Please let me know the name of a company supplying an instrument for single fiber.
Also need to understand the working principle for such instrument.
The Wilhelmy method can, in principle be used for such prupose. One actual realization:
http://www.kruss.de/products/tensiometers/k100/force-tensiometer-k100sf/
Sorry, this note was pertinent to surface energy measurement of single fibers
Depending upon the size of the fiber, you can opt for different techniques. AFM can also be a useful techique for determining the COF
COF can not be a characteristic of any material since it depends on many factors such as mating material, load, speed, lubrication condition etc. That means that you can measure it only with respect to specific conditions. AFM will only give a lateral force which, strictly speaking is not the friction force. Also how do you can fix the fiber in a holder to avoid bending. I believe the one of the solutions will be making a CF coating and measuring the friction coefficient using a tribometer.
@Tarasov
Can you clearly explain why the lateral force measured using AFM is not the friction force?
I am also not able to follow as to what do you mean by bending of the fiber while mounting for AFM testing.
Can you please elaborate?
In my opinion, the lateral force magnitude mostly depends on the mechanical component of friction force while real full scale friction force is a superposition of many components related to adhesion, plastic and elastic deformation, mechanical interaction, etc. Again nanoindentor interacts with the surface only at nanoscale real contact areas which is not the same effect as for contacting the real components. Also if you use nanoscratching test you fall into conditons of plastic ploughing or cutting and again you can not say for true that the force you obtain is a real friction force.
To measure the friction factor of isolated fiber you have apply some load and this load will bend or break your fiber because of it low axis stability and strength. In this case you also would not be able to measure the real friction force. I do not see a way to measure more or less real friction force from either AFM or nanosctratch test on the CF.
More practical way will be fixing the CF on some surface in the form of brush (or nap) coating and measuring the friction force under real contact area conditions. Also if you add some lubricant you may play with very interesting things like very low (or zero) friction which may occur as a result of stabilization of a lubricant layer by fibers agains turbulent mode in the vicinity of surface.
As mentioned above by Dr. Tarasov, friction force at a macro scale is the combination of many factors. Thus, effect of surface roughness and waviness would be contributing in the calculation of the friction force at the microscale. However, at the Nanoscale, the effect of such irregularities will reduces and the friction force calculated will be mostly influenced by the material itself, rather than surface texture. Thus, in my opinion AFM and Nano-scratching studies could provide better estimate of the friction and hence COF.
Secondly, with the use of appropriate magnitude of force (which is of course well controllable in both AFM and Nanoindentor) the concerns related to breaking and bending of the fiber can be easily avoided.
Friction is a system property, as depicted in the previous answers. You need to supply all relevant details of the intended application for the friction model. That is, what two materials are contacting, what geometry, length scale, RH, temperature and so on. Coulomb's/Amonton's law for dry friction only holds approximately for macroscopic rigid bodies.
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