Solvation Forces in Liquid Crystals

One could expect that liquid crystals confined between two solid surfaces should lead to strong oscillatory forces since they show long-range order even in the bulk. This is indeed the case, and solvation forces have been observed in various liquid crystalline systems [1128,1130-1134]. In liquid crystals, the presence of an interface generates a smectic layering, while the bulk fluid is still isotropic or nematic. Such layers are called presmectic layers. 

To describe interfacial forces caused by orientation correlations, Landau theory in combination with a mean field approach has been applied. In Landau theory, the free energy density of a system is expanded in a power series of the order parameters and their derivatives. A description of the theory is beyond the scope of this book and the reader is referred to textbooks on the statistical physics of liquids. It was first applied to describe hydration forces (see below) [1135,1136]. De Gennes applied it to liquid crystals [1137], and sometimes the theory is referred to as Landau-de Gennes theory. 

Depending on the boundary condition for ordering at the surfaces and the kind of structure of the liquid, different equations describe the force. For example, the presmectic force between a sphere of radius JJp and a plane made of an identical material (symmetric case) with fixed orientation of the liquid molecules directly in contact with the surfaces is [1128, 1131, 1134] 

1) Lev Davidovich Landau, 1908-1968. Russian theoretical physicist, professor in Moskau, Nobel Prize in physics, 1962. 

2) Pierre-Gilles de Gennes, 1932-2007. French physicist, professor in Paris, Nobel Prize in physics, 1991. 

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