Juel Mikkel Metzsch, Malthe-Sørenssen Anders, Sveinsson Henrik Andersen
The Njord Centre, Department of Physics, University of Oslo, Oslo 0316, Norway.
Institute for Geotechnical Engineering, Department of Civil, Environmental and Geomatic Engineering, ETH Zürich, Zürich 8093, Switzerland.
Proc Natl Acad Sci U S A. 2025 Jul 15;122(28):e2501728122. doi: 10.1073/pnas.2501728122. Epub 2025 Jul 11.
The friction force typically increases linearly with normal load with a constant of proportionality called the coefficient of friction. Most materials exhibit a positive friction coefficient, so that an increase in the normal load leads to an increase in the friction force. Recently, materials with negative friction coefficients have been observed at meticulously constructed interfaces due to an interplay between superstructures at heterojunctions, out-of-plane buckling, or the ordering of thin water films. However, the magnitude of the negative friction coefficient is typically much less than [Formula: see text], the geometries are highly restrictive, and the mechanisms are difficult to scale to larger systems. Here, we show that a friction coefficient of [Formula: see text]0.08 can be obtained for a graphene sheet modified with kirigami-inspired cuts when inducing sheet tension through a normal load coupling. We use molecular dynamics simulations to show that the frictional behavior of kirigami cut sheets exhibits a strong nonmonotonic dependence on in-plane strain, while only weakly influenced by normal loading. We argue that the strong influence of strain arises from changes in commensurability between the graphene sheet and the substrate when the sheet deforms and buckles out of plane. We propose a simple nanomachine design that couples normal loading and sheet tension which enables the realization of different frictional laws, including a negative coefficient of friction. This represents a unique approach to creating tunable frictional surfaces and opens up applications in sheet-like systems across scales.
摩擦力通常随法向载荷呈线性增加，其比例常数称为摩擦系数。大多数材料表现出正的摩擦系数，因此法向载荷的增加会导致摩擦力增加。最近，由于异质结处的超结构、面外屈曲或薄水膜的有序排列之间的相互作用，在精心构建的界面处观察到了具有负摩擦系数的材料。然而，负摩擦系数的大小通常远小于[公式：见原文]，几何形状具有高度限制性，且这些机制难以扩展到更大的系统。在这里，我们表明，当通过法向载荷耦合诱导片材张力时，对于用受kirigami启发的切口修饰的石墨烯片材，可以获得[公式：见原文]0.08的摩擦系数。我们使用分子动力学模拟表明，kirigami切口片材的摩擦行为表现出对面内应变的强烈非单调依赖性，而仅受法向载荷的微弱影响。我们认为，应变的强烈影响源于石墨烯片材与基底之间的可公度性在片材发生面外变形和屈曲时的变化。我们提出了一种简单的纳米机器设计，该设计将法向载荷与片材张力耦合，从而能够实现不同的摩擦定律，包括负摩擦系数。这代表了一种创建可调摩擦表面的独特方法，并为跨尺度的片状系统开辟了应用前景。
