This award supports theoretical research and education to investigate phase transitions between smectic structures in liquid crystals, specifically the smectic-A - smectic-C transition. Recently, there has been significant activity in the synthesis and experimental study of technologically promising new chiral liquid crystal materials. These materials exhibit Sm-A - Sm-C transitions with several unusual properties including very strong fluctuation effects, minimal layer contraction, and colossal electro-optical response, which is particularly appealing to the liquid crystal display (LCD) industry. The transition in most of the materials is either continuous and near a tricritical point, or first order.
The central aim of this project is to study the chiral Sm-A - Sm-C transition. One focus of the research will be the transition at a tricritical point and the roles played by fluctuations and chirality. The project will also involve analysis of a novel chiral biaxial Sm-A phase, which may occur between the uniaxial Sm-A phase and the Sm-C phase. Biaxiality is a significant issue in the field of liquid crystals, particularly in the search for the elusive and technologically desirable biaxial nematic phase. Another component of the project will be the analysis of the surface electroclinic effect near a first order chiral Sm-A - Sm-C transition. The surface electroclinic effect is a phenomenon, of particular importance in the design of ferroelectric LCDs, whereby the chiral Sm-A - Sm-C transition can be locally induced near a surface. There is reason to believe that the surface electroclinic effect is particularly sensitive near the chiral first order Sm-A - Sm-C phase boundary.
This project has both broad scientific and technological implications. It will further the fundamental understanding of phase transitions. Additionally, a theoretical framework for the properties of the unusual new materials discussed above will be technologically valuable in terms of optimizing their applications and guiding the future synthesis of desirable new materials. The wide-ranging applicability of liquid crystals in science and industry adds value to their study, particularly as the nation focuses increasingly on scientific and technological education and innovation. In particular, this award will support the training of undergraduate students in condensed matter and liquid crystals research.
NON-TECHNICAL SUMMARY
This award supports theoretical research and education on liquid crystals. Liquid crystals are a fascinating class of soft materials that have a rich variety of internal structures with molecules in spatial arrangements less organized than solid crystals but with directional patterns formed by the orientation of the constituent molecules. So, they can exhibit a range of phases intermediate between liquid and crystalline. These phases can be classified according to the types of order and patterning in their molecular arrangements. Liquid crystal materials exhibit transitions between different molecular arrangements or phases. An everyday example of a phase transition is between water and ice. The rich variety of liquid crystal order and phase transitions has led to considerable scientific interest in their properties. In addition to the fundamental scientific motivation to understand liquid crystals, there is significant technological incentive. Liquid crystals have many wide-ranging applications including LCDs, more explicitly liquid crystal displays, a multibillion dollar industry.
This award will facilitate the investigation of a particular phase transition between layered structures in liquid crystals: the smectic-A - smectic-C transition. It will also allow the investigation of a possibly new phase, as well as the surface effects on liquid crystals. This last part of the project is important in the design of LCDs.
This project has both broad scientific and technological implications. It will further the fundamental understanding of phase transitions. Additionally, a theoretical framework for the properties of the unusual new materials discussed above will be technologically valuable in terms of optimizing their applications and guiding the future synthesis of desirable new materials. The wide-ranging applicability of liquid crystals in science and industry adds value to their study, particularly as the nation focuses increasingly on scientific and technological education and innovation. In particular, this award will support the training of undergraduate students in condensed matter and liquid crystals research.
Liquid crystals are a fascinating class of soft materials that exhibit a range of phases intermediate between liquid and crystalline. These phases can be classified according to the types of order and patterning in their molecular arrangements. Liquid crystal materials exhibit transitions between phases. An everyday example of a phase transition is between water and ice. In addition to the fundamental scientific motivation to understand liquid crystals, there is significant technological incentive. Liquid crystals have many wide-ranging applications including LCDs, i.e. liquid crystal displays, a mutlibillion dollar industry. This award facilitated the investigation of a particular phase transition between layered structures in liquid crystals: the smectic-A – smectic-C transition. As shown in Figure 1, in both the smectic-A (Sm-A) and smectic-C (Sm-C) phases the rod-like molecules (shown in red) form layers. This layering is highlighted using blue lines in Figure 1. The alignment of the molecules acts as a polarizer (like that in sunglasses) and control of the direction alignment using an electric field allows the liquid crystal to display as bright or dark. This is the basic principle of a liquid crystal display (LCD). In going from the Sm-A to the Sm-C phase at a special temperature, the molecules tilt relative to the layers, which corresponds to a tilting of the polarizer direction, which has the potential to be exploited for improved LCD quality. This potential has motivated the synthesis of many new liquid crystal compounds, many of which exhibit a Sm-A to the Sm-C transition with unusual properties. It was our aim to develop models in order investigate the mechanisms behind some of these unusual features. A summary of these investigations follows: (1) Analysis of the critical behavior of the Sm-A∗ – Sm-C∗ transition at a tricritical point. As the Sm-A to the Sm-C is approached, the system will exhibit large critical fluctuations. Critical fluctuations are a feature of many phase transitions, including water-steam, whereby patches of the phase about to be entered begin to develop. Thus, while in the Sm-A phase, patches of Sm-C develop. Many new liquid crystal compounds exhibit a Sm-A to the Sm-C transition that is very near a tricritical point. This is a point at which the transition goes from continuous to abrupt, and at which the critical fluctuations are especially large. Our analysis showed that these fluctuation effects are spectacular. In particular they lead to large scale layer undulations, which grow so large that right at the transition temperature the layering itself is destroyed. As a result of these large layer fluctuations, the compressibility decreases significantly, i.e., the layers become easier and easier to squeeze together. (2) Control of the surface electronic effect in the Sm-A phase near a ?rst order Sm-A – Sm-C transition. In the Sm-A phase of some materials, pre-tilting of the molecules can occur near the surface of an LCD- this is known as the surface electroclinic effect (SEE). As one moves away from the surface, the degree of tilt decreases to zero. This variation of the tilt within the LCD can lead to difficulties, so it is often desirable to eliminate it. We developed a model to analyze the SEE, in particular how it can be controlled by applying an external electric field and/or varying the temperature. As a result of this analysis we have made several interesting predictions. Firstly, it is possible to eliminate and reverse the surface tilt via the application of an electric field. Secondly, it was shown that stresses can result from a difference in the layer spacing at the surface and in the center of the LCD. This means that for certain ranges of applied electric field, the difference in surface and bulk layer spacing can result in layer buckling, thus reducing the overall quality of the LCD. (3) Manipulation of the helical superstructure in chiral smectics Many recently synthesized smectics are chiral, i.e., they have an enantiomeric excess (an imbalance between the number of right handed and left handed molecules- picture screws instead of rods in Figure 1). In the Sm-C phase, the chirality of the material manifests itself as a helical superstructure in which the direction of the tilt traces out a helix as one moves from layer to layer. One can visualize this helix as the tip of the molecules tracing out the rail of a spiral stair case running up through the layers. We have investigated how this helix can be manipulated using temperature and electric field and showed that it should be possible for the system to exhibit reentrance: by ramping up the electric field, the system may go from not having a helical superstructure (being "unwound") to having a helical superstructure (being "wound") to being unwound. The system may even exhibit double reentrance: as the temperature is lowered the system may exhibit the sequence unwound-wound-unwound-wound.
