Tools to Probe Nanoscale Surface Phenomena in Cellulose Thin Films: Applications in the Area of Adsorption and Friction - The Nanoscience and Technology of Renewable Biomaterials - page 99

where V is the speed of the moving (sliding) material (for example fiber); η b is the bulk
viscosity of the lubricant and P is the pressure applied between the two sliding surfaces.
In full-film lubrication (aka hydrodynamic lubrication) the surfaces are separated by
a thick lubricant film. Ideally there is no wear of the solid surfaces and the friction
is determined by the rheology, surface chemistry, and intermolecular forces of the bulk
lubricant. During boundary lubrication regime the load is carried by the surface asperities
and the lubricant film and the friction behavior is determined by the dynamic properties
of the boundary film. In the intermediate mixed region both the bulk lubricant and
the boundary film do play key roles. Under these conditions the properties of the
adsorbed components and the chemistry and dynamics of the interfacial region between
the tribosurfaces are of utmost importance.
In the Stribeck curve, the bulk viscosity η b applies to all the cases considered, from
wide to narrow gaps between the sliding surfaces. However, in reality, the local or
microscopic effective viscosity η eff may be quite different from the bulk viscosity η b
especially in the case of very confined systems of ultra narrow gaps (Cho, Cai et al .
1997).
Luengo, Israelachvili and Granick proposed a set of improved Stribeck-type curves
that are based on experimental data typical in engineering conditions. The corresponding
generalized map of friction force against sliding velocity in various tribological regimes
were also discussed (Luengo, Israelachvili et al . 1996). In the boundary layer film η eff
is noted to be much higher that the bulk value, η b . As the shear rate increases a point
is reached where the effective viscosity starts to drop with a power-law dependence
on the shear rate. As the shear rate further increases, a second Newtonian plateau is
encountered. At higher loads η eff continues to grow with load and transition to sliding
at high velocity is discontinuous and usually of the stick-slip type. While this chapter
covers the general topic of adsorption and lubrication, our emphasis in the next sections
will be the chemistry and adsorbed layer state of polymeric surfactants. Issues related
to roughness, asperities and others are not considered here.

4.6 Boundary Layer Lubrication

In the boundary lubrication regime, the load is carried by a lubricant thin film. A typical
lubricant film usually has a thickness of 100 nm or lower, i.e., only several hundreds of
molecules thick (Guddati, Zhang et al . 2006; Guo, Li et al . 2006; Izumisawa and Jhon
2006). Studying the structure of lubricant thin films and how the molecules organize
during the lubrication process is of utmost importance. In this regime physisorption (as
opposed to chemisorption) is a dominant effect since during fiber processing the lubricant
film is not always intended to be retained onto the surface (in some cases the lubricant
on fiber surfaces could interfere with successive processes or uses of the fiber). The
robustness or strength of adsorbed layer of lubricants during fiber processing is an issue
that has not been addressed systematically.

4.6.1 Thin Films: Property Changes and Transitions

As discussed above, the properties of lubricant thin films change depending on their
distance from the surface. When the thickness of the adsorbed film is comparable to the