Sliding on wet sand
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ORIGINAL PAPER
Sliding on wet sand Rinse W. Liefferink1 · Mojgan Aliasgari2 · Nahid Maleki‑Jirsaraei2 · Shahin Rouhani3 · Daniel Bonn1 Received: 10 October 2019 © The Author(s) 2020
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. Keywords Wet granular material · Friction · Hardness
1 Introduction 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, however, too much water results in a muddy puddle [1–4]. 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 [5–8]. 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 Rinse W. Liefferink and Mojgan Aliasgari have contributed equally to this work. * Rinse W. Liefferink [email protected] 1
Institute of Physics, University of Amsterdam, Science Park 904, 1098 XH Amsterdam, The Netherlands
2
Complex Systems Laboratory, Physics and Chemistry Department, Alzahra University, P.O. Box 1993893973, Tehran, Iran
3
Department of Physics, Sharif University of Technology, P.O. Box 11365‑9161, Tehran, Iran
fraction [10–12]. However, with continuously increasing the water fraction the capillary bridges starts to coalesce which decreases the shear modulus again. The consequence of the nonmonotonic behaviour of the shear modulus for increasing water fraction is evident when walking over a beach, but also interesting because it governs for instance the force necessary for sliding a sledge over sand. It was reported recently that when a sledge is pulled over a water-sand mixtu
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