Liquid to solidlike transitions of molecularly thin films under shear

Abstract

We have measured the shear forces between two molecularly smooth solid surfaces separated by thin films of various organic liquids. The aim was to investigate the nature of the transitions from continuum to molecular behavior in very thin films. For films whose thickness exceeds ten molecular diameters both their static and dynamic behavior can usually be described in terms of their bulk properties, but for thinner films their behavior becomes progressively more solidlike and can no longer be described, even qualitatively, in terms of bulk/continuum properties such as viscosity. The solidlike state is characterized by the ordering of the liquid molecules into discrete layers. The molecular ordering is further modified by shear, which imposes a preferred orientation. All solidlike films exhibit a yield point or critical shear stress, beyond which they behave like liquid crystals or ductile solids undergoing plastic deformation. Our results on five liquids of different molecular geometry reveal some very complex thin-film properties, such as the quantization of various static and dynamic properties, discontinuous or continuous solid–liquid transitions, smooth or stick–slip friction, and two-dimensional nucleation. Quantitatively, the ‘‘effective’’ viscosity in molecularly thin films can be 105 times the bulk value, and molecular relaxation times can be 1010 times slower. These properties depend not only on the nature of the liquid, but also on the atomic structure of the surfaces, the normal pressure, and the direction and velocity of sliding. We also conclude that many thin-film properties depend on there being two surfaces close together and that they cannot be understood from a consideration of a single solid–liquid interface. The results provide new fundamental insights into the states of thin films, and have a bearing on understanding boundary friction, thin-film lubrication and the stress–strain properties of solids at the molecular level.