Juliette Cayer-Barrioz

Microscopic Mechanism for Shear Thickening of Non-Brownian Suspensions

We propose a simple model, supported by contact-dynamics simulations as well as rheology and friction measurements, that links the transition from continuous to discontinuous shear thickening in dense granular pastes to distinct lubrication regimes in the particle contacts. We identify a local Sommerfeld number that determines the transition from Newtonian to shear-thickening flows, and then show that the suspensions volume fraction and the boundary lubrication friction coefficient control the nature of the shear-thickening transition, both in simulations and experiments.

In Situ Film-Forming and Friction-Reduction Mechanisms for Carbon-Nanotube Dispersions in Lubrication

Current requirements in automotive lubrication impose extremely complex formulation. For environmental reasons, it is important to reduce or eliminate the presence of sulphur and phosphorus contained in tribological additives. For that purpose, multi-walled carbon nanotubes have been dispersed in oil in various concentrations. The lubrication mechanisms of such dispersions in mixed and EHL regimes have been investigated by means of the IRIS tribometer that allows us simultaneous contact visualization, film thickness and friction measurement under controlled contact kinematics. The lubricant film-forming capability has been determined as a function of the entrainment velocity and the nanotube content: the presence of carbon nanotubes within the contact results in a local increase in the film thickness and it can be shown that the contact acts as a filter of carbon-nanotube aggregates. Introduction of sliding results in a diminution of the number of aggregates passing through the contact. Moreover, a reduction in friction and a drift in the wear onset have been observed under controlled contact kinematics: this behaviour originates from the transient propagation of carbon-nanotube aggregates through the contact and a friction law is proposed taking into account the contact heterogeneity.

Friction Dynamics of Confined Weakly Adhering Boundary Layers

The nanotribological behavior of self-assembled monolayers is investigated. The latter accommodate friction through transient relaxation and dilatancy effects whose kinetics depends on the structure of the confined layers. Thus, the molecular ordering onto the surfaces controls the level and the stability of the friction coefficient. Moreover, the behavior of these systems is theoretically accounted for using a model based on the kinetics of formation and rupture of adhesive bonds between the two shearing surfaces with an additional viscous term.

Polymorphism of Natural Fatty Acid Liquid Crystalline Phases

We study the phase behavior in water of a mixture of natural long chain fatty acids (FAM) in association with ethylenediamine (EDA) and report a rich polymorphism depending on the composition. At a fixed EDA/FAM molar ratio, we observe upon dilution a succession of organized phases going from a lamellar phase to a hexagonal phase and, finally, to cylindrical micelles. The phase structure is established using polarizing microscopy, SAXS, and SANS. Interestingly, in the lamellar phase domain, we observe the presence of defects upon dilution, which SAXS shows to correspond to intrabilayer correlations. NMR and FF-TEM techniques suggest that these defects are related to an increase in the spontaneous curvature of the molecule monolayers in the lamellae. ATR-FTIR spectroscopy was also used to investigate the degree of ionization within these assemblies. The successive morphological transitions are discussed with regards to possible molecular mechanisms, in which the interaction between the acid surfactant and the amine counterion plays the leading role.

Stribeck and Traction Curves Under Moderate Contact Pressure: From Friction to Interfacial Rheology

Friction in the full-fluid-film regime is generally attributed to the shear response of the lubricated interface under pressure. However, the associated rheological law remains under debate. Stribeck and traction experiments were carried out for model fluids and complex aged fully formulated fluids using the IRIS tribometer, which allows simultaneous measurement of film thickness and friction force under controlled contact kinematics. This leads to the establishment of rheograms that collapse over a wide range of shear rate for moderate contact pressure. A theoretical methodology is then proposed to determine the rheology of the confined interfacial film in a unique way, the latter being discussed in terms of the Eyring and Cross physical non-Newtonian models.

The Combined Role of Soot Aggregation and Surface Effect on the Friction of a Lubricated Contact

Abstract One of the considered research paths to reduce friction loss consists in optimizing the interactions between surfaces and lubricants. The latter may significantly change with the lubricant ageing. In this framework, the tribological behaviour of aged formulated lubricant is analysed for various low-speed reciprocating motions and with different nature of surfaces. This paper focuses on soot aggregate formation processes in a lubricated contact and on their correlation to friction. Although no aggregates have been observed in pure rolling conditions, pure sliding conditions may lead to the appearance of aggregates moving through the contact as a function of the nature of the surfaces. The analysis of their displacement within the contact is used to discuss their interactions with the surfaces. Moreover, we show that the velocity and the dwell time of the aggregate depend on the sliding speed. The morphology of these aggregates evolves over time, affecting friction behaviour. An additive law combining a contribution from the shear of the aggregates with another one due to the shear of a thin lubricant film surrounding the aggregate is then proposed to interpret friction origin and friction evolution with time of shear. The aggregate motion also varies with the nature of the surfaces: in particular, DLC–DLC couple reduces aggregation phenomena and maintains a low friction without apparent wear.

Direct, Robust Technique for the Measurement of Friction between Microspheres

Friction between microscopic objects controls many macroscopic phenomena. For instance, the friction between microasperities determines the tribology of rough surfaces in contact and in relative motion. Additionally, the friction between microparticles is responsible for many aspects of the rheological response of granular media, ranging from microscale contacts at the single-particle level to macroscopic flow properties of sheared, dry granular systems and non-Brownian suspensions. We propose a new, precise, and robust method, based on lateral force microscopy, to measure the coefficient of friction between microspheres quantitatively and without complex data processing. We have successfully applied this method to the contact between silica spheres in liquid with and without a polymer coating.

Friction of Textured Surfaces in EHL and Mixed Lubrication: Effect of the Groove Topography

The influence of the groove topography, especially their top width and their orientation upon the friction in elastohydrodynamic lubrication (EHL) and mixed lubrication regimes was characterized by means of Stribeck curves. Friction levels with groove surfaces were linked to optical measurements of the film thickness distribution inside the contact and compared with data for a smooth surface. In EHL, results indicate no overall friction modification. Nonetheless in this regime, the local film thickness becomes, under a critical top width, lower than the smooth surface leading to a shift of the EHL/mixed regime transition. In mixed regime, the friction is much higher than the smooth surface and narrower tops cause more severe friction than wider one. If the orientation of the grooves does not modify significantly the friction, transverse grooves (perpendicular to the entrainment direction) enable the formation of local dimples in mixed regime, which locally increase the lubrication efficiency.

Friction Control at The Molecular Level: From Superlubricity to Stick-Slip

Publisher Summary When two rubbing surfaces are separated by a bulk layer of lubricating fluids, such as oils in gears or in automotive engines, the lubricant facilitates the relative motion of the solids and reduces friction. Then the shearing of the lubricant accommodates the sliding velocity and the frictional dissipation is mainly dependent on viscosity. Thus, to increase the lifetime of contacts, tribologists need to quantify the basic properties of the lubricant and the surfaces it is separating. The dynamic friction regime appears as analogous to the plastic flow of a confined amorphous solid. The glassy state of the adhesive joint is obvious when the bulk solids are themselves amorphous, but it should be so for ordered solids because of defects in the bulk or the frequent presence of adsorbates. In some cases, layering occurs in the interfacial material, and the situation is similar to a liquid–crystal transition. Usually, the liquid/solid transition occurs without any ordering and belongs to a large class of structural transitions. For some lubricants, either glassy or layering transition can be observed depending on temperature, dwell time, surface roughness, and commensurability of the surface and the film.

From Full-Film Lubrication to Boundary Regime in Transient Kinematics

Friction and lubrication regimes are usually characterized using generalized Stribeck curves. Lubricated friction experiments were carried out using the IRIS tribometer, which allows simultaneous measurement of film thickness distribution in the high-pressure zone and friction force by controlling, accurately and independently, the motions of solids. Transient kinematics combining pure rolling, acceleration, pure sliding and deceleration phases was chosen in order to get access to friction and lubrication mechanisms. It is shown that these transient operating conditions cover all the lubrication regimes that are traditionally obtained through steady-state Stribeck curves. The analytical modelling proposed here emphasizes the role of the local shear rate distribution in full-film regime and the influence of surface in the mixed and boundary regimes.