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Why is the energy on the surface of a solid always higher than its interior? Do you know the secret behind it?
In materials science, the surface energy of solids is an important concept in understanding the properties of solid materials. When analyzing the crystal structure and surface properties of solids, it is not difficult to find that the energy on the surface of a solid is always higher than the energy inside. Why is this happening? This question not only involves face, but also goes deep into the microscopic atomic structure and chemical bonding.
The existence of surface energy arises from the difference in bonding between surface atoms and internal atoms: surface atoms are not as tightly connected to their neighboring atoms as internal atoms.
When a solid material is cut, this action causes the solid's internal structure to break and create new surfaces. This is because inside a solid, the bonds between atoms are stable and each atom is surrounded by other atoms, forming a strong network structure. The situation is different for surface atoms. They are not fully integrated with the surrounding atoms. This incomplete bonding causes the surface atoms to have a higher energy than the internal atoms, so we would think that the surface energy of a solid is always higher than its internal energy.
This "excess energy" represents unrealized bonds and is one of the main reasons for the high energy of solid surfaces.
Measurement and evaluation of surface energy
Depending on what they need to know, scientists have developed a variety of methods to measure the surface energy of solids. One of the most common methods is the contact angle test. This method calculates the surface energy of a solid surface by measuring its contact angle with liquid penetration. When the contact angle is small, it means that the liquid penetrates well into the solid surface, and its surface energy is high; conversely, when the contact angle is large, it means that the solid's attraction to the liquid is weak and the surface energy is relatively low.
The convenience of this test is that it does not require too much experimental equipment and can be applied to a variety of different materials, making it convenient for academic research and industrial applications.
Calculation of solid surface energy
Take the deformation of a solid as an example. When a solid is subjected to stress, the changed surface energy can be regarded as "the energy required to create unit surface area." This concept helps us understand how the physical properties of solids change under different conditions. For example, using Density Functional Theory (DFT) we can predict the surface energy of solids and further understand the property changes during cooling, heating and deformation of materials.
In addition, through experiments on solids at high temperatures, their surface energy can also be measured more accurately. In this case, the solid exhibits different flow properties, thus changing its surface area while maintaining almost the same volume.
Interface energy and wettability
Another noteworthy aspect is the interface energy, which has a significant impact on the thermodynamic parameters of the material. The "wetness" of a liquid on a solid becomes apparent when we consider a drop of liquid laying flat on a solid surface. This is further related to the surface energy of the solid, as different surface energies lead to different wetting behavior of the liquid.
Wetting is not only a macroscopic phenomenon, it is also based on microstructural interactions, such as the affinity of atoms to contact surfaces.
Conclusion
Why the surface of a solid always has higher energy than its interior is rooted in the characteristics of its atomic structure, unrealized bonds, and the reactions of solids in different environments. The study of surface energy is not only an important topic in materials science, but also has implications for various engineering applications. As we further explore these phenomena, we can’t help but think: In the future of materials science, how can surface energy properties be used more efficiently to create more efficient materials?
