Presmectic ordering

Keywords Atomic force microscopy Liquid crystal Surface-induced ordering Presmectic ordering Landau-de Gennes... 

Monitoring Prenematic and Presmectic Ordering by Force-Distance Spectroscopy... 

A fit of the force curves with the sum of the prenematic (Eq. 2) and presmectic force (Eq. 4) permits to determine the correlation length of prenematic and presmectic ordering, the anchoring energies wi, W2, as well as the repeat distance ao (see Table 1). 

The full lines in Fig. 3.8 are best fits to the presmectic interaction, based on the LdG theory [22,24] with an addition of the van der Waals force [2,14]. Just like in previous nematic cases, the mesoscopic LdG theory superbly describes the structural force between the surfaces. The fitting of the measured presmectic forces to the theory allows for the determination of many important surface parameters, that are difficult to obtain using other methods. For example, the amplitude of the smectic order at the surface and the smectic correlation length can be obtained directly. For 8CB on silanated glass, one obtains a typical surface smectic order parameter iPfi which is of the order of 0.1 and the smectic correlation length y, which is in perfect agreement with X-ray data of Davidov et al. [25]. In addition to that, it has been observed that the smectic order is coupled to the nematic order and this amplifies the presmectic interaction close to the isotropic-nematic phase transition [23]. 

Moreau L, Richetti P, Barois P (1994) Direct measurement of the interaction between two ordering surfaces confining a presmectic film. Phys Rev Lett 73 3556... 

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. 

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]... 

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