Helen F. Gleeson

Anomalously low twist and bend elastic constants in an oxadiazole-based bent-core nematic liquid crystal and its mixtures; contributions of spontaneous chirality and polarity

The splay, twist, and bend elastic constants of an oxadiazole based bent-core liquid crystal have been measured as functions of temperature throughout the nematic phase. The splay elastic constant (K11) behaves similarly to other bent-core systems, though it exhibits rather high values, varying between 8–14 pN. Further, anomalously low, temperature-independent values of both the twist and bend constants (K22 and K33) are found. The combination of such low values of both constants (∼0.15 pN and ∼1.2 pN) and temperature independence has not been observed in any type of nematic material previously. This unusual behaviour has been further investigated using two independent theoretical approaches; a molecular theory based on atomistic simulations of the real molecular structure and an analytical molecular-field theory. Computational calculations show good agreement with the experimental results for both the twist and bend constants. The statistical theory introduces a term related to chiral fluctuations that affects the twist constant, while the inclusion of polarity is known to explain low values of the bend constant. The additional terms enable one to understand the origin of the low values of both elastic constants and to describe their weak temperature dependence. It also offers an explanation for the spontaneous chiral segregation observed in some nematic bent-core systems. Mixtures of this unusual oxadiazole material are made with another similar material (C5-Ph-ODBP-Ph-OC12) that has been extensively studied that exhibits somewhat higher twist and bend elastic constants to explore the behaviour as a function of concentration.

The identification of the sign and strength of disclinations in the schlieren (nucleated domain) texture of the nematic phase, by optical microscopy

ABSTRACT The optical texture of the nematic phase, variously known as the schlieren, structure à noyuax or nucleated domain texture, was identified over a century ago as being an array of point singularities. When viewed between crossed polars, patterns of dark brushes radiate from each point nucleus. The sign and strength of each nucleus can be uniquely determined from the changes in the orientation of these brushes when either the sample or the crossed polars are rotated, from two formulae given by Chadrasekhar in 1977. However, these were given with little exemplification and have been largely overlooked. Consequently, the majority of the discussions given in current literature are either incomplete and confusing or, in some cases, incorrect. Here, we provide a detailed explanation of the textures and their behaviour as viewed with the most commonly used experimental geometry (i.e. with a rotating sample and stationary polars). Graphical Abstract

From understanding structures in antiferro- ferri and ferroelelectric liquid crystals to an unusual electro-optic effect in a bent-core nematic; a celebration of innovative materials

ABSTRACT The field of liquid crystals is truly multidisciplinary with numerous examples of virtuous circles of interaction between chemistry, physics, theory and engineering resulting in breakthroughs in both fundamental understanding and novel applications. This paper, written to mark John Goodby’s 65th birthday, offers a personal perspective of the synergy between chemistry and physics from a collaboration that has spanned three decades. The first part of the paper reviews some of the physics insights that resulted from chiral liquid crystals fundamental to understanding structures in ferroelectric, ferrielectric and antiferroelectric systems. The second part of the paper describes some of the remarkable consequences of the anomalous elasticity and flexoelectricity found in the nematic phases of bent-core materials. In particular, we present unusual bowing of disclination lines in the nematic phase of a bent-core material in the presence of a field. Finally, the paper summarises some future prospects relating for bent-core materials. The paper by no means captures the amazing breadth of contribution that John’s chemistry has made to the subject, but aims to exemplify how his generous collaborative approach coupled with innovative chemistry and physical insight has led to paradigm changes in our subject. Graphical Abstract

Creation and topological charge switching of defect loops on a long fibre in the nematic liquid crystal

ABSTRACT We demonstrate new type of topological defects on a homeotropic fibre aligned perpendicular to the nematic director in a planar nematic cell. Contrary to expectations we can create defect loops which are encircling the fibre along its short axis and are strongly tilted with respect to the fibre. Such loops are always accompanied by two topological solitons, which emanate from the loop and propagate to the left- and right-hand side of the fibre. Unlike previously reported closed loops of either positive and negative charge, encircling the fibre parallel to the nematic director, these loops can carry either positive or negative charge, or can be charge neutral and very stable. We show how to switch the charge of individual loops from positive to neutral and negative charge by adding unit monopoles of appropriate topological charge. We demonstrate new type of interaction of dipolar colloids with these new topological entities on a fibre. Graphical Abstract

Understanding liquid crystal order parameters deduced from different vibrations in polarised Raman spectroscopy

ABSTRACT Polarised Raman Spectroscopy (PRS) has been used to measure order parameters in liquid crystalline materials for decades. However, it is well known that different values of the order parameters are deduced for the same material when different vibrational modes are used in the analysis. This is an undesirable discrepancy that has somewhat hindered the use of the technique. Here we use two Raman active bands namely the phenyl (1606 cm−1) and cyano (2220 cm−1) stretching modes in the nematic phase of 5CB (4-cyano-4ʹ-pentylbiphenyl) as the example to explore the origin of such discrepancy. Two approaches are proposed in the data a nalysis taking either non-axial or non-cylindrical symmetric vibration into consideration. Together with a systematic discussion based on experimental data, we can conclude that whether or not the vibration satisfies the conditions associated with cylindrical symmetry is the correct physical explanation for the discrepancy in the order parameters. Graphical Abstract

Understanding local forces in electrophoretic ink systems: utilizing optical tweezers to explore electrophoretic display devices

Optical tweezers can be used as a valuable tool to characterize electrophoretic display (EPD) systems. EPDs are ubiquitous with e-readers and are becoming a commonplace technology where reflective, low-power displays are required; yet the physics of some features crucial to their operation remains poorly defined. We utilize optical tweezers as a tool to understand the motion of charged ink particles within the devices and show that the response of optically trapped electrophoretic particles can be used to characterize electric fields within these devices. This technique for mapping the force can be compared to simulations of the electric field in our devices, thus demonstrating that the electric field itself is the sole governor of the particle motion in an individual-particle regime. By studying the individual-particle response to the electric field, we can then begin to characterize particle motion in ‘real’ systems with many particles. Combining optical tweezing with particle tracking techniques, we can investigate deviations in many particle systems from the single-particle case.

Liquid crystals through experiments (IoP concise physics)

Prof. Čepič is a renowned expert in both liquid crystal research and in education and this book represents a delightful and very useful convergence of these two areas. It is designed to be used by teachers and in my opinion will be extremely useful to teachers at all levels – school teachers, postgraduates who may be demonstrating concepts for the first time to undergraduates, early-career researchers (postgraduates, postdocs and academics) who are trying to engage the public in their research and the ideas it encompasses and university teachers who are trying to explain concepts around soft matter and liquid crystals to undergraduates. The book achieves this by careful explanation of the relevant liquid crystal science, imaginative suggested experiments, well-thought out hints for the teachers and a structured approach that will encourage learning.