What is interface force?

The force that combines two forces is called interface force. It can be an intermolecular force, an electrostatic attraction, or a chemical bond. Calculations show that when two solids are close to each other within 0.4nm, that is, the effective short range of the intermolecular force is reached, the intermolecular force can reach 10 8 ~ 10 ⁹ N ^/ m ² (100 ~ 1000MPa), even if Both can be bonded without adhesive. The tensile strength of various materials is shown in the table.

But in fact, the smoothest surface that can be artificially produced still has an unevenness of about 20.0nm. In this way, the contact between the two solid planes is only the local contact of the raised part, and there are few contact points, and the area is not enough to generate adhesive force. In order to achieve full contact between these concave and convex surfaces, adhesives are required. The role of the adhesive is to change the solid-solid contact into a liquid-solid contact. Utilizing the characteristic that the liquid can flow, the uneven surface can be flattened, thereby increasing the contact area between the interfaces to improve the bonding strength.

The mechanical strength after bonding depends not only on this interfacial force, but also on the mechanical properties of the interface region and the two bulk phases. If the fracture occurs in the bulk phase away from the interface or in a thin layer close to the interface, it is called cohesive fracture; then the bond can be considered to have sufficient strength. When fracture occurs in the interface region, it is called interfacial fracture, which indicates that the adhesion is weak.

Since the role of the adhesive is to transform the solid-solid contact interface into a liquid-solid interface, the wetting of the liquid and the solid is very important.