Mecanisme D\u27action Des Sulfonates De Calcium Hyperbasiques En Regime De Lubrification Limite

The formation mechanism of boundary film obtained with the overbased calcium sulfonates was investigated.;Firstly a physico-chemical analysis of the overbased calcium sulfonates was achieved. The structure, the size and the polydispersity of the micelles formed by the additive in oil, and the nature of the interactions existing between them were studied using Small Angle X-Ray Scattering (SAXS). Different parameters which can influence the micellar structure such as pressure, temperature and water, were also studied.;Secondly, the formation of the boundary film was studied using an alternating tribometer. The Infrared Reflexion Absorption Spectroscopy by Polarization Modulation (PM IRRAS), the Scanning Electron Microscopy (SEM) and the Atomic Force Microscopy (AFM) were used to investigate the boundary film.;The overbased calcium sulfonates form in oil micelles consisting of a calcium carbonate core of 2.0 nm surrounded by a calcium sulfonate layer of between 0.9 and 2.2 nm in thickness depending on the concentration. In concentrated solutions, the micelles are subjected to steric repulsions caused by the cluttering of the sulfonate chains. In diluted solutions, an attractive energy of the order of 1 kT exists between the micelles leading to their aggregation. When water is added, this attractive energy is increased by a factor of 10. When the additive is submitted to friction constraints, its micellar structure is modified. The film formation is related to the adsorption of the calcium carbonate on the surface and to the expulsion of the sulfonate chains surrounding the micelle cores.;The film formation is achieved during the first seconds of friction and it grows by agglomeration and crystallization of calcium carbonate cores bared of their calcium sulfonate layer. The boundary film partially recovers the friction surface and its thickness increases during the first hour of friction. The maximum local film thickness may reach 1 micron. The crystallization phenomenon carries out principally on the prominent parts of the film and leads to the formation of calcite particles with very polydispersed sizes, of between few ten and few hundred nanometers. In presence of water, the film formation kinetics is slower and the calcium carbonate partially crystallizes

Selectins Ligand Decorated Drug Carriers for Activated Endothelial Cell Targeting

New active particulate polymeric vectors based on branched polyester copolymers of hydroxy-acid and allyl glycidyl ether were developed to target drugs to the inflammatory endothelial cell surface. The hydroxyl and carboxyl derivatives of these polymers allow grafting of ligand molecules on the polyester backbones at different densities. A known potent nonselective selectin ligand was selected and synthesized using a new scheme. This synthesis allowed the grafting of the ligand to the polyester polymers, preserving its binding activity as assessed by docking simulations. Selectin expression on human umbilical cord vascular endothelial cells (HUVEC) was induced with the pro-inflammatory bacterial lipopolysaccharide (LPS) or with the nonselective inhibitor of nitric oxide synthase L-NAME. Strong adhesion of the ligand decorated nanoparticles was evidenced in Vitro on activated HUVEC. Binding of nanoparticles bearing ligand molecules could be efficiently inhibited by prior incubation of cells with free ligand, demonstrating that adhesion of the nanoparticles is mediated by specific interaction between the ligand and the selectin receptors. These nanoparticles could be used for specific drug delivery to the activated vascular endothelium, suggesting their application in the treatment of diseases with an inflammatory component such as rheumatoid arthritis and cancer

Compliant Surfaces under Shear: Elastohydrodynamic Lift Force

International audienceIn this work, we have investigated the behavior under shear and compression of mica surfaces coated with poly(N-isopropylacrylamide) cationic microgels. We have observed the emergence of velocity dependent, shear-induced normal forces, which can be large enough to entrain a fluid film that separates the surfaces out of contact, driving the dynamic system from conditions of boundary to hydrodynamic lubrication. By implementing a feedback-loop control on the surface separation, we were able to quantify the magnitude of the lift force as a function of the surface separation and driving speed. Our results illustrate how elastohydrodynamic effects can play an important role in the lubrication of compliant surfaces, providing pathways for control of friction and wear

Lubrication with Soft and Hard Two-Dimensional Colloidal Arrays

Normal and friction forces between immobilized two-dimensional (2D) homogeneous non-close-packed colloidal arrays made of different particles are compared in aqueous media. Soft pH-responsive (microgels) and nonresponsive hard (silica) particles of different sizes were used to create the 2D arrays. The results show that the friction of soft responsive structured layers can be successfully modulated by varying the pH, with a friction coefficient varying by nearly 3 orders of magnitude (10^–2 to 1). This important change in lubricating properties is mainly correlated with the particle swelling behavior, i.e., the friction coefficient decreasing exponentially with an increase in the swelling ratio regardless of the size, surface coverage, and degree of ionization of the particles. In addition, the robustly attached microgel particles were able to sustain high pressure (up to 200 atm) without significant surface damage. The 2D arrays of nonresponsive hard particles also gave rise to a very low friction coefficient (μ ≈ 10^–3) under similar experimental conditions and could sustain a larger pressure without damage (≤600 atm). The low friction dissipation observed between the hard arrays resulted from a rolling mechanism. Even though rolling requires nonimmobilized particles on the substrates, the results show the importance of attaching a certain proportion of particles on the surfaces to reduce friction

Interaction between Compliant Surfaces: How Soft Surfaces Can Reduce Friction

International audienceWe describe how a long-range repulsive interaction can surreptitiously modify the effective geometry of approaching surfaces, if compliant, with significant consequences on friction. We investigated the behavior under shear and compression of mica surfaces coated withpoly (N-isopropylacrylamide) pNIPAM-based cationic microgels. We show that local surface deformations as small as few nanometers must be considered to understand the response of such surfaces under compression and shear, in particular when the range of action of normal and friction forces are significantly different, as is often the case for macromolecular lubrication. Under these conditions, a subtle interplay between normal forces and surface compliance may significantly reduce friction increment by limiting the minimum approach of the surfaces under pressure. We found that stiffening of compressed microgels confined in the region of closest approach make i

Boundary Lubricant Polymer Films: Effect of Cross-Linking

We have studied the adsorption and lubricant properties of a multifunctional triblock copolymer poly­(l-lysine)-<i>b</i>-poly­(acrylic acid)-<i>b</i>-poly­(l-lysine). In particular, we investigated the nature of the layer adsorbed under different conditions of polymer and salt concentration and the lubricant properties of the polymer layer before and after its chemical cross-linking by bridging the poly­(acrylic acid) blocks. We found that the amount of polymer adsorbed is controlled by the ionic strength and the polymer concentration in the solution. In all cases, the self-assembled polymer layer is a poor lubricant before cross-linking, but the cohesion and load-carrying ability of the layer are substantially improved by this reaction. However, the chemically cross-linked coating has a limited deformation capacity as a consequence of its permanent network nature, and irreversible damage is observed after excessive strain of the film