Molecular dynamics simulation of the Stribeck curve: Boundary lubrication, mixed lubrication, and hydrodynamic lubrication on the atomistic level

Lubricated contact processes are studied using classical molecular dynamics simulations for determining the entire range of the Stribeck curve.Therefore,the lateral movement of two solid bodies at different gap height are studied.In each simulation,a rigid asperity is moved at constant height above a flat iron surface in a lubricating fluid.Both methane and decane are considered as lubricants.The three main lubrication regimes of the Stribeck curve and their transition regions are covered by the study:Boundary lubrication(significant elastic and plastic deformation of the substrate),mixed lubrication(adsorbed fluid layer dominates the process),and hydrodynamic lubrication(shear flow is set up between the surface and the asperity).We find the formation of a tribofilm in which lubricant molecules are immersed into the metal surface—not only in the case of scratching,but also for boundary lubrication and mixed lubrication.The formation of a tribofilm is found to have important consequences for the contact process.Moreover,the two fluids are found to show distinctly different behavior in the three lubrication regimes:For hydrodynamic lubrication(large gap height),decane yields a better tribological performance;for boundary lubrication(small gap height),decane shows a larger friction coefficient than methane,which is due to the different mechanisms observed for the formation of the tribofilm;the mixed lubrication regime can be considered as a transition regime between the two other regimes.Moreover,it is found that the nature of the tribofilm depends on the lubricant:While methane particles substitute substrate atoms sustaining mostly the crystalline structure,the decane molecules distort the substrate surface and an amorphous tribofilm is formed.

Generalized laws of refraction and reflection at interfaces between different photonic artificial gauge fields

Artificial gauge fields the control over the dynamics of uncharged particles by engineering the potential landscape such that the particles behave as if effective external fields are acting on them.Recent years have witnessed a growing interest in artificial gauge fields generated either by the geometry or by time-dependent modulation,as they have been enablers of topological phenomena and synthetic dimensions in many physical settings,e.g.,photonics,cold atoms,and acoustic waves.Here,we formulate and experimentally demonstrate the generalized laws of refraction and reflection at an interface between two regions with different artificial gauge fields.We use the symmetries in the system to obtain the generalized Snell law for such a gauge interface and solve for reflection and transmission.We identify total internal reflection(TIR)and complete transmission and demonstrate the concept in experiments.In addition,we calculate the artificial magnetic flux at the interface of two regions with different artificial gauge fields and present a method to concatenate several gauge interfaces.As an example,we propose a scheme to make a gauge imaging system-a device that can reconstruct(image)the shape of an arbitrary wavepacket launched from a certain position to a predesigned location.

Limits of topological protection under local periodic driving

The bulk-edge correspondence guarantees that the interface between two topologically distinct insulators supports at least one topological edge state that is robust against static perturbations.Here,we address the question of how dynamic perturbations of the interface affect the robustness of edge states.We illuminate the limits of topological protection for Floquet systems in the special case of a static bulk.We use two independent dynamic quantum simulators based on coupled plasmonic and dielectric photonic waveguides to implement the topological Su-Schriefer-Heeger model with convenient control of the full space-and time-dependence of the Hamiltonian.Local time-periodic driving of the interface does not change the topological character of the system but nonetheless leads to dramatic changes of the edge state,which becomes rapidly depopulated in a certain frequency window.A theoretical Floquet analysis shows that the coupling of Floquet replicas to the bulk bands is responsible for this effect.Additionally,we determine the depopulation rate of the edge state and compare it to numerical simulations.

Artificial gauge field switching using orbital angular momentum modes in optical waveguides

The discovery of artificial gauge fields controlling the dynamics of uncharged particles that otherwise elude the influence of standard electromagnetic fields has revolutionised the field of quantum simulation.Hence,developing new techniques to induce these fields is essential to boost quantum simulation of photonic structures.Here,we experimentally demonstrate the generation of an artificial gauge field in a photonic lattice by modifying the topological charge of a light beam,overcoming the need to modify the geometry along the evolution or impose external fields.In particular,we show that an effective magnetic flux naturally appears when a light beam carrying orbital angular momentum is injected into a waveguide lattice with a diamond chain configuration.To demonstrate the existence of this flux,we measure an effect that derives solely from the presence of a magnetic flux,the Aharonov-Bohm caging effect,which is a localisation phenomenon of wavepackets due to destructive interference.Therefore,we prove the possibility of switching on and off artificial gauge fields just by changing the topological charge of the input state,paving the way to accessing different topological regimes in a single structure,which represents an important step forward for optical quantum simulation.

Photosensitive Material Enabling Direct Fabrication of Filigree 3D Silver Microstructures via Laser-Induced Photoreduction

Dear Editor Laser-induced photoreduction(LPR)as a direct fabrication technique that promises to be one of the most versatile routes for fabricating highly conductive 3D metallic microstructures on-chip(e.g.,metamaterials,electro-mechanical systems,and high-frequency components like antennas).