Ploughing friction on wet and dry sand

The friction for sliding objects over partially water-saturated granular materials is investigated as a function of the water volume fraction. We find that ploughing friction is the main sliding mechanism: The slider leaves a deep trace in the sand after its passage. In line with previous research and everyday experience, we find that the friction force varies nonmonotonically with the water volume fraction. The addition of a small amount of water makes the friction force sharply drop, whereas too much added water causes the friction force to increase again. We present a ploughing model that quantitatively reproduces the nonmonotonic variation of the friction force as a function of water volume fraction without adjustable parameters. In this model, the yield stress of the water-sand mixture controls the depth to which the hemisphere sinks into the sand and the force that is required to plough through the water-sand mixture. We show that the model can also be used for other ploughing friction experiments, such as an ice skate that leaves a ploughing track on ice

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