Have you ever had the impression that heavy items of furniture start to take root – that after years standing in the same place, they’re harder to slide to a new position? Do your best wine glasses, after standing many months unused in the cabinet, seem slightly stuck to the shelf? Has the fine sand in the kids’ play tray set into a lump?
If so, you’re not just imagining it. The friction between two surfaces in contact with each other does slowly increase over time. But why? A paper by two materials scientists at the University of Wisconsin in Madison, USA, suggests that the surfaces could actually be slowly chemically bonding together.
There are already several other explanations for this so-called “frictional ageing” effect. One is simply that two surfaces get squashed closer together. But a curious thing about friction is that the frictional force opposing sliding doesn’t depend on the area of the contacting surfaces. You’d expect the opposite to be the case: more contact should create more friction. But in fact two surfaces in apparent contact are mostly not touching at all, because little bumps and irregularities, called asperities, prop them apart. That’s true even for apparently smooth surfaces like glass, which are still rough at the microscopic scale. It’s only the contacts between these asperities that cause friction.
If two surfaces are pushed more firmly together, the asperities can get deformed and flattened so that more of the surfaces touch one another, and the friction then gets stronger. This sort of squashing and increased contact can also happen over time without any extra pressure, because the materials slowly sag, a process called creep that occurs for most materials.
Another cause of frictional ageing is that water vapour can condense into liquid in all the tiny crevices at the interface. Even if air is not humid enough for water droplets to form on a flat surface, they can condense in very small gaps, a phenomenon called capillary condensation. The surface tension of the water then holds the surfaces in place and increases friction. This effect may make powders clump together and resist flowing, because there are lots of small gaps between grains where water can condense.
Yet these effects can’t explain all kinds of frictional ageing. For example, last year a team of US researchers showed that it can happen between two surfaces of silicon dioxide (silica, which is basically sand or window glass), even when the pressure pushing them together is too low to induce creep. They figured that the surfaces must be gradually adhering because of the formation of chemical bonds between them.
It is this process that Yun Liu and Izabela Szlufarska at Wisconsin have set out to understand. For one thing, they wondered how such bonds might form, given that physical contact between materials doesn’t necessarily lead to chemical bonding. And they wanted to know why the friction increases over time according to a very specific mathematical relationship, namely that the frictional force is proportional to the logarithm of the time elapsed (which means that the increase is very gradual).
Liu and Szlufarska focused on silica too, and for good reason. One of the most important contexts for frictional ageing is in earthquake zones, where rock surfaces deep in the earth are pressed against one another along a geological fault. The likelihood of a fault slipping, creating a quake, depends on the friction there, and so it’s important to understand how this friction changes over time. Most rocks are silicate minerals, which are rather similar to silica.
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.