Jumping in Steps as Molecules are Squeezed Out

The key question about adhesion of surfaces in the presence of contaminating molecules is what happens as the last few molecules are squeezed out from between the surfaces. It has been known since Langmuir s time that certain contaminant molecules can stick very strongly to a surface in a monolayer and that further multilayers can build up with progressively weaker bonds as shown in Fig. 6.13. 

Bowden studied this for 30 years especially to find the conditions for removing the final tenacious bonded layer. ° In one of the last papers before he died, Bowden described experiments to clean-up surfaces of diamond, sapphire, quartz and other oxides. This was important for space exploration because the harsh clean conditions of space could cause bearings to seize up. Bowden and his 

I igure 6.13. (a) Cloan surface (b) monolayer of cooUmination (c) multilayer contamination. 

In this case, the ultimate adhesion of the mica surfaces was weak, with an adhesion energy of 11 This was beneficial because no damage was then 

But the most significant result of this experiment, and of subsequent work by Horn and his coUeagues on other solvent molecules, was the demonstration that the systen could now sit stably at several different stages of adhesion, shown by the zero force points in Fig. 6.14. In other words, there is not just one single 

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