Nature has produced water-based lubricant systems that far outclass the best of most man-made devices. Biolubrification involves complex hydrodynamic flow in the vicinity of soft interfaces as fluid membranes in a wide length scales. A major challenge resides thus in understanding the relations connecting these different scales. This is the main goal of our project which combine the expertise on tribology, biotribology and rheology experiments. We have developed an original set-up which allow us to combine well defined tribology and velocimetry experiments.We have first investigated tribology of lipid monolayer and tri-layer at solid/air interface. A first surprising result is the importance of the humidity rate on the shear plane location and on the tribological properties which exhibit a non-monotonic behavior. To have a better understanding of these results, we need to have hight resolution structural information on monolayer and tri-layer under different humidity rate by using neutron reflectometry. This is the purpose of this proposal.