<p>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&rsquo;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).</p>
<p>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&rsquo;s  important to understand how this friction changes over time. Most rocks  are silicate minerals, which are rather similar to silica.</p>
<p>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.&nbsp;</p>