Synovial joint lubrication – does nature teach more effective engineering lubrication strategies?

Nature shows numerous examples of systems which show energy efficiency, elegance in their design and optimum use of materials. Biomimetics is an emerging field of research in engineering and successes have been documented in the diverse fields of robotics, mechanics, materials engineering and many more. To date little biomimetics research has been directed towards tribology in terms of transferring technologies from biological systems into engineering applications. The potential for biomimicry has been recognised in terms of replicating natural lubricants but this system reviews the potential for mimicking the synovial joint as an efficient and durable tribological system for potential engineering systems. The use of materials and the integration of materials technology and fluid/surface interactions are central to the discussion

Magnetic Field Control of the Optical Spin Hall Effect

We investigate theoretically the effect of an external magnetic field on polarization patterns appearing in quantum microcavities due to the optical spin Hall effect (OSHE). We show that increase of the magnetic field perpendicular to the plane of the cavity resulting in the increase of the Zeeman splitting leads to the transition from azimuthal separation of polarizations to their radial separation. This effect can be straightforwardly detected experimentally.Comment: 9 pages, 6 figure

Control of electronic transport in graphene by electromagnetic dressing

We demonstrated theoretically that the renormalization of the electron energy spectrum near the Dirac point of graphene by a strong high-frequency electromagnetic field (dressing field) drastically depends on polarization of the field. Namely, linear polarization results in an anisotropic gapless energy spectrum, whereas circular polarization leads to an isotropic gapped one. As a consequence, the stationary (dc) electronic transport in graphene strongly depends on parameters of the dressing field: A circularly polarized field monotonically decreases the isotropic conductivity of graphene, whereas a linearly polarized one results in both giant anisotropy of conductivity (which can reach thousands of percents) and the oscillating behavior of the conductivity as a function of the field intensity. Since the predicted phenomena can be observed in a graphene layer irradiated by a monochromatic electromagnetic wave, the elaborated theory opens a substantially new way to control electronic properties of graphene with light.Comment: Published versio

Providing adhesion for a miniture mobile intra-abdominal device based on biomimetic principles

This paper investigates the surface adhesion characteristics required for a miniature mobile device to move around the abdominal cavity. Such a device must be capable of adhering to the tissue lining and move freely across the upper surface of the insufflated abdomen. Accordingly, the potential of utilising bioinspired solutions to facilitate wet adhesion is assessed

New insights on the decomposition mechanism of Molybdenum DialkyldiThioCarbamate (MoDTC): a Raman spectroscopic study

Molybdenum DialkyldiThioCarbamate (MoDTC) is a friction modifier that has been used in automotive engines for many years. However, its exact decomposition mechanism within tribocontacts is not fully understood. In this study, an attempt has been made towards understanding the mechanism of MoDTC decomposition in steel/steel contacts by employing Raman spectroscopy. Results show that the main MoDTC decomposition products are MoS2, FeMoO4 and sulphur-rich molybdenum compounds, MoSx (x>2), in contrast to the previously reported MoS2 and MoO3. Formation of these products is dependent on tribological parameters. Raman results from this study indicate that the Mo6+ species previously observed in X-ray Photoelectron Spectroscopy (XPS) analysis are probably from FeMoO4 and not MoO3. This paper presents an alternative reaction pathway for MoDTC decomposition in steel/steel contacts with MoSx as an intermediate product and MoS2 as the final product. FeMoO4 is formed from a side reaction of iron oxides with molybdenum compounds at low temperatures and low MoDTC concentrations. Results also show that friction is dependent on the nature of decomposition products at the tribocontact. Knowledge of MoDTC decomposition reaction pathway will enable the friction performance of MoDTC lubricants to be optimized

Effect of lubricant ageing on lubricants' physical and chemical properties and tribological performance; Part I: effect of lubricant chemistry

Purpose Ageing has been known to affect the performance of lubricants. However, there is a lot of debate as to whether ageing is beneficial or detrimental to the wear performance of lubricants. Design/methodology/approach The purpose of this research is to evaluate the effect of ageing on the viscosity, total acid number and Fourier-transform infrared spectroscopy information of a series of lubricants. The tribological performance (friction and wear) of the aged lubricants is also studied, followed by X-ray photoelectron spectroscopy analysis on the selected post-test samples to study the tribochemical features of the tribofilm. Findings The results show that ageing has a different impact on lubricants and tribological performances based on the physical and chemical properties. Originality/value These findings will be compared with the research on the role of water in lubricant ageing (Part II of this study)

Transient processes of MoS₂ tribofilm formation under boundary lubrication

A tribochemistry study that involves the application of Raman spectroscopy surface analysis has been undertaken to understand the time-dependent tribochemical reactions, for lubrication by Molybdenum dialkyl-dithiocarbamate (MoDTC) occurring in boundary lubricated conditions. Under the conditions of rubbing and high temperature, time-resolved Raman spectroscopy results show the intermediate steps that lead to the MoDTC additive to be tribochemically structured on the wear scar of the contacting surface. A MoS2 tribofilm with a lattice layer structure is observed on the wear scar whenever the lowest friction was achieved. An apparent shift of the A1g and E2g Raman modes, indicating qualitative and quantitative information on the MoS2 tribofilm formed, is observed to be related to low friction. Detailed analyses of Raman spectra obtained on wear scars at different test durations and temperatures indicate that both temperature and rubbing are needed for the formation of low friction MoS2 tribofilm

Effect of base oil polarity on the micropitting behaviour in rolling-sliding contacts

An automated micropitting test rig that uses the proven three-point contact configuration was applied to study the effect of base oil polarity on micropitting behaviour in rolling-sliding contacts. The tribological tests using one polar (ester) and one non-polar (polyalpha olefin [PAO]) base oil mixed with four different additives were first done in a micropitting rig to study the friction, wear, and micropitting performance. The tested specimen after tribological tests were examined by an optical microscope to study the micropitting on the surface. The X-ray photoelectron spectroscopy (XPS) was applied to obtain the related tribochemical information from the reaction layer. The results showed that higher wear and less micropits formed when testing with ester-based lubricants. This was related to the higher affinity to the steel surface of polar base oil molecules that can compete with additives to attach to the surface, resulting in a thinner reaction layer and shorter chain phosphates