Anchoring Transitions

The use of dendrimers as supports to anchor transition metal catalysts has attracted considerable attention over the past decades [48] (see also Chapter 4 of this book). Several groups studied the use of dendrimers immobilised on insoluble supports [49], and this type of material meet the requirements for catalysis in interphases. Alper reported the use of diphosphine functionalised polyamidoamine (PAMAM) dendrimers... 

R. and Abbott. N.L., 2001. Principles for measurement of chemical exposure based on recognition-driven anchoring transitions in liquid crystals. Science, 293, pp. 1296-1299. 

In nematic liquid crystals, the viscosity depends on the relative orientation between the shear gradient and the orientation of the nematic phase. Close to a surface, the orientation is usually governed by surface orientational anchoring [77]. Anchoring transitions, for instance induced by the adsorption of an analyte molecule to the surface [78], can therefore be easily detected with the QCM [79,80]. This reorientation induced by adsorption amounts to an amplification scheme the expected shift in the resonance frequency and bandwidth... 

Chapter 6 provides an extensive review of the uses of carbon as a catalyst, with particnlar emphasis being placed on cases in which active sites have been properly identified and activity correlations established. The special case of nitrogen-doped carbons and their catalytic activity in oxidation reactions is discussed in Chapter 7, and Chapter 8 covers the heterogenization of homogeneous catalysts by anchoring transition-metal complexes onto the snrface of suitable carbon materials. 

J.K. Vij, A. Kocot, T.S. Perova, Order parameter, alignment and anchoring transition in discotic liquid crystals. Mol. Cryst. liq. Cryst. 397, 231-244 (2003)... 

T.S. Perova, J.K Vij, A. Kocot, Observation of an anchoring transition in a discotic liquid crystal. Europhys. Lett. 44, 198-204 (1998)... 

T. Brunet, O. Thiebaut, E. Charlet, H. Bock, J. Kelber, E. Grelet, Anchoring transition in confined discotic columnar liquid crystal films. Eur. Phys. Lett. 93(16004), 1-6 (2011)... 

Patel, J.S., Yokoyama, H. Continuous anchoring transition in liquid crystals. Nature 362, 525-527 (1993)... 

Sometimes the change in the director orientation at the surface occurs spontaneously when temperature is varied. Reorientation of the director also takes place when a liquid crystal is placed on a substrate covered with a thin film of the other material. Changing the thickness of the film, we can reorient the liquid crystal in the bulk. In both cases, the director is forced to accept only those orientations which are compatible with the symmetry of the substrate. Thus, the reorientation has much in common with the phase transition phenomena and the process is usually called the anchoring transition [2] or the local Prederiks transition [3]. 

The anchoring transitions can be jump-like or continuous (first and second order). They can be driven by temperature, dielectric properties [89], the thickness of intermediate orienting layers (as discussed above), the density of orienting layers, or the composition of a liquid crystal (for more details see B. Jerome [2]). 

J.-B. Fournier and P. Galatola, Coarse-grained surface energies and temperature-induced anchoring transitions in nematic liquid crystals, Phys. Rev. Lett., 82,4859 [1999]. 

There are several examples of experimentally observed anchoring transitions (see J6r6me [4] and Pieranski and Jerome [10], and references therein). Since anchoring transitions occurring at the surface of a liquid crystal cell induce a change of director configuration in the cell, these transitions can be used to switch a liquid crystal device between different states [24, 25],... 

This type of calculation has been performed successfully, both by using a general form of the surface energy [22] and experimentally determined surface orientational distributions [66, 67], in order to explain the anchoring directions and anchoring transitions observed on different substrates. These calculations were made within the framework of the Landau-de Gennes... 

In general, liquid crystal molecules do not have the D200 symmetry of the uniaxial nematic phase. Since an interface acts as a field, its presence can provide polar order. Such surface polar ordering, confined to a single molecular layer, has been observed [99]. Surface SHG can also be used to probe the orientational distribution at the surface, and anchoring transitions [100]. 

In addition to transitions between different phases of liquid crystals these are also orientational phase transitions within a single phase. A wellknown example is the Fredericksz transition in nematics. Less well known are the anchoring transitions discussed by Pieranski in this volume. 

Definitions of anchoring transitions a) space of intensive variables... 

Fig. 8. Anchoring transition at the E8/gypsum interface. Nucleation from a point defect. 

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