Using molecular dynamics (MD) we study the dependency of the contact mechanics on the sliding speed when an elastically soft slab (block) is sliding on a rigid substrate with a ${\rm sin} (q_0 x)$ surface height profile. The atoms on the block interact with the substrate atoms by Lennard-Jones potentials. We consider contacts with and without adhesion. We found that the contact area and the friction force are nearly velocity independent for small velocities ($v < 0.25 \ {\rm m/s}$) in spite of the fact that the shear stress in the contact area is rather non-uniform. For the case of no adhesion the friction coefficient is very small. For the case of adhesion the friction coefficient is higher, and is mainly due to energy dissipation at the opening crack tip, where rapid atomic snap-off events occur during sliding. Adhesion experiments between a human finger and a clean glass plate were carried out, and for a dry finger no macroscopic adhesion occurred. We suggest that the observed decrease in the contact area with increasing shear force results from non-adhesive finger-glass contact mechanics, involving large deformations of a complex layered material.
We report a numerical study accounting for the electrical response of a slipping multi-contact interface and of their variations with the sliding speed. We characterize these variations by analyzing the voltage noise across the contact from a mechanical model based on the roughness interaction. Our model focuses only on the mechanical aspects of the slipping interface and explores its electrical consequences (electrical noise) versus the sliding speed. Numerical results are compared with experimental data of the resistance noise versus sliding speed: we find a good agreement between theory and…
Liquid crystal elastomers contract along their director on heating and recover on cooling, offering great potential as actuators and artificial muscles. If a flat sheet is programmed with a spatially varying director pattern, it will actuate into a curved surface, allowing the material to act as a strong machine such as a grabber or lifter. Here we study the actuation of programmed annular sheets which, owing to their central hole, can sidestep constraints on area and orientation. We systematically catalogue the set of developable surfaces encodable via axisymmetric director patterns, and unco…
Experiments on fluid systems in micro-/nano-scale solid conveyors have shown a violation of the no-slip assumption that have been adopted by the classical fluid mechanics. To correct this mechanics for the fluid slip, various approaches have been proposed to determine the slip boundary conditions. However, these approaches have revealed contradictory results for a variety of systems, and a debate on the mechanisms and the conditions of the fluid slip/no-slip past solid surfaces is sustained for a long time. In this paper, we establish the hybrid continuum-molecular modeling (HCMM) as a general…
We study the nominal (ensemble averaged) contact pressure $p(x)$ acting on a cylinder squeezed in contact with an elastic half-space with random surface roughness. The contact pressure is Hertzian-like for $α< 0.01$ and Gaussian-like for $α> 10$, where the dimensionless parameter $α= h_{\rm rms}/δ$ is the ratio between the root-mean-square roughness amplitude and the penetration for the smooth surfaces case (Hertz contact).
The phenomenon of corrugated surfaces is a known technical problem of tribological systems; considerable work has been published in the past on the aspect of rail corrugation of railway systems. Less known is a similar phenomenon observed within the cylinder-piston system of advanced automotive engines using aluminium cylinders. This paper investigates the condition leading to cylinder corrugation in the piston/cylinder system. Material investigations strongly indicate that heat in the contact is playing a major role. Using basic analytical relationships from contact mechanics, the condition r…
Lubrication forces depend to a high degree on elasticity, texture, charge, chemistry, and temperature of the interacting surfaces. Therefore, by appropriately designing surface properties, we may tailor lubrication forces to reduce friction, adhesion and wear between sliding surfaces or control repulsion, assembly, and collision of interacting particles. Here, we show that variations of slippage on one of the contacting surfaces induce a lift force. We demonstrate the consequences of this force on the mobility of a cylinder traveling near a wall and show the emergence of particle oscillation a…
Using molecular dynamics (MD) we study the dependency of the contact mechanics on the sliding speed when an elastic block (cylinder) with a ${\rm cos} (q_0 x)$ surface height profile is sliding in adhesive contact on a rigid flat substrate.The atoms on the block interact with the substrate atoms by Lennard-Jones (LJ) potentials, and we consider both commensurate and(nearly) incommensurate contacts. For the incommensurate system the friction force fluctuates between positive and negative values, with an amplitude proportional to the sliding speed, but with the average close to zero. For the com…
Frictional weakening by vibrations was first invoked in the 70's to explain unusual fault slips and earthquakes, low viscosity during the collapse of impact craters or the extraordinary mobility of sturzstroms, peculiar rock avalanches which travels large horizontal distances. This mechanism was further invoked to explain the remote triggering of earthquakes or abnormally large landslides or pyroclastic flows runout. Recent experimental and theoretical work pointed out the velocity of vibration as the key parameter which governs frictional weakening in sheared granular media. Here we show that…
