Liquids under Shear Explored by Neutron Scattering: A Problem in Lubrication Max Wolff, Rainer Hock and Andreas Magerl Lehrstuhl für Kristallographie und Strukturphysik, Universität Erlangen-Nürnberg, Bismarckstr. 10, 91054 Erlangen, Germany. Bernhard Frick Institut Max von Laue-Paul Langevin, Avenue des Martyrs, 38042 Grenoble, France. Hartmut Zabel Lehrstuhl für Festkörperphysik, Ruhr-Universität-Bochum, Universitätsstr. 150, 44780 Bochum, Germany. ABSTRACT Considering the atomic scattering cross sections for neutrons they are an excellent tool to investigate lubrication problems. Two different shear cells have been built to investigate both the dynamics and structural properties of liquids under shear: one cell has been optimised for quasielastic and inelastic neutron scattering while another one has been designed for reflectivity and diffraction work. The dynamical aspects have been studied on the high-resolution backscattering instrument (IN16 at Institut Laue-Langevin (ILL)). Data with a commercial motor oil as a sample have been taken in contact with an aluminium boundary showing directly the developing anisotropy of diffusion under shear. Furthermore within the same set-up it has been possible to monitor the macroscopic velocity distribution including surface slip. In addition, a diffraction experiment has been carried out, demonstrating from a measurement of the position and the profile of the graphite 002 reflection that the ordering of macroscopic graphite particles in a flowing liquid can be studied with neutrons and an ordering with a tilt angle of the particles of 5° to the flow has been determined. INTRODUCTION It is estimated that about 6 % of the gross national product of the USA are lost due to friction and wear [1]. In spite of this fact the detailed mechanism of lubrication is not well understood up to now. Considering the high penetration power of neutrons for many engineering materials and the large scattering cross section of hydrogen contained in most lubricants neutrons appear to be an excellent tool to investigate lubricants in massive environments like cryostats, furnaces or shear devices under conditions relevant for applications. Arguing along this line it has been shown that the macroscopic flow of lubricants can be observed by neutron backscattering [2, 3, 4]. The investigations have revealed different velocity distributions between a fixed and a rotating disc depending on the adhesion to the surface. Such differences in the macroscopic flow should relate with structural changes as well as with changes in the microscopic diffusion processes, especially at the solid liquid interface. This contact region has recently attracted much attention. Differences from the normally assumed non slip boundary condition of liquids have been found by light scattering [5] and atomic force microscopy [6]. An unusual ordering of macroscopic particles near the surface under flow has also been reported by x-ray scattering [7]. In the present work we investigated the macroscopic velocity distribution of a commercial motor oil between a fixed and a rotating disk for a temperature of 18°C via neutron backscattering measurement, Mat. Res. Soc. Symp. Proc. Vol. 710 © 2002 Materials Research Society DD2.9.1