Mechanical properties of very thin surface films

Abstract

This paper comprises two parts: the first briefly describes some earlier work on the determination of the shear strength of a calcium stearate bimolecular layer deposited between two mica surfaces. The second part deals with some more recent studies on the mechanical properties of thin liquid films sandwiched between a rubber and glass surface. The rubber surface was soft and optically smooth so that certain liquids squeezed between it and polished glass formed a very thin film of uniform thickness. Squeeze film studies made with this system revealed the importance of repulsive forces exerted by electrically-charged double layers residing on the solid surfaces. These forces were able to support the externally applied pressure so that an equilibrium film some 200 Å thick was obtained.

When rubber and glass surfaces were sheared in the presence of an electrolyte solution, double-layer repulsion helped to support the normal load and so protect the surfaces from serious abrasion. The presence of a soap in the solution led to further protection by an oriented monolayer on the rubber surface. Measurements suggested that these two factors operated to provide effective lubrication—the electrical repulsive forces to keep surfaces apart and the SDS monolayer for increased protection at points of intimate contact where the separating fluid had been locally penetrated.