Alberta Ferrarini tells Journal of Materials Chemistry about her hot paper.

Could you explain the significance of your article to the non-specialist? 
Liquid crystals possess properties that are analogous to those of crystals. For example, nematic liquid crystals exhibit flexoelectricity, that is an induced polarisation when the uniformity of the alignment is disturbed; this is clearly analogous to the piezoelectricity of crystals. The flexoelectricity is thought to be important for certain liquid crystal displays and so materials for these displays need to have large flexoelectric coefficients. Here theory is used to understand, at a molecular level, the flexoelectricity found for liquid crystal dimers in which rigid groups are linked by a flexible spacer. This has provided the first semi-quantitative understanding of their flexoelectric behaviour in terms of the realistic molecular geometry and charge distribution for selected conformations of the spacer.

What has motivated you to conduct this work?
Our primary motivation was the development of a tool with which to relate the realistic molecular structure of a liquid crystal to its flexoelectric behaviour. In this way we aimed to create a theory-led approach to the rational design of new nematic materials with large flexoelectric coefficients. Several reasons led us to select liquid crystals dimers as a benchmark of the theory: they possess large flexoelectric coefficients, significantly larger than the corresponding monomers and the origin of this difference was not clear. Indeed, the high flexoelectricity, combined with other properties of dimers, makes them attractive for display applications.

Where do you see this work developing in the future?
Our next task is to extend the computations to include conformational averages of the flexoelectricity coefficients which is a non-trivial task given the large number of conformers. This averaging is important to facilitate a complete comparison with experiment. It is also essential to explore the role played by two molecular factors in determining the flexoelectric coefficients. These are the electrostatic distribution within the mesogenic groups, which need not be the same, and the geometric structure of the dimer, in particular its shape which is largely determined by the groups linking the mesogenic groups to the spacer. We also intend to apply our method to other classes of mesogens, in particular to the so called bent-core mesogens which, among other unusual properties, have been claimed to possess huge flexoelectricity. One intriguing prediction of our theory for liquid crystal dimers is the large contribution made to individual flexoelectric coefficients by the quadrupolar charge distribution in the mesogenic molecules. If this prediction is confirmed, then liquid crystal displays based on individual coefficients and not the sum as in present devices, where the quadrupolar contribution is zero, will merit development.

Are there any particular challenges facing future research in this area?
The flexoelectric effect is a material property exhibiting a strong dependence upon the molecular structure and this relation is non-trivial because of the interplay of shape and charges. This is an example of a common problem for the behaviour of complex materials. Sophisticated methods are now available for the prediction of single molecule properties; it is challenging to convey this information into the prediction of the properties of materials. To this purpose, suitable theoretical approaches need to be developed, able to connect different length- and time-scales. This entails the distinction between the relevant variables, which require a detailed representation, from unnecessary degrees of freedom, which can be treated at a coarse grained level and the combination of methods appropriate for the different scales.

 

Alberta Ferrarini  is Associate Professor in the Chemical Sciences Department at the University of Padova, Italy. Her research interests focus on the development of theoretical methods and computational tools for the molecular interpretation of equilibrium and dynamical properties of liquids and complex fluids. They have been applied to materials science and biologically motivated problems. Examples include: dielectric and elastic properties of liquid crystals; order and dynamics of phospholipids in bilayers; organization of lyotropic suspensions of proteins, DNA and viruses; chirality amplification in liquid crystals.

 

Cristina Greco was born in Verona, Italy, in 1981. She received her First Level Degree in Materials Science from the University of Padova, Italy, in 2006. She is currently studying at the University of Padova to attain her Masters Degree in Materials Science and Engineering.

 

Geoffrey Luckhurst is Professor Emeritus in the School of Chemistry at the University of Southampton where he had previously held the Chair of Chemical Physics. He has wide interests in the behaviour of liquid crystals at both molecular and macroscopic levels. Included in these is the design and synthesis of liquid crystal dimers, their physical characterisation and the development of theory and simulation to understand their properties.