The Orientational order in So-Called de Vries Materials
Artikel i vetenskaplig tidskrift, 2009

There is a great interest in smectic materials that show no layer shrinkage (NLS) in the transition from smectic A to smectic C. Such materials are often discussed in terms of ode Vries materialso or ode Vries behavioro after A. de Vries, who proposed different mechanisms for this NLS behavior, involving a significant tilt of the individual molecules in the smectic A phase. According to the original proposition of de Vries, the molecules are already tilted in this kind of smectic A phase, with a large constant tilt and the same tilt direction in each layer but random tilt direction between different layers. Despite the individual molecular tilt the smectic A phase remains uniaxial and the transition to the biaxial smectic C state is seen as a global ordering in the tilt directions. The model thus ad hoc predicts that there is zero layer shrinkage at the A - C transition. We refer to this model as the ohollow cone distributiono. As later pointed out by de Vries there are, however, other possible models for describing a tilt disorder (thermodynamically unavoidable) in the A phase. Nevertheless, the hollow cone has been a widely accepted model in the literature and is repeatedly referred to in discussing de Vries behavior or even taken as a basis for theories describing it. We discuss different smectic A orientational distribution functions that could be related to de Vries behavior or de Vries transitions. We find that two opposite models have comparable predictive power but only one gives a consistent picture together with existing data. Our conclusion is that the smectic A - smectic C transition can have a continuously changing character from a opure tilto to a opure de Vrieso, and we illustrate the orientational distribution functions in the A and C phases for these two limiting cases. We find that de Vries behavior is not related to any exotic distribution of hollow cone or similar kind in the A phase, but instead to an unusual combination of low nematic order and high smectic order in the de Vries smectic A. As the technical interest in such materials is considerable, a directed effort toward the synthesis of new optimized materials would be important.

De Vries materials

Smectic A

Nematic and smectic order

Orientational distribution


Sven Lagerwall

Chalmers, Mikroteknologi och nanovetenskap (MC2), Fotonik

Per Rudquist

Chalmers, Mikroteknologi och nanovetenskap (MC2), Fotonik

Frank Giesselmann

Universitat Stuttgart

Molecular Crystals and Liquid Crystals

1542-1406 (ISSN) 1543-5318 (eISSN)

Vol. 510 1282-1291



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