Wettability is the preference of a liquid to be in contact with a solid surrounded by another fluid (liquid or gas). Depending on the application, wettability can be wanted or not. Take for example a newly waxed car. The purpose of adding the wax is to prevent the water from spreading and to prevent corrosion of the car. In this case, the aim is clearly to reduce the wettability. Then, on the other hand, good wettability is needed when for example coating is applied on the surface (wax on top of the car).
Intermolecular forces define the degree of wettability
Adhesive and cohesive forces between the solid and the liquid determine the wettability. Cohesive forces affect in between the same type of molecules. In a liquid or solid, the molecules are pulled towards each other because of the cohesion between the molecules. Adhesion is the interaction between unlike molecules. The balance between these forces is what defines the degree of wettability. For example, if a drop of water on the solid substrate beads up, ethanol on the same substrate is likely to spread. This is because in the water there are strong cohesive forces called hydrogen bonds that pull the molecules towards each other. In ethanol, van der Waals forces are responsible for cohesive interactions and are much weaker than hydrogen bonds. This will cause ethanol to spread more easily on solid surfaces.
Solid also matters. Solids can be divided into high and low-energy solids. High energy solids such as glass, ceramics, and metals are held together by chemical bonds (covalent, ionic and metal) which are very strong. This causes high excess energy on the surface of the solid i.e. the name high energy solid. Most of the liquids will wet the high energy solid completely as that will lead to a decrease in interfacial energy. Low-energy solids, such as many of the polymers, especially Teflon are harder to wet.
Wettability can be measured through contact angle
Contact angle is a measure of wettability. A drop of liquid is placed on the solid and contact angle can be optically measured. Typically, 90° contact angle is considered as a threshold value. When the contact angle is above 90° the wettability is bad, when it is below 90° the wettability is good. Complete wetting is achieved when the contact angle is zero, although in practice when contact angles are below 5° the surface is typically considered to be completely wetted. When water is used as a measuring liquid, the surface is called hydrophobic when the contact angle is above 90° and hydrophilic when it is below the same value.
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Contact lens should support a continuous tear film for optimum visual clarity. It should also resist the sorption of tear components as deposition can lead to decreased visual clarity and discomfort. Wettability of the surface affects both the tear film as well as biocompatibility of the lens.
In industry, wettability is important for example when manufacturing electronics or when planning how to extract oil from the oil reservoir. In all these applications, knowing the contact angle value gives a strong indication on the performance of the product or a successfulness of the process.
Contact angle is easy to understand visually. It is the angle a droplet forms in contact with a solid surface. Thermodynamically, it is a balance between different forces acting on a three-phase contact point where solid, liquid and air meet.