Cohesion and adhesion
From Thermal-FluidsPedia
Cohesion is the intermolecular attractive force between molecules of the same kind or phase. For a solid, cohesion is significant only when the molecules are extremely close together; for instance, once a crack forms in a metal structure, the two edges of the crack will not rejoin even if pushed together. The rejoining can't occur because gas molecules attach to the fractured surface, preventing the cohesive intermolecular attraction from occurring.
The fundamental basis for viscosity observed in fluids is cohesion within the fluids. Viscosity is the resistance of a liquid or a gas to shear forces; it can be measured as a ratio of shear stress to shear strain. As a significant factor in the analysis of fluid flows, viscosity depends on temperature. Generally, as temperature increases the viscosity of a gas increases, while that of a liquid decreases.
Adhesion is the intermolecular attractive force between molecules of a different kind or phase. An example of adhesion is the phenomenon of water wetting a glass surface. Intermolecular forces between the water and the glass cause the wetting. In this case, the adhesive force between the water and the glass is greater than the cohesive forces within the water. The opposite case can also occur, where the liquid is repelled from the surface, indicating that cohesion in the liquid is greater than adhesion between the liquid and the solid. For example, when a freshly waxed car sits in the rain, the raindrops bead on the surface and then easily flow off.
References
- Faghri, A., and Zhang, Y., 2006, Transport Phenomena in Multiphase Systems, Elsevier, Burlington, MA
- Faghri, A., Zhang, Y., and Howell, J. R., 2010, Advanced Heat and Mass Transfer, Global Digital Press, Columbia, MO.