ORIGINAL PAPER Raman Scattering from Confined Liquid Films in the Sub-Nanometre Regime David A. Beattie Æ Sarah A. Winget Æ Colin D. Bain Received: 6 December 2006 / Accepted: 1 March 2007 / Published online: 12 May 2007 Ó Springer Science+Business Media, LLC 2007 Abstract Raman spectroscopy has been used to study the confinement of octamethylcyclotetrasiloxane (OMCTS) and n-hexadecane between a prism and a lens at contact pressures of 40 MPa. Both lens and prism surfaces were optically smooth but not atomically flat. A total internal reflection geometry was employed to provide the sensi- tivity needed to detect liquid films of sub-nanometre thickness. For both liquids, the thickness of the residual film under load corresponded to less than a monolayer of liquid. The Raman spectra of the confined liquids were identical or very similar to that of the bulk liquid. Similar results were obtained for hexadecane confined between two surfaces coated with a Langmuir-Blodgett monolayer of a fatty acid. We infer that the liquids do not form a boundary layer under these conditions but rather they are squeezed out of the contact under the modest pressure applied. We ascribe the residual signal to liquid trapped in scratches or other defects on the surfaces. Keywords Spectroscopy  Raman  Boundary lubrication  Nanotribology  Hexadecane  OMCTS Introduction The behaviour of thin liquid films confined between solid surfaces is of great interest in the study of lubrication. In the mixed lubrication regime, between elasto-hydrody- namic lubrication and boundary lubrication, the thickness of the film of lubricating fluid is of molecular dimensions [1]. The last 20 years have seen the development of a number of methods for obtaining information from such thin liquid films confined between two surfaces. The dominant techniques for studying liquid films are scanning probe microscopy (especially atomic force microscopy (AFM)) [2] and the surface force apparatus (SFA) [3]). Both of these techniques measure the microscopic force (both normal and lateral) between surfaces. Numerous experiments using these techniques and idealized surfaces have suggested that fluids form ordered layers when con- fined between atomically flat surfaces, with stepwise thinning of the film and dramatic changes in the viscosity of the film [4–10]. Theoretical calculations have been performed by a number of authors to explore the behaviour of liquids in confined systems, along with calculations of the mechani- cal properties of the confined film. The results from cal- culations for model globular molecules [11–13] and linear molecules [14, 15] confined between atomically smooth surfaces reproduce the layering phenomena and altered shear response observed in SFA and AFM experiments. However, input of surface roughness into the simulations results in a disappearance of the confinement phenomena [16–18]. These results are more in line with those from D. A. Beattie  S. A. Winget  C. D. Bain Physical and Theoretical Chemistry Laboratory, University of Oxford, South Parks Road, Oxford, OX1 3QZ, UK Present Address: D. A. Beattie Ian Wark Research Institute, University of South Australia, Mawson Lakes, Adelaide, SA 5095, Australia Present Address: S. A. Winget Agnes Scott College, 141 E. College Ave., Decatur, GA 30030, USA Present Address: C. D. Bain (&) Department of Chemistry, Durham University, South Road, Durham, DH1 3LE, UK e-mail: c.d.bain@durham.ac.uk 123 Tribol Lett (2007) 27:159–167 DOI 10.1007/s11249-007-9214-0