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 100

Boundary
liquid

Solid Bulk liquid

• Viscosity
• Elasticity
• Relaxation
time

Continuum
properties

Å nm µ m

Figure4.6 Schematicdiagramofhowtheeffectiveviscosity,elasticityandrelaxationchange
with thickness of a lubricant film. Reprinted from Cho, Cai etal. Copyright (1997) with
permissionfromElsevier.

dimensions of the lubricant molecules, the properties of the thin film are quite different
than those of the bulk medium (Cho, Cai et al . 1997). As shown in Figure 4.6, the
effective viscosity, elasticity and relaxation time increase with diminishing thickness
and diverge when the film thickness is sufficiently small. At these dimensions classical
continuum considerations, which can be applied to the bulk phase, do not hold for
thin films.
The diffusion coefficient of finish molecules in thin films also diverges when compared
with that in the bulk. Mukhopadhyay et al . (Mukhopadhyay, Zhao et al . 2002) found
that the molecular diffusion coefficient decreases exponentially from the edges towards
the center in systems under Hertzian contact. Hertzian contact is an ideal model to
describe deformation and lubrication. In Hertzian contact only small deformation occurs
in the contact areas as contacting bodies are elastic. Granick et al . (Mukhopadhyay, Bae
et al . 2004; Granick and Bae 2006) studied the influence of shear behavior on polymer
interfacial diffusion. According to their results shear did not substantially modified the
Brownian diffusion.
Phase behaviors of lubricants may change in confined conditions and that is one of the
main reasons why properties of thin films differ from those of the bulk. Confinement-
induced phase states of lubricant layers could change from liquid-like to an amorphous
state and then to a solid-like state (Yoshizawa, Chen et al . 1993). While low friction
is exhibited by solid-like and liquid-like layers, high friction is exhibited by amor-
phous layers. A change of some controlling variables such as temperature and humidity
may shift the phase status from the solid-like towards the amorphous or liquid-like
states. Confinement-induced solidity of lubricant was observed by Denirel and Granick
(Demirel and Granick 2001) by placing octamethyl cyclotetrasiloxane (OMCTS) liquids
between two rigid mica plates and decreasing their spacing below ca . 10 molecular
dimensions of the lubricant. This phenomena was also observed by Israelachvili and
coworkers (Israelachvili, Luengo et al . 1996; Luengo, Schmitt et al . 1997) by shearing
polybutadiene (PBD) of 7000 Daltons. They found that at low shear rates PBD exhibited
bulk-like properties in films thicker than 200 nm while in thinner films (200-220 nm)