Understanding and tuning sliding friction

From pushing a bookcase over the floor to the relative motion of tectonic plates or sliding a newly hewn statue over land - as the ancient Egyptians did - to ice skating on frozen canals, a resistance against sliding counteracts all these movements and tries to hold the surfaces in place. Leonardo Da Vinci already observed that this friction force increases linearly with the normal force, or with the mass of the sliding object. The ratio of the friction force and the normal force is known as the friction coefficient μ and can be defined for the specific sliding system. However, it is complex to predict a friction coefficient and even more difficult to control it.
In this thesis, we make a contribution to answering the seemingly simple question, ‘What controls sliding friction?' We aimed to bridge the gap between macroscopically observed sliding friction and the underlying microscopic behaviour at the interface between the sliding surfaces. We performed sliding experiments using various shapes --- spheres, plates, model ice skates --- and various degrees of surface roughness --- as smooth as a magnifying glass or as rough as sandpaper --- to measure the friction force. We focused on three very different types of surfaces, namely wet sand, ice, and a collection of artificial surfaces whose geometry we can precisely control, to gain a better understanding of the sliding friction and, where possible, control over sliding friction.