A review of in situ methodologies for studying elastohydrodynamic lubrication

Research in elastohydrodynamic lubrication (EHL) was, at its very beginning, essentially numerical, and even analytical. This area has, however, significantly evolved due to the necessity of validating the first models experimentally. Moreover, it soon became clear that the elastohydrodynamic (EHD) problem would be a topic of much more complex nature than the basic one involving perfectly smooth surfaces, Newtonian fluid, isothermal flow, etc. The experimental approach was the natural choice to make progress in offering the possibility of comparisons, first at the global level of the contact and, over time, at higher and higher spatial resolutions. The in situ analysis of elastohydrodynamic contacts was launched and everyone knows how much its contribution to the field has been and remains important. The present paper takes the task of reviewing the different in situ experimental techniques that have proven to be able to map the key-parameters in living EHD contacts. They are presented both in chronological order and according to the physical mechanism behind them, namely electrical, optical and acoustic methods. A last section presents some opportunities and recommendations for future works, with the aim to improve further our understanding of EHL.