Friction anisotropy of violet phosphorene and its surface structure direction identification

Violet phosphorene, a recently determined semiconducting two-dimensional elemental structure, is a promising electronic and optoelectronic material. The nano-tribological properties of violet phosphorene nanoflakes are essential for their micro device applications. A friction anisotropy has been demonstrated for the violet phosphorene nanoflakes by lateral force microscope due to the sub-nanorod components of violet phosphorus. The friction forces of the violet phosphorene nanoflakes have been demonstrated to be valley along sub-nano rod direction and peak across the sub-nanorod direction with a period of 180°, resulting in a fast identification of the surface structure direction of violet phosphorene. The friction of violet phosphorene nanoflakes has also been shown to increase with increasing scanning pressure. However, it is not sensitive to scanning speed or layers. The friction of the violet phosphorene nanoflakes have also been demonstrated to increase when exposure to air for hours. The friction and adhesion features of violet phosphorene nanoflakes provide valuable foundation for violet phosphorene based devices.

Modeling of Polymer Friction on Boundaries of Solids and Inside Materials

Friction models are proposed for anisotropic and heterogeneous dry friction on boundaries of polymer solids. Unit vectors and oriented angles of sliding velocities, radii of curvature and unit normal vectors of sliding trajectories are taken as independent variables in constitutive equations of anisotropic and heterogeneous friction. Heterogeneous dry friction of a polymer pin in pin-on-disc tests is illustrated in the case of Archimedean spiral trajectory. Individual molecular chains composing polymer materials can move inside the material with a high degree of friction anisotropy. The resistance of macromolecule motion is considered with respect to micromechanical models of macromolecules, their kinematics, and friction laws. Two approaches are applied for modeling of anisotropic friction inside polymer materials: continuum-based models (anisotropic viscous friction) and micromechanical models (anisotropic dry friction). Examples of macromolecule dry friction are considered under conditions of spinning and sliding of a disc-like macromolecule and snake-like sliding of a long macromolecule.

Interfacial Friction Anisotropy in Few-Layer Van der Waals Crystals

Friction anisotropy is one of the important friction behaviors for two-dimensional (2D) van der Waals (vdW) crystals. The effects of normal pressure and thickness on the interfacial friction anisotropy in few-layer graphene, h-BN, and MoSe2 under constant normal force mode have been extensively investigated by first-principle calculations. The increase of normal pressure and layer number enhances the interfacial friction anisotropy for graphene and h-BN but weakens that for MoSe2. Such significant deviations in the interfacial friction anisotropy of few-layer graphene, h-BN and MoSe2 can be mainly attributed to the opposite contributions of electron kinetic energies and electrostatic energies to the sliding energy barriers and different interlayer charge exchanges. Our results deepen the understanding of the influence of external loading and thickness on the friction properties of 2D vdW crystals.

Getting grip in changing environments: the effect of friction anisotropy inversion on robot locomotion

Legged locomotion of robots can be greatly improved by bioinspired tribological structures and by applying the principles of computational morphology to achieve fast and energy-efficient walking. In a previous research, we mounted shark skin on the belly of a hexapod robot to show that the passive anisotropic friction properties of this structure enhance locomotion efficiency, resulting in a stronger grip on varying walking surfaces. This study builds upon these results by using a previously investigated sawtooth structure as a model surface on a legged robot to systematically examine the influences of different material and surface properties on the resulting friction coefficients and the walking behavior of the robot. By employing different surfaces and by varying the stiffness and orientation of the anisotropic structures, we conclude that with having prior knowledge about the walking environment in combination with the tribological properties of these structures, we can greatly improve the robot’s locomotion efficiency.

Frictional anisotropy in casted seismic faults: or how to 3D print a fault to better characterize it

<p>Anisotropic phenomena have long been studied in the vicinity of seismic faults. It has for instance been shown that both in situ pore fluids and seismic mechanical waves travel at different velocities along various directions of a fault zone. Yet, while more and more complexity and disorder in seismic models are introduced to better understand earthquakes, frictional anisotropy is only rarely regarded. In many other domains than geophysics, however, such anisotropy in solid friction is believed to be crucial. For instance, the tribology of rubber tires, skis or advanced adhesives is improved when those are designed to have a preferential frictional direction. But numerous natural systems also benefit from such anisotropy: is is notably essential in the motion of numerous animal skins and in the efficient hydration of some plants. In most cases, these frictional anisotropies derive from the existence of preferential topographic orientations on, at least, one of the contact surfaces, with scales for such structural directivity that can be multiple and various. Seismic faults also exhibit such preferential directions in their topography: unique rock crystals, such as antigorite, can already display some frictional anisotropy, fault zones are  initiated by early fractures that often propagates through layered sediments, generating ramp-flat morphology in their surfaces and, finally, mature faults are marked by grooves of various wavelengths due to the slip induced erosion.</p><p> </p><p>In this work, we study how the morphology of faults affects their stability, as understood by their Coulomb static coefficient of friction. In particular we study its anisotropy with the slip direction. To do so, we make use of the 3D-printing technology and print actual fault surfaces, that were measured in the field. We perform friction experiments with gypsum casts of these 3D-printed faults, as mineral-like materials might deform differently under shear than plastic materials. With these experiments, we show that the friction coefficient along seismic faults is highly anisotropic, with slip motions that are easier in, but not limited to, the direction of the main grooves. This anisotropy could for instance be paramount to better predict the next direction of rupture along some faults under a varying stress state. In some cases, it could indeed not only be related to the orientation of the main regional stress, but also to the structural anisotropy, and  depending on stress and friction anisotropy, along which orientation a rupture criterion will first be exceeded.</p>