The slow adhesion of minerals is also important for the stability of historical stone buildings, which are held together as much by friction as by mortar. And yet another reason to be interested specifically in the stickiness of silica is that bonding between very thin films of this material is important in the manufacture of silicon chips for microelectronics.
Using computer simulations of silica’s chemical behaviour, the Wisconsin pair studied how two surfaces held very close together change over time. In other words, they calculated how atoms at the surface may react with one another. If there is a little water vapour around, the silicon atoms at the surfaces react with water molecules to become ‘capped’ with hydroxyl chemical groups, composed of an oxygen and hydrogen atom. But if two of these hydroxyl-capped silicon atoms come close together, they can react further, spitting out a water molecule and leaving a lone oxygen atom bridging the two silicons. In other words, these oxygen bridges can bind the two surfaces together.
The more bonds that form, the greater the frictional force, and Liu and Szlufarska showed that the chance of a new bond forming at a particular silicon atom depends on whether there are already bonds bridging neighbouring atoms. It is this inter-dependence of bond formation that leads to the number of bridging bonds increasing in proportion to the logarithm of the time passed – just as is seen experimentally. So this bonding alone is enough to explain what is seen experimentally for frictional ageing – even if other processes, such as creep and capillary condensation, might also occur.