This award to Case Western Reserve University by the Solid State Materials Chemistry program in the Division of Materials Research is to enhance our understanding about the nature of interaction between surfaces and liquid crystal, especially in the realm of controlled polar tilt. The main goal of the project includes novel approaches to alignment using ink-jet technology, to quasiperiodic dielectric lasers based on liquid crystals, and to surface chirality-induced polarization phenomena. Advances in these areas are expected to transform our conceptions about, and methodology toward, liquid crystal orientation control and applications. Additionally, the proposed work seeks to achieve significantly improved control and much deeper understandings of anchoring well beyond the harmonic approximation and of reverse tilt domains and associated topological defects. In this light, the proposed work has several objectives interconnected by the common theme of surface manipulation. As part of this project, two different aligning polyimides will be used to control pretilt angle of the liquid crystal on length scales as small as a few micrometers. These studies are expected to develop a novel electrically-switchable, polarization-independent blazed grating. The proposed studies with quasiperiodic dielectric structures via surface orientation control are expected to serve as photonic band gap devices, and may be used as highly tunable mirrorless lasers.
The scientific broad impact of the proposal could be very strong in understanding the basic science behind the interactions of liquid crystals with alignment layers and the basic soft condensed matter physics about these alignments and interactions. The properties of new gratings that are planned would have interesting applications as lasing media and new devices such as laser beam steering and adaptive optics. The proposed teaching, training and outreach activities of students from High School to Postdoctoral levels would have a significant impact in developing a strong pool of future scientists. As part of this research project, new graduate and undergraduate courses such as ?Optics of Complex Fluids? and ?The Physics of Liquid Crystals? will be further developed and expanded, and will be incorporated in to Case Western University curriculum.
Our work focuses on surface phenomena and control in liquid crystals, which is important not only in fundamental surface science, but also has important implications in display technology. More recently our work has evolved to include the study of chirality, especially at interfaces. An object is "chiral" if it cannot be superimposed on its mirror image by a combination of translations and rotations. The classical example is one’s hands: The mirror image of the left hand is the right hand. One cannot rotate the hands so that the left and right hands overlap. With chirality come many important physical and biological phenomena. For example, most drugs are chiral, and the plane of polarization rotates as light traverses a chiral medium. The project began by focusing on the interaction of liquid crystals with both untreated and treated substrates, i.e., substrates coated with a thin polymer layer that subsequently was rubbed mechanically either with a commercial rubbing cloth, or scribed with the stylus of an atomic force microscope, thereby facilitating nanoscopic patterns that serve as an "easy axis" for the orientation of the liquid crystal molecules. We first examined topological defects in the liquid crystal when the molecules were tilted with respect to the surface -- this is referred to as "pretilt" (image 3). We measured the robustness of these defects to temperature changes through phase transitions, and demonstrated how the defects are affected by the initial amount of pretilt. We also examined how the pretilt is affected by the polymer baking regimen. Perhaps most important was that we demonstrated that a mixture of two types of polymers, one that promotes planar liquid crystal alignment and one that promotes vertical alignment, can be used to create a tilted alignment at the surface. Our ability to tailor the pretilt angle was contrary to the then conventional wisdom, and facilitates development of faster displays with improved optical performance. In the high temperature isotropic phase any orientational order imposed in the liquid crystal by a surface decays to zero as one transits into the liquid crystal's bulk . For more than 30 years it was assumed that the interaction between the surface and liquid crystal is highly localized, and as a result the order should decay exponentially. Using the ultrahigh resolution technique of optical nanotomography that was developed in my lab, we showed that the order does not decay spatially exponentially, but rather decays with an initial shoulder before the onset of a rapid quasi-exponential decay (image 2). This result demonstrates that the surface interaction is not local, but actually extends across the interface over several nanometers. During the course of our work using patterned substrates we observed a curious phenomenon: the appearance of light and dark stripes under a polarizing microscope. Experimentation revealed that we had, unknowingly, created a quasi-two-dimensional chiral pattern in the substrate, which then transmitted its chiral properties into the adjacent nonchiral liquid crystal. With this realization we set out to develop a number of chiral patterns that could be scribed into the surface, and investigated the resulting chiral behavior of the liquid crystal at the interface. This work ultimately may lead to new methods to separate left and right handed enantiomers at surfaces. Recently our work on surfaces and chirality led to a fundamental breakthrough. Since antiquity, chirality has been a "bottom - up" phenomenon, where the chiral molecules result in chiral macroscopic behavior. Here we deal with chirality from the opposite direction: We applied a macroscopic torsional strain, i.e., we twisted the cell (image 1), to induce a deracemization of the initially racemic conformer distribution. The electrooptic signature -- this is the so-called "electroclinic effect" -- along with a simple mathematical model, showed that the director twist induces a deracemization of the initially equal number of right and left handed chiral molecular conformers (‘‘top-down’’ chiral induction). Using this experimental geometry we now are able to separate contributions to the electroclinic effect between those of chiral origin and those from the rub-induced surface topography. Being able to separate these two phenomena allows us to quantify the "strength" of the rub-induced two-fold symmetry. There broader impacts of our work are many. In addition to training students from the postdoctoral through the high school levels, I presented numerous public lectures in the U.S., Europe, Asia, and South America. All students have gone on to industrial or academic jobs. Our scientific results have had important consequences in the display industry, not only in terms of treating surfaces to achieve desired pretilt, but also to minimize unwanted topological defects. Our work on chirality has demonstrated how one can create a chiral surface environment by mechanical means, and has led to an understanding of chiral induction; these results have important consequences in pharmaceuticals, chiral chemistry, and potential new surface-controlled display devices.
