Amparo Galindo, of Imperial College London's Department of Chemical Engineering, tells Soft Matter about her hot paper.

1. What have you achieved with this work?
Our objective was to study the influence of confinement on a discotic liquid crystal including the capillary process arising in slab geometry. The study is focused on the I-N transition region, and we use Monte Carlo molecular simulations in the isothermal-isobaric (NPT) and Gibbs ensembles. We confirm the predictions of Sheng, in that confinement causes a capillary transition; i.e., a change in the coexistence density (or coexistence pressure) of a liquid crystal fluid. A key result has been to observe both capillary isotropisation, in which disordered isotropic phases are promoted, and capillary nematisation, in which ordered nematic phases are promoted, for different types of walls.

2. Could you explain the significance of your article to the non-specialist?
The ability of certain molecules to form liquid crystalline phases, intermediate between solids and liquids, has been known for more than 100 years.  These materials are now at the centre of the multimillion display and optoelectronics industry, and their application invariably involves the interaction of the liquid crystal material with a surface. The solid surface causes a breaking in the symmetry of the system that alters the structure of the liquid crystal phase, and due to this effect, the key to successful novel applications and materials relies on the rational control of the position and orientation of the molecules in the presence of surfaces and under confinement. We have used computer simulation models to study the effect of different types of confining planar walls; the difference is in the 'chemistry' of the walls: either fully repulsive, or hard, or absorbing. If the walls are hard, they are seen to promote ordered states, and a so-called capillary nematisation transition is seen, if the walls are absorbing, a layer of absorbed molecules embedded in the wall acts as a rough wall for the rest of the molecules and it induces disordered states and a capillary isotropisation transition.

3. What has motivated you to conduct this work?
In the last few years discotic liquid crystals have become a subject of major interest, partly due to the recent commercialisation of nematic discotics in the production of optical compensation films for displays by Fuji, and to the high charge-carrier mobilities observed in columnar phases, which lead towards practical technological applications, such as in photovoltaic cells, light-emitting diodes and electrically tuneable cholesteric mirrors. In addition, the investigation of the stability of these crystalline phases also finds relevance in the self assembly of biomolecules in living cells.  For instance the nucleosomes consist of disc-like protein aggregates (10 nm diameter), which form chiral columnar structures. There are however few studies involving discotic liquid crystals (prolate or rod-like molecules are more commonly the subject of liquid crystal studies, partly, although not only, because theoretical approaches are more accurate for prolate particles). Our interest in this area stems partly from such lack of studies of discotic phases, and also from the development of so-called super-hydrophobic materials, where a very rough surface prevents wetting of the surface, hence our interest in the effect of different walls on the phase behaviour of these materials.

4. Where do you see this work developing in the future?
There are a number of future avenues for this work: one would be to attempt to study the nematic-columnar transition in confinement, although we expect this to be very challenging as it is very difficult to equilibrate very ordered states in confined geometries. The interest here comes from the high charge mobility observed in columnar phases which is leading to many practical applications, it'd be interesting to understand the phase behaviour and how it is affected by confinement. A second route for future work, which should be more attainable, is the study of confinement in non-planar geometry, this was the initial motivation for this work, and it remains our long term goal. 

5. Are there any particular challenges facing future research in this area?
An important challenge in the study of liquid crystals by computer simulation is to consider atomistic models. In our work we use simplified, coarse-grained models, which are crucial to further our understanding of the fundamental physical phenomena, but it is also important to acknowledge the important effect of the detailed chemistry of the molecules. A number of groups are considering such models. In addition, more computer simulations of confined fluids in complex geometries should be carried out.