D. A. Coleman

Chiral Isotropic Liquids from Achiral Molecules

Packing Bananas and Boomerangs Assembling achiral molecules typically generates achiral domains. However, odd things can happen when the molecules are banana-or boomerang-shaped—their cores can twist out of plain to form left- or right-handed helices, which can then pack into chiral domains that will polarize light (see the Perspective by Amabilino). Hough et al. (p. 452) show that if you make the situation even more complex by frustrating the packing of adjacent layers, you can create a material that appears to be macroscopically isotropic with only very local positional and orientational ordering of the molecules but still shows an overall chirality. In a second paper, Hough et al. (p. 456) also show that if you change the chemistry of the molecules to allow for better overall packing, you can create a situation where helical filaments form that also tend to pack in layered structures. However, the frustration between the two types of packing leads to macroscopically chiral and mesoporous structures. Banana-shaped molecules lacking handedness form a macroscopically isotropic fluid that still has overall chirality. A variety of simple bent-core molecules exhibit smectic liquid crystal phases of planar fluid layers that are spontaneously both polar and chiral in the absence of crystalline order. We found that because of intralayer structural mismatch, such layers are also only marginally stable against spontaneous saddle splay deformation, which is incompatible with long-range order. This results in macroscopically isotropic fluids that possess only short-range orientational and positional order, in which the only macroscopically broken symmetry is chirality—even though the phases are formed from achiral molecules. Their conglomerate domains exhibit optical rotatory powers comparable to the highest ever found for isotropic fluids of chiral molecules.

Electro-optic characteristics of de Vries tilted smectic liquid crystals: Analog behavior in the smectic A* and smectic C* phases

Chiral smectic A liquid crystal materials of the de Vries type (with molecules tilted relative to the layer normal) exhibit analog field-induced (electroclinic) optic axis rotation accompanied by an increase in birefringence. We identify two such de Vries smectic A* materials and use them to develop and test models for these characteristic electro-optic effects. These materials also exhibit colossal analog field-induced optic axis rotation in the lower temperature smectic C* phase, a consequence of polarization charge stabilization, and of polarization screening of the applied field in the liquid crystal.

The case of thresholdless antiferroelectricity: polarization-stabilized twisted SmC* liquid crystals give V-shaped electro-optic response

We have studied the three-component liquid crystal mixture reported to exhibit ‘thresholdless antiferroelectricity’ [Inui et al., J. Mater. Chem., 1996, 6, 671]. We find that the thresholdless or V-shaped switching is obtained in the absence of antiferroelectricity. This analog electro-optic response is due to the field-induced switching of a twisted smectic C* structure stabilized by polar surface interactions and by electrostatic bulk polarization charge interactions. The latter confine the director twist to thin surface regions leaving the bulk of the cell uniform, which gives good extinction at zero field. In sufficiently thin cells, such thresholdless switching can in fact be followed down to much lower temperatures, where the bulk would be antiferroelectric, but is maintained in the cells in the ferroelectric state by hysteresis from surface action.

The first bent-core mesogens exhibiting a dimorphism B7–SmCPA

New homologous achiral five-ring bent-core mesogens with a lateral fluorine substituent at the central core and chlorine substituents at the terminal rings have been synthesized. Two mesophases could be distinguished by polarizing microscopy, calorimetry, X-ray, and electro-optical investigations. The high-temperature phase was found to be an antiferroelectric SmCP phase whereas the low-temperature phase could be identified as a ferroelectric B 7 phase. This phase sequence was observed for the first time. NMR measurements as well as optical studies indicate that both phases have a helical superstructure.

Unraveling the Mystery of “Thresholdless Antiferroelectricity”: High Contrast Analog Electro-Optics in Chiral Smectic Liquid Crystals

A liquid crystal mixture reported to exhibit “thresholdless antiferroelectricity” has been studied in experiments employing freely suspended liquid crystal films, bookshelf cell electro-optics, total internal reflection from bookshelf cell solid-liquid crystal interfaces, and computer simulation. The results show that the so-called “V-shaped” analog electro-optic response is due to the field-induced switching of a twisted smectic C* structure, stabilized by strong polar surface interactions and having the twist confined to thin surface regions by polarization space charge effects. This leaves the bulk of the cell uniform, which gives good extinction at zero field.

Optimal parameter identification for discrete mechanical systems with application to flexible object manipulation

We present a method for system identification of flexible objects by measuring forces and displacement during interaction with a manipulating arm. We model the objects structure and flexibility by a chain of rigid bodies connected by torsional springs. Unlike previous work, the proposed optimal control approach using variational integrators allows identification of closed loops, which include the robot arm itself. This allows using the resulting models for planning in configuration space of the robot. In order to solve the resulting problem efficiently, we develop a novel method for fast discrete-time adjoint-based gradient calculation. The feasibility of the approach is demonstrated using full physics simulation in trep and using data recorded from a 7-DOF series elastic robot arm.

Spontaneous formation of horizontal chevrons in smectic-C* liquid crystals

We report an unusual structural formation observed in surface-stabilized ferroelectric liquid crystal cells with asymmetrically treated surfaces of 3-Glycidoxypropyl trimethoxysilane and nylon. On cooling the material W415 from the smectic-A* to the smectic-C* phase, the bookshelf smectic layers spontaneously rearrange to form a horizontal chevron structure, manifested optically as a periodic texture of uniform stripes. The polarization is perpendicular to the boundary plates and reverses direction from stripe to stripe, while the director is oriented in the plane of the cell and alternates symmetrically about the stripe direction. In an applied field the cells show a V-shaped (analog) electro-optic response.

Robotic Manipulation for Identification of Flexible Objects

This paper provides preliminary insight into stiffness profile identification of a complex flexible object by robotic manipulation. The object is in the shape of the letter ‘Y’, chosen to resemble a living plant. The object is approximately modelled as a spring mass system. The robot manipulates the object with one or two arms grasped at the ends of the ‘Y’, and makes visual measurements which locates the object’s position in space. Identification results from an optimization approach are compared for both one and two arm manipulation and sensing with and without vision. The results are not consistent with the expected physical object’s properties due to a failure to observe the motion dependence between the object’s connected segments. The result provides insight into the problem of assessing the minimal information needed to identify the stiffness of a flexible object, an issue of importance to automated approaches.

Liquid-crystal-solid interface structure at the antiferroelectric-ferroelectric phase transition

Total internal reflection is used to probe the molecular organization at the surface of a tilted chiral smectic liquid crystal at temperatures in the vicinity of the bulk antiferroelectric-ferroelectric phase transition. Data are interpreted using an exact analytical solution of a real model for ferroelectric order at the surface. In the mixture T3, ferroelectric surface order is expelled with the bulk ferroelectric-antiferroelectric transition. The conditions for ferroelectric order at the surface of an antiferroelectric bulk are presented.