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 102

cartilage lubrication. These polysaccharides (outermost cartilage layer) were not expec-
ted to be the responsible molecule for the great lubricity of cartilage. However, the
authors found that they may contribute to the loadbearing and wear protection in these
surfaces. Their study showed that a low coefficient of friction is not a requirement for,
or necessarily a measure of, wear protection.

4.7 Techniques to Study Adsorption and Friction Phenomena

A common function of thin films in boundary and mixed lubrication regimes is to offer
friction reduction and wear protection. A better understanding of thin film lubrication
will improve our knowledge of how lubricants work and this knowledge can be used to
develop superior lubricant formulations and help to predict tribological failures.
In the last few decades, rapid advancements in analytical instrumentation and tech-
niques as well as the expansion in computing power have offered an unprecedented
opportunity to unveil the behavior of lubricant polymers under boundary lubrication con-
ditions (at the atomic/molecular or nano levels). For example, atomic force microscopy
(AFM) with lateral force capabilities can measure the friction between a substrate and
a sharp tip with contact areas of a few to several hundred atoms. In fact, the lateral
resolution of LFM can be less than an atomic spacing (Behary, Ghenaim et al . 2000;
Breakspear, Smith et al . 2003). The surface force apparatus (SFA) can measure the
forces between atomically flat surfaces as their separation is varied with Angstrom level
resolution. The friction and adhesion can be studied as a function of the chemistry
and thickness of the material between the surfaces (Hu and Granick 1998; Sulek and
Wasilewski 2006; Drummond, Rodriguez-Hernandez et al . 2007; McGuiggan, Gee et al .
2007; Zappone, Ruths et al . 2007; Zhang, Hsu et al . 2007). Computer simulation has
also played an important role in interpreting and explaining the findings from these exper-
imental methods. Computer simulations and theoretical investigations have shed much
light on the molecular details underlying both structural and dynamic behavior of liquids
in the highly confined regime (Akagaki and Kato 1988; Kong, Tildesley et al . 1997).
From a molecular perspective lubricant molecules adsorb on a metal or organic surface
as ordered or oriented chains. The interactions of solid surfaces and lubricant films could
be categorized as physical adsorption or chemical reaction (Hsu 2004). As the thickness,
the adsorption mass and structure of the adsorbed layer are crucial to the performance
of lubrication (Rabinowi 1967; Grudev and Bondaren 1973; Visscher and Kanters 1990;
Gilmour, Paul et al . 2002) in situ techniques that can measure these phenomena are
needed. Surface Plasmon Resonance (SPR) and Quartz Crystal Microbalance (QCM)
are well-established noninvasive methods capable of providing a wealth of information
about interfacial phenomena in situ, in real time and in fluid media (Stockbridge 1966;
Nomura, Okuhara et al . 1981; Nomura and Okuhara 1982; Kanazawa and Gordon 1985a,
1985b; Johannsmann, Mathauer et al . 1992; Liedberg, Nylander et al . 1995; Rodahl,

Hook et al . 1995; Rodahl and Kasemo 1996a, 1996b; Mak and Krim 1997; Homola,

Ye e et al . 1999; Bailey, Kanazawa et al . 2001; Bruschi and Mistura 2001; Bailey,

Kambhampati et al . 2002; Wang, Mousavi et al . 2003; Krim, Abdelmaksoud et al .