Surface forces in thin layers of liquid crystals as probed by Surface Force Apparatus - SFA

The Surface Force Apparatus (SFA) probes the surface properties of nanome- ter thin layers of liquids by measuring the forces acting on the confining sur- faces. It has been developed in the 70’s [1], allowing the first direct confir- mation of the Derjaguin-Landau-Verwey-Overbeek (DLVO) theory of surface forces in classical colloidal systems. SFA has provided important contribu- tions in the domain of polymers (depletion forces), colloids and organic solu- tions (hydrophobic forces, layering structuration of the interface) and, more recently, friction forces on the nanometer scale (see ref. [2] for a complete re- view). In liquid crystals specific forces are expected, in view of the particular structure and order of different liquid crystalline phases. First, when there is a long-range positional or orientational order in a confined system, the free energy depends on surface separation, which always produces some long- range elastic force, also considered as a structural force. An example is the constrained smectic sample, described in Sec. 3. Second, the surfaces may en- hance or lower the order parameter in a thin boundary layer of a few tenths of nanometers. A close proximity of two identical surfaces causes overlapping of these layers, producing short-range surface-order forces. An example is the system described in sec. 4, where the surfaces induce positional order in a non- layered nematic. Third, the nano-confinement suppresses a number of modes of orientational fluctuations and produces fluctuation forces, by a mechanism similar to that producing Casimir forces between non-charged metals [3].