Abstract
We present sliding experiments of a sledge on wetted sand and describe that the frictional response is controlled by the penetration hardness of the granular medium. Adding a small amount of water to sand increases the hardness which results in a decrease of the sliding friction. Pouring even more water to sand results in a decrease of the hardness and a subsequent increase of the friction. This inverse correlation between hardness of a wetted granular material and its frictional response to sliding is found to be due to ploughing of the sledge. When the load of the sledge exceeds the penetration hardness of the water-sand mixture the granular material is irreversibly deformed, which is evident by a trace of the slider left after its passage. The penetration hardness sets how deep the trace of the slider is which, in turn, controls the ploughing force. Consequently, increasing the hardness of the water-sand mixtures makes pulling a sledge over it easier. In addition, we quantify the critical shear strain which sets the transition of an elastic to plastic response of (wet) granular materials which enables us to directly relate the shear modulus, in the elastic regime, to the hardness, in the plastic regime.Graphic abstract
Highlights
The mechanical response of sand is largely dominated by the amount of water added to it
The friction coefficient of a sledge sliding over a water-sand mixture defined as the friction force over the normal force is measured for increasing water volume fractions w
A roughly inverse correlation between the penetration hardness and sliding friction coefficient is obtained, which is due to ploughing
Summary
The mechanical response of sand is largely dominated by the amount of water added to it. With a small amount of water, a pile of sand can be made into a sandcastle where, too much water results in a muddy puddle [1,2,3,4]. Pouring water in sand enables the formation of capillary liquid bridges between the grains where the curvature of the liquid interface leads to a capillary pressure [5,6,7,8]. The capillary bridges act as a cohesive force between the grains which, for example, enables building a sandcastle [1, 2]. The elastic response on deformation can be quantified by the increase of the elastic shear modulus G′ with increasing water volume. Liefferink and Mojgan Aliasgari have contributed to this work
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