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

中文翻译：

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在湿沙上滑动

我们提出了在湿沙上的雪橇的滑动实验，并描述了摩擦响应是由粒状介质的渗透硬度控制的。向沙子中添加少量水会增加硬度，从而降低滑动摩擦。将更多的水倒入沙中会导致硬度降低，并随之增加摩擦。发现润湿的粒状材料的硬度与其对滑动的摩擦响应之间的这种反相关性是由于雪橇的犁入造成的。当雪橇的载荷超过水沙混合物的渗透硬度时，颗粒状材料将发生不可逆的变形，这可以通过滑块通过后留下的痕迹来证明。穿透硬度决定了滑块的痕迹深度，进而，控制耕犁力。因此，增加水沙混合物的硬度使得在其上拉雪橇变得容易。另外，我们量化了临界剪切应变，该临界剪切应变设定了（湿）粒状材料的弹性到塑性响应的转变，这使我们能够直接将弹性状态下的剪切模量与塑性状态下的硬度相关联。

图形摘要