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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.