Friction
Cut to Chase
Introduction
Friction is an everyday physical phenomina and yet the precise mechanisms underlying it at the microscopic scale are still not completely understood. Research into friction is essential for advanced nanotechnology.
What is Friction?
Friction is the name given to the force that opposes the motion of one body sliding over another. It is called friction when the objects are solid, viscosity in liquids. It always opposes the motion of an object.
Static Friction
At an interface between two objects, the surfaces may appear to be smooth, however on a microscopic scale the surfaces are pitted and jagged. When another similar surface is in contact with it, the irregularities and peaks interlock. Australian Physicist, Frank Philip Bowden who worked on friction is quoted as saying: "Putting two solids together is rather like turning Switzerland upside down and standing it on Austria—the area of contact would be small."
The static friction depends on:
- The two materials and the nature of their surfaces. Rougher surfaces have higher values of friction but Machinists know that two very smooth, clean metal surfaces may stick to each other firmly and be very difficult to slide apart. This makes sense in terms of a model in which friction is described as arising from chemical bonds between the atoms of the two surfaces at their points of contact: very flat surfaces allow more atoms to come in contact
- The force pushing the two surfaces together. Pushing the surfaces together causes the more asperities to come together and increases the surface area in contact with each other
The Limiting Friction is the largest value of the static friction.
F=μN where F is the frictional force, μ is the coefficient of friction and N is the normal force
Dynamic Friction or Kinetic Friction
Once an object has started to move, the irregularities of the surface must run over each other.
Nanotribology
Nanotribology is the study of friction and wear processes on the nanometer scale. In force microscopy, friction forces arise when the probing tip slides over the sample surface with a well-defined normal load. They are detected as torsion of the cantilever sustaining the tip. On some materials friction can be traced down to the atomic scale.
