Researchers from the Department of Mechanical Engineering of Katholieke Universiteit (KU) Leuven conducted a nanoscale study on friction. They observed that an object not only slides over another object during friction but also moves up and down slightly. The phenomenon was termed as lift-up hysteresis.
During late 17th and mid 18th century, French physicists Guillaume Amontons and Charles August Coulomb struggled to derive an explanation for frictional resistance. For example, it is difficult to move a cabinet across a floor when compared to a chair due to the frictional resistance. Actually, the cabinet’s bottom and the floor move against one another, however, the weight of the cabinet is perpendicular to the cabinet’s bottom and the floor. Due to this ‘normal’ movement, the cabinet and the floor tend to move together instead of moving in the opposite direction, which results in frictional resistance. On the other hand, if the cabinet and the chair are moved uphill using wheels, more force will be required to move the cabinet.
Amontons and Coulomb explained this phenomenon through ‘bump hypothesis’ theory, where friction was explained through the roughness of two surfaces, the asperities. When the objects are at rest, the nooks and crannies of one surface settle upon another surface. During friction, the asperities act as slopes which help the movement of the objects in the opposite direction.
However, in the 20th century, the theory failed to satisfy the laws of thermodynamics, since it did not explain about the loss of energy due to friction. A new adhesion theory was developed based on the electro-chemical tendency of the pure surfaces, which tend to stick to each other due to the asperities. During movement, the asperities of the two object surfaces are broken and rejoined somewhere else. Additionally, friction is influenced by acceleration and speed too. Yet, inconsistencies prevailed in this theory as well.
Currently, an atomic level research on friction was carried out by Professor Farid Al-Bender of KU Leuven and his group to find out the cause for friction. The research group used highly accurate displacement and friction sensors to examine materials like brass, plastic and paper at various speeds. The results validated the adhesion theory; however, the ‘normal’ movement, developed by Amontons and Coulomb, has to be considered as well. The experiment also measured the up and down movement of the objects, which was around 5 to 50 nm. The research group concluded that both ‘normal’ movement and adhesion interactions cause friction.
The research paper was published in the journal Tribology Letters.
Source: http://www.kuleuven.be