Introduction
Surface-free energy is crucial in studying materials science and interfacial phenomena. It refers to the excess energy at the surface of a material compared to its bulk. This excess energy arises due to the deviation of surface atoms or molecules from their equilibrium positions and the interactions between these surface entities. Understanding surface-free energy is essential in various fields, including chemistry, physics, materials science, and engineering.
Key Concepts
1. Definition:
Surface free energy is the excess energy per unit area at the surface of a material. It is denoted by the symbol γ (gamma) and is measured in energy per unit area units, such as J/m².
2. Origin of Surface Free Energy:
At the molecular or atomic level, molecules or atoms on the surface of a material experience different forces than those in the bulk. The imbalance of intermolecular forces leads to the existence of surface-free energy.
Surface free energy results from molecules or atoms’ tendency to minimize their exposure to the surface and maximize interactions with neighbouring molecules or atoms.
3. Surface Tension:
Surface tension γ (gamma) is closely related to surface free energy and is often used interchangeably. However, surface tension is associated explicitly with liquids and represents the force acting parallel to the surface per unit length.
For a liquid, surface tension is the work required to increase the area of the liquid surface by a unit amount.
4. Contact Angle:
The contact angle formed by a liquid on a solid surface is related to surface free energy. It describes the angle at which the liquid-vapor interface meets the solid surface. The Young equation relates the contact angle (θ) to the surface tensions of the solid (γ solid), liquid (γ liquid), and vapour (γ vapour) phases: γ solid – γ liquid cos (θ) = γ vapour.
5. Wettability:
Surface free energy influences the wettability of a solid surface by a liquid. A high surface free energy of a solid typically leads to better wetting by a liquid.
6. Applications:
Understanding surface-free energy is crucial in various applications, including coatings, adhesives, and biomaterials. For example, in coating technologies, achieving good adhesion often relies on matching or modifying the surface-free energy of the substrate and coating.
7. Measurement Techniques:
Various techniques are employed to measure surface free energy, including contact angle measurements, the Wilhelmy plate method, and the pendant drop method. Surface-free energy plays a fundamental role in governing the behaviour of materials at interfaces. Its understanding is vital for designing materials with specific surface properties, optimizing adhesion in various applications, and advancing materials science, chemistry, and engineering technologies. The balance between cohesive forces within a material and adhesive forces at its surface is intricately tied to surface-free energy, making it a key parameter in studying interfacial phenomena
