Solid Surfaces—Adsorption - Surface and Colloid Chemistry: Principles and Applications - page 126

Solid

Lubricant

Solid

FIGure 5.12 Friction reduction due to lubrication by adsorbed film (bilayer) of amphiphiles

(as lubricant).

The solid surfaces that one finds are manufactured by different methods: sawn, cut,
turned, polished, or chemically treated. All of these procedures leave the solid sur-
face rough, in varying degrees. In industry, one finds many methods that can create
the roughness.
Polishing is also an important application area of the surface chemistry of solids.
The surface layer produced after polishing may or may not remain stable after expo-
sure to its surroundings (air, other gases, oxidation). The polishing industry is much
dependent on surface molecular behavior.

5.6.3.2 Friction (between Solid1-Solid2)
Friction is defined as the resistance to sliding between two bodies (Figure 5.12).
Friction is lowered using a suitable lubricant (specific for each system; Biresaw
and Mittal, 2008). In cases where the solids are very close, the surface roughness
becomes the determining factor. That means the resistance is higher between two
rough surfaces. The degree of plasticity or deformation of the solids will also effect
the friction. Further, the lubricant will give high sliding resistance if its viscosity is
high. Thus, when one solid is sliding or rubbing against another, there are a variety
of parameters. These parameters are also called “tribology” (related to rubbing).
The magnitude of friction can be appreciably reduced if boundary lubrication
decreases the force field. This may be achieved by adsorbed films (Figure 5.12).
However, one may expect that this is a highly idealized model, and some modifica-
tions will be necessary.

5.7 SurFace tenSIon oF SolId PolymerS
The surface tension of polymers (synthetic polymers such as plastics, biopolymers
such as proteins and gelatin) is indeed of much interest in many areas. In industry
where plastics are used, the adhesion of these materials to other materials (such as
steel, glass) is of much interest. The adhesion process is very complex since the
demand on quality and control is very high. This is also because adhesion systems are
part of many life-sustaining processes (such as implants, etc.). The forces involved in
adhesion need to be examined, and we will consider some typical examples in the
following text.