidity in experimental setups to obtain valid measurement results.

In which applications does wettability play a role?

The wettability of a solid surface in contact with a liquid is important in many applications. Developments range from self-cleaning surfaces to more efficient coatings.

• Wettability significantly affects the adhesion of paints and coatings to surfaces, the distribution of liquids in microstructures, and the quality of printed materials – in particular, the wettability properties of printing paper directly influence the quality of the print image.
• In medical research and practice, wettability is used to investigate implants and to manufacture medical devices.
• In materials science, wettability is an important parameter for characterizing material surfaces.
• For adhesive processes, wettability is crucial, as the adhesive must wet the surfaces to be joined well in order to ensure strong adhesion.
• In the food industry, wettability plays a role in packaging production.
• In the automotive industry, coatings are used to make surfaces water-repellent and to prevent corrosion.
• In microfluidics, wettability properties are used to control liquids in micro-scale channels.