Solid Surfaces in Contact under Extreme Environmental Conditions

The forces of interaction between smooth silica surfaces in contact has been investigated in order to contribute to an understanding of soil behavior in the lunar environment. The normal cohesion between two fused silica spheres (0.1 cm diameter) was measured under ultra-high vaccum conditions (10⁻⁹ torr) and in the presence of water vapor (10⁻⁹ to 18 torr). The values obtained (5-25 dynes) are essentially independent of the environmental conditions and may be accounted for in terms of the van der Waals “body” forces.

In the absence of physically adsorbed water the force required to separate two smooth fused silica spheres in tangential contact is significantly higher than the normal cohesion. The formation of chemical bonds across the unprotected contact interface as a result of micro-sliding is suggested by this observation.