Qunyang Li

Frictional Characteristics of Atomically Thin Sheets

Thin Friction The rubbing motion between two surfaces is always hindered by friction, which is caused by continuous contacting and attraction between the surfaces. These interactions may only occur over a distance of a few nanometers, but what happens when the interacting materials are only that thick? Lee et al. (p. 76; see the Perspective by Müser and Shakhvorostov) explored the frictional properties of a silicon tip in contact with four atomically thin quasi–two dimensional materials with different electrical properties. For all the materials, the friction was seen to increase as the thickness of the film decreased, both for flakes supported by substrates and for regions placed above holes that formed freely suspended membranes. Placing graphene on mica, to which it strongly adheres, suppressed this trend. For these thin, weakly adhered films, out-of-plane buckling is likely to dominate the frictional response, which leads to this universal behavior. A universal trend is observed for the friction properties of thin films on weakly adhering substrates. Using friction force microscopy, we compared the nanoscale frictional characteristics of atomically thin sheets of graphene, molybdenum disulfide (MoS2), niobium diselenide, and hexagonal boron nitride exfoliated onto a weakly adherent substrate (silicon oxide) to those of their bulk counterparts. Measurements down to single atomic sheets revealed that friction monotonically increased as the number of layers decreased for all four materials. Suspended graphene membranes showed the same trend, but binding the graphene strongly to a mica surface suppressed the trend. Tip-sample adhesion forces were indistinguishable for all thicknesses and substrate arrangements. Both graphene and MoS2 exhibited atomic lattice stick-slip friction, with the thinnest sheets possessing a sliding-length–dependent increase in static friction. These observations, coupled with finite element modeling, suggest that the trend arises from the thinner sheets’ increased susceptibility to out-of-plane elastic deformation. The generality of the results indicates that this may be a universal characteristic of nanoscale friction for atomically thin materials weakly bound to substrates.

Organogel as durable anti-icing coatings

A durable organogel anti-icing material via swelling cross-linked poly(dimethylsiloxane) with liquid paraffin is reported. The surface of the organogel is covered by a thin released layer of paraffin due to the osmotic pressure, which acts as a lubricant and reduces the ice adhesion greatly. Results show that the ice adhesion on the surface of the organogel is as small as 1.7±1.2 kPa (at −30°C) and the low ice adhesion remains even when the temperature is lowered to −70°C. The surface with lubricating liquid paraffin layer exhibits excellent durability, as it shows an ultralow ice adhesion after 35 cycles of icing/deicing and 100 days of exposure in ambient environment.中文摘要本论文报道了一种通过液体石蜡溶胀聚二甲基硅氧烷制备的持久耐用的油凝胶除冰涂层. 由于油凝胶表面被一层渗透压驱动的石蜡缓释层覆盖, 这层缓释层可作为润滑层, 大大降低油凝胶表面的冰粘附强度. 实验结果显示, 油凝胶表面是一种具有超低冰粘附的防结冰材料, 在−30°C测试温度条件下的冰粘附强度是1.7 ± 1.2 kPa, 且温度降低到−70°C时, 其冰粘附强度不高于10 kPa. 在连续100天的使用期间, 经过35次的结冰/除冰循环操作, 具有润滑层的油凝胶表面仍然保持了持久的低粘附特性. 较低的冰粘附和较宽的低温适用窗口, 以及持久的耐用性, 使其在依靠重力或风力的低粘附被动除冰领域具有很大的潜力和实际应用价值.

Frictional ageing from interfacial bonding and the origins of rate and state friction

Earthquakes have long been recognized as being the result of stick–slip frictional instabilities. Over the past few decades, laboratory studies of rock friction have elucidated many aspects of tectonic fault zone processes and earthquake phenomena. Typically, the static friction of rocks grows logarithmically with time when they are held in stationary contact, but the mechanism responsible for this strengthening is not understood. This time-dependent increase of frictional strength, or frictional ageing, is one manifestation of the ‘evolution effect’ in rate and state friction theory. A prevailing view is that the time dependence of rock friction results from increases in contact area caused by creep of contacting asperities. Here we present the results of atomic force microscopy experiments that instead show that frictional ageing arises from the formation of interfacial chemical bonds, and the large magnitude of ageing at the nanometre scale is quantitatively consistent with what is required to explain observations in macroscopic rock friction experiments. The relative magnitude of the evolution effect compared with that of the ‘direct effect’—the dependence of friction on instantaneous changes in slip velocity—determine whether unstable slip, leading to earthquakes, is possible. Understanding the mechanism underlying the evolution effect would enable us to formulate physically based frictional constitutive laws, rather than the current empirically based ‘laws’, allowing more confident extrapolation to natural faults.

The evolving quality of frictional contact with graphene

Graphite and other lamellar materials are used as dry lubricants for macroscale metallic sliding components and high-pressure contacts. It has been shown experimentally that monolayer graphene exhibits higher friction than multilayer graphene and graphite, and that this friction increases with continued sliding, but the mechanism behind this remains subject to debate. It has long been conjectured that the true contact area between two rough bodies controls interfacial friction. The true contact area, defined for example by the number of atoms within the range of interatomic forces, is difficult to visualize directly but characterizes the quantity of contact. However, there is emerging evidence that, for a given pair of materials, the quality of the contact can change, and that this can also strongly affect interfacial friction. Recently, it has been found that the frictional behaviour of two-dimensional materials exhibits traits unlike those of conventional bulk materials. This includes the abovementioned finding that for few-layer two-dimensional materials the static friction force gradually strengthens for a few initial atomic periods before reaching a constant value. Such transient behaviour, and the associated enhancement of steady-state friction, diminishes as the number of two-dimensional layers increases, and was observed only when the two-dimensional material was loosely adhering to a substrate. This layer-dependent transient phenomenon has not been captured by any simulations. Here, using atomistic simulations, we reproduce the experimental observations of layer-dependent friction and transient frictional strengthening on graphene. Atomic force analysis reveals that the evolution of static friction is a manifestation of the natural tendency for thinner and less-constrained graphene to re-adjust its configuration as a direct consequence of its greater flexibility. That is, the tip atoms become more strongly pinned, and show greater synchrony in their stick–slip behaviour. While the quantity of atomic-scale contacts (true contact area) evolves, the quality (in this case, the local pinning state of individual atoms and the overall commensurability) also evolves in frictional sliding on graphene. Moreover, the effects can be tuned by pre-wrinkling. The evolving contact quality is critical for explaining the time-dependent friction of configurationally flexible interfaces.

Fluorination of Graphene Enhances Friction Due to Increased Corrugation

The addition of a single sheet of carbon atoms in the form of graphene can drastically alter friction between a nanoscale probe tip and a surface. Here, for the first time we show that friction can be altered over a wide range by fluorination. Specifically, the friction force between silicon atomic force microscopy tips and monolayer fluorinated graphene can range from 5-9 times higher than for graphene. While consistent with previous reports, the combined interpretation from our experiments and molecular dynamics simulations allows us to propose a novel mechanism: that the dramatic friction enhancement results from increased corrugation of the interfacial potential due to the strong local charge concentrated at fluorine sites, consistent with the Prandtl-Tomlinson model. The monotonic increase of friction with fluorination in experiments also demonstrates that friction force measurements provide a sensitive local probe of the degree of fluorination. Additionally, we found a transition from ordered to disordered atomic stick-slip upon fluorination, suggesting that fluorination proceeds in a spatially random manner.

Molecular Dynamics Simulation of Atomic Friction: A Review and Guide

This paper reviews recent progress in molecular dynamics simulation of atomic-scale friction measured by an atomic force microscopy. Each section of the review focuses on an individual condition or parameter that affects atomic friction including materials, surfaces, compliance, contact area, normal load, temperature, and velocity. The role each parameter plays is described in the context of both experimental measurements and simulation predictions. In addition, the discussion includes an overview of the research communitys current understanding of observed effects, guidelines for implementation of those effects in an atomistic simulation, and suggestions for future research to address open questions. Taken together, this review conveys the message that friction at the atomic scale is affected by many interrelated parameters and that the use of molecular dynamics simulation as a predictive tool can be accomplished only through careful model design.

Robust ultra-low-friction state of graphene via moiré superlattice confinement

Two-dimensional (2D) materials possess outstanding lubrication property with their thicknesses down to a few atomic layers, but they are easily susceptible to sliding induced degradation or ubiquitous chemical modification. Maintaining the superior lubricating performance of 2D materials in a harsh working environment is highly desirable yet grandly challenging. Here we show that by proper alignment of graphene on a Ge(111) substrate, friction of graphene could be well preserved at an ultra-low level even after fluorination or oxidation. This behaviour is experimentally found to be closely related to the suppression of molecular-level deformation of graphene within the moiré superlattice structure. Atomistic simulations reveal that the formation of an interconnected meshwork with enhanced interfacial charge density imposes a strong anchoring effect on graphene even under chemical modification. Modulating molecular-level deformation by interfacial confinements may offer a unique strategy for tuning the mechanical or even chemical properties of 2D materials.

Functional map of biological and biomimetic materials with hierarchical surface structures

Many biological materials utilize hierarchical surface structures to achieve their wetting-based functions, e.g. self-cleaning and antifogging. In this paper, a classification method is proposed for both biological and artificial materials with hierarchical surface structures to establish the functional map of various hierarchical surface structures. From the viewpoint of geometric features, the constituent building elements on functional surfaces are categorized into dimensional classes both at the micrometer and nanometer scales. Following this classification, one can correlate the biological functions, especially those related to superhydrophobicity, with surface morphologies. In addition to natural biological tissues, we also briefly review the fabrication techniques for realizing these superhydrophobic structures in the laboratory. This dimensionality classification may serve as a guide for future analysis, design and preparation of surfaces with tuned functions achieved by geometrical morphologies.

Bioinspired Solid Organogel Materials with a Regenerable Sacrificial Alkane Surface Layer

In nature, lifetime-long functionalities of land plant leaves rely on the regenerability as well as the solid feature of the epicuticular wax layer. Inspired by the regenerable solid epicuticular wax on land plant leaf surfaces, herein a type of solid organogel material with regenerable sacrificial alkane surface layer is reported. This type of surface material is demonstrated to be of great practical importance for tackling solid deposition, such as anti-icing, antigraffiti, and antifouling, since the deposited foreign materials can be easily removed together with the alkane surface layer. Significantly, the solid alkane layer does not contaminate nearby surfaces due to its solid nature in both working and stand-by conditions, which is completely different to liquid-infused materials.

Friction, slip and structural inhomogeneity of the buried interface

An atomistic model of metallic contacts using realistic interatomic potentials is used to study the connection between friction, slip and the structure of the buried interface. Incommensurability induced by misalignment and lattice mismatch is modeled with contact sizes that are large enough to observe superstructures formed by the relative orientations of the surfaces. The periodicity of the superstructures is quantitatively related to inhomogeneous shear stress distributions in the contact area, and a reduced order model is used to clarify the connection between friction and structural inhomogeneity. Finally, the movement of atoms is evaluated before, during and after slip in both aligned and misaligned contacts to understand how the interfacial structure affects the mechanisms of slip and the corresponding frictional behavior.