<p>Have you ever had the impression that heavy items of furniture start  to take root &ndash; that after years standing in the same place, they&rsquo;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&rsquo; play tray set into a lump?</p>
<p>If  so, you&rsquo;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 <a href=\"http://prl.aps.org/abstract/PRL/v109/i18/e186102\">slowly chemically bonding together</a>.</p>
<p>There are already several other explanations for this so-called  &ldquo;frictional ageing&rdquo; 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&rsquo;t depend on the area of the  contacting surfaces. You&rsquo;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&rsquo;s true even  for apparently smooth surfaces like glass, which are still rough at the  microscopic scale. It&rsquo;s only the contacts between these asperities that  cause friction.</p>
<p>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.</p>
<p>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.</p>
<p>Yet these effects can&rsquo;t explain all kinds of frictional ageing. For example, last year <a href=\"http://www.nature.com/nature/journal/v480/n7376/full/nature10589.html\">a team of US researchers showed</a> 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.</p>