Origin of friction hysteresis on monolayer graphene

Load-dependent friction hysteresis is an intriguing phenomenon that occurs in many materials, where the friction measured during unloading is larger than that measured during loading for a given normal load. However, the mechanism underlying this behavior is still not well understood. In this work, temperature-controlled friction force microscopy was utilized to explore the origin of friction hysteresis on exfoliated monolayer graphene. The experimental observations show that environmental adsorbates from ambient air play an important role in the load dependence of friction. Specifically, the existence of environmental adsorbates between the tip and graphene surface gives rise to an enhanced tip-graphene adhesion force, which leads to a positive friction hysteresis where the friction force is larger during unloading than during loading. In contrast to positive friction hysteresis, a negative friction hysteresis where the friction force is smaller during unloading than during loading is observed through the removal of the environmental adsorbates upon in situ annealing. It is proposed that the measured friction hysteresis originates from the hysteresis in the contact area caused by environmental adsorbates between the tip and graphene. These findings provide a revised understanding of the friction hysteresis in monolayer graphene in terms of environmental adsorbates.

Analysis of Vertical Load Transfer Mechanism of Assembled Lattice Diaphragm Wall in Collapsible Loess Area

Based on analysis of the formation mechanism and characteristics of the negative friction in collapsible loess areas, this study investigates the load transfer law of a wall-soil system under a vertical load, establishes the vertical bearing model of a lattice diaphragm wall, and analyzes the vertical bearing capacity of an assembled latticed diaphragm wall (ALDW) in a loess area. The factors influencing the vertical bearing characteristics of the ALDW in a loess area are analyzed. The vertical bearing mechanism of the lattice diaphragm wall in the loess area is investigated. The failure modes of the ALDW in the loess area are mainly shear failure of the soil around the wall and failure of the wall-soil interface. In the generation and development of negative friction, there is always a point where the relative displacement of the wall-soil interface is zero at a certain depth below the ground; at this point, the wall and soil are relative to each other. The collapsibility of loess, settlement of the wall and surrounding soil, and rate and method of immersion are the factors affecting the lattice diaphragm wall. The conclusions of this study provide a reference for the design and construction of ALDWs in loess areas.

Predicting the Healthy Operation of Heavy Oil Well Casings in Permafrost Regions

This study explores the distribution of stress and deformation on casings in heavy oil recovery wells and the distribution of stress in the thaw bulb in permafrost areas. Considering the expansion of the thaw bulb, the simulation analysis method is used to explore the internal mechanisms of vertical settlement displacement development and stress redistribution within thawed soil and casing. Calculation results show the following: (a) The maximum settlement of the thawed soil and the casing was positively correlated with the expansion of the thaw bulb. Although the settlement of the thawed soil was greater than that of the casing, the initial increase in maximum settlement difference between the thawed soil and the casing eventually tended to be constant due to stabilization of the thaw bulb’s expansion. (b) The size of the thaw bulb directly affects the redistribution of internal stress in thawed soil, leading to different distribution rules for the vertical displacement of thawed soil and casing with depth. (c) Beyond a certain formation depth, the vertical stress of thawed soil gradually transits from a tensile stress state to a compressive stress state. The depth of a soil layer whose horizontal stress value is initially greater than its vertical stress value will gradually deepen with an increase in thaw bulb radius. (d) There is no significant negative friction on the lateral wall of casing in yield state, but significant negative friction exists on the lateral wall of casing in elastic state. The vertical stress of casing in elastic state increased gradually with the increase of casing depth, due to the existence of continuous negative friction and dead weight.

High-performance PDMS‐based triboelectric nanogenerator fabricated using surface-modified carbon nanotubes via pulsed laser ablation

Poly-(dimethylsiloxane) (PDMS) has been used as a negative friction layer in triboelectric nanogenerator (TENG) owing to its high electronegativity and flexibility. With advantage of the PDMS, various materials design to...

Model test study of bending moment and negative skin friction for batter rock-socketed piles under surface load

Batter rock-socketed piles (BRSP) foundation is one of common foundations, such as port engineering or cross-sea bridge, while there are few studies on negative skin friction for BRSP. A series of model tests are conducted to explore negative skin friction of BRSP which are embedded in thick soft clay. The effects of the inclined angle of piles and soil consolidation time to negative friction resistance and the bending moment of BRSP are analyzed. The test results show that: the development of negative friction and bending moment BRSP have pronounced time effect; the longer the consolidation time is, the slower the axial force and bending moment intensify. The ultimate pile shaft axial force and bending moment increases nonlinearly concerning the inclined angle of piles. And the “neutral point” position and peak point of bending moment is always located at 0.9~1.0 times soil depth.

Negative friction memory induces persistent motion

We investigate the mean-square displacement (MSD) for random motion governed by the generalized Langevin equation for memory functions that contain two different time scales: In the first model, the memory kernel consists of a delta peak and a single-exponential and in the second model of the sum of two exponentials. In particular, we investigate the scenario where the long-time exponential kernel contribution is negative. The competition between positive and negative friction memory contributions produces an enhanced transient persistent regime in the MSD, which is relevant for biological motility and active matter systems. Graphical abstract

Dynamical friction with radiative feedback – II. High-resolution study of the subsonic regime

ABSTRACT Recent work has suggested that the net gravitational force acting on a massive and luminous perturber travelling through a gaseous and opaque medium can have same direction as the perturber’s motion (an effect sometimes called negative dynamical friction). Analytic results were obtained using a linear analysis and were later confirmed by means of non-linear numerical simulations which did not resolve the flow within the Bondi sphere of the perturber, hence effectively restricted to weakly perturbed regions of the flow. Here we present high-resolution simulations, using either 3D Cartesian or 2D cylindrical meshes that resolve the flow within the Bondi sphere. We perform a systematic study of the force as a function of the perturber’s mass and luminosity, in the subsonic regime. We find that perturbers with mass M smaller than a few Mc ∼ χcs/G are subjected to a thermal force with a magnitude in good agreement with linear theory (χ being the thermal diffusivity of the medium, cs the adiabatic sound speed, and G the gravitational constant), while for larger masses, the thermal forces are only a fraction of the linear estimate that decays as M−1. Our analysis confirms the possibility of negative friction (hence a propulsion) on sufficiently luminous, low-mass embryos embedded in protoplanetary discs. Finally, we give an approximate expression of the total force at low Mach number, valid both for subcritical (M < Mc) and supercritical (M > Mc) perturbers.