The Significance of Bradleys Rule

The discovery that solid bodies jump into adhesive contact under the influence of the molecular attractions was enormously significant. This experiment was certainly known to Tomlinson in 1928 and was studied both by Obreimoff and Derjaguin and Abrikossova some time later. Once you see this phenomenon, you become convinced that molecular adhesion exists. When you observe it in different situations (on mica, on glass, on metals, on polymers) then you realise it is a universal observation that applies to all bodies when there are no contaminant molecules to stop the adhesive interaction of the particles. 

Consider the wheel of a truck, which is one meter in diameter. This does not adhere significantly to the road surface, and can be lifted up without significant sticking. However, applying Bradley s rule that the adhesion force should be the 

Consider now a bacterium which is 1 jim in diameter. This now behaves in a completely different way to the truck tire, as seen in Fig. 4.14. Because of the rapid decline in gravitational force with diameter, the weight of a bacterium is now extremely small, less than 1 pN, or less than a single weak chemical bond force. But adhesion, according to Bradley s rule, has not declined so fast and is around 1 pN for a smooth bacterium or 1 nN for a rough surface contact. Thus a bacterium in dry conditions will always stick to a surface and cannot behave like a track tire which exhibits zero adhesion. A key problem with dry bacteria is releasing them from surfaces. When wet, the adhesion is deaeased substantially, as described in Chapter 6, but bacteria can still show strong adhesive behavior even under that condition (see Chapter 12). 

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