Nanofluidics: Molecularly thin lubricant layers under confinement

Abstract

Confined liquid films with a thickness in the range of a few molecular diameters exhibit different mechanical properties than in the bulk. With the technique of a 2-dimensional (2D) imaging surface forces apparatus (SFA) we investigated in detail the layer by layer thinning of a thin liquid film confined between two atomically smooth surfaces upon pressing them towards each other with increasing load. The dynamics of a series of subsequent squeeze-out processes of individual layers were analyzed. Using a simple hydrodynamic model, we extracted the thickness-dependence of the viscosity. For the system investigated here—the model lubricant Octamethylcyclotetrasiloxane (OMCTS) confined between ultraclean, recleaved mica surfaces—we found that the viscosity increased by a factor of 10 with decreasing the film thickness from 6 to 2 layers. We decomposed the friction into two components, one describing the sliding of liquid layers on top of the substrates, and the other describing liquid-on-liquid sliding. The latter contribution was found to agree closely with expectations based on the bulk viscosity, whereas the former was approximately 35 times higher for the present system.

Keywords:

• Surface forces apparatus
• Octamethylcyclotetrasiloxane
• Confined liquid films
• Microelectromechanical systems
• Elastohydrodynamics
• Nanotribology

Acknowledgements

We would like to thank S. Herminghaus, B. Persson, S. Granick and J. Klein for fruitful discussions and comments. We acknowledge financial support by the German Science Foundation under grant number Mu 1472/2-1 within the priority program “Wetting and Structure Formation at Interfaces”.