Boundary conditions in fractionated lubrication

The concept of fractionated lubrication is proposed, to cover sliding contacts in which the fluid load carrying capacity is "split" into geometrically disjoint zones, whose supply conditions are independent. In fractionated lubrication, the lubricant supply of local carrying zones is weak (or zero), and not controlled. Local pressure and shear rate are generally high. Specific questions arise when trying to model fractionated lubrication, namely: Phenomena occurring at the immediate vicinity of a load carrying zone (starved lubricant supply, free boundaries...) act as lateral boundary conditions, and influence the flow rate in the carrying zone and consequently the local pressure, Wettability of the solid surfaces influences the wall boundary conditions for the problem of the fluid flow. These questions are expanded, and situated relative to the theories of lubrication and wettability. A model problem has been defined, which emphasises the influence of both wall boundary conditions and lateral ones; it consists of a thin liquid drop which is trapped between two plates and submitted to shear by the differential tangential speed of the plates; the surface tension of the fluid and the wettability of the solid surfaces influence the boundary conditions in the model problem. An outline of experimental and theoretical exploratory tools to tackle the model problem is presented. Preliminary experimental results show that analyses based on classical wetting theory do not allow to predict the behaviour of sheared drops, thus demonstrating the originality of the problem. In some cases apparent slip at the wall was observed. A theoretical model has been developed parallel to the experimental approach. Wall boundary conditions of slip proportional to the shear stress have been introduced in the Reynolds equation. The lateral boundary conditions describe the stress conditions at the menisci, and the flow between meniscus and wall.