Liquids’ ability to wet a solid surface has widespread importance in many everyday products and industrial processes. In everyday life, wettability is experienced when you wax your car or wear your Gore-Tex jacket on a rainy day. In an industrial setting, understanding wettability is essential for designing and optimizing processes such as coating, painting, lubrication, and even medical diagnostics and treatments.
Wettability is a fundamental concept in surface science that describes the ability of a liquid to maintain contact with a solid surface. Wettability refers to the degree to which a liquid can spread over or adhere to a solid surface. It is determined by the balance between adhesive forces (attraction between the liquid and the solid surface) and cohesive forces (attraction within the liquid molecules). When adhesive forces dominate, the liquid spreads, leading to high wettability. Conversely, when cohesive forces dominate, the liquid tends to form droplets, resulting in low wettability.
Wettability is affected by many factors. Both the liquid and the solid properties play a role.
Surface tension of the liquid: Surface tension is a property of a liquid that gives rise from intermolecular interactions at the air-liquid interface. In the case of water, the bonds in the bulk of the liquid are fairly strong hydrogen bonds, which leads to high surface tension compared to liquids with weaker van der Waals forces between molecules. High surface tension leads to lower wettability at the solid surface. In water-based formulations, surfactants are often used to break hydrogen bonds, leading to lower surface tension and thus better wettability.
Surface free energy of the solid: Surface free energy is a property of a solid and equivalent to the surface tension of the liquid. High surface energy materials tend to be more wettable. Metals and glass typically have high surface energy, while plastics and other non-polar materials have low surface energy. Surface treatment, like the plasma treatment of a polymer, is often used to increase the surface free energy and thus improve wettability.
Surface roughness: Surface roughness directly affects the wettability of the surface. Rough surfaces will typically enhance the wetting behavior arising from the chemical composition of the surface. For example, if you place a water drop on a surface that is completely smooth but has some affinity towards water, adding surface roughness will increase this affinity.
The most common method to quantify wettability is by measuring the contact angle. The contact angle is the angle formed between the tangent to the liquid surface at the point of contact with the solid and the solid surface itself. Here's how it works:
Wettability is a crucial property that influences how liquids interact with solid surfaces. By understanding the basics of wettability, including how to measure and control it, we can optimize a wide range of processes and applications. Whether it's improving the durability of a coating, enhancing oil recovery, or developing advanced medical devices, the principles of wettability are integral to innovation and efficiency in numerous fields.
If you would like to learn more about wettability, please register for Attension User Day held online on the 3rd of June!
Susanna is an Application Scientist at Biolin Scientific. In her PhD thesis, she developed fabrication methods for a new type of inorganic-organic polymers. Microfabricated polymer chips were utilized as tool for biomolecule separation in analytical chemistry.
