PhD Student, Case Western Reserve University
Development of programmable, stimuli-responsive, shape and color changing cholesteric LC elastomers.
Biological systems employ anisotropy to enable selectivity in motion, transport, or add functionality, like structural color. Resembling the structure-function relationships found in nature, cholesteric liquid crystals (CLCs) are inherently and selectively reflective, and shown to exhibit stimuli-induced changes in coloration particularly in compositions prepared with low-molar mass liquid crystals. Here, we report on the ability to pattern and imprint both color and shape (topography) change in monolithic elements prepared from cholesteric liquid crystalline elastomers. The materials examined here are of the main-chain subclass, synthesized via photopolymerization. The spatial anisotropy was initiated via photoalignment of dye molecules on substrate surfaces. We will elucidate subtle nuances in the fundamental nature of the mechanics that differentiate the stimuli-response of these materials from nematic liquid crystalline elastomers. The ability to simultaneously and concurrently regulate the color as well as the direction of reflected light could open up interesting applications in textiles, optics, and sensing.
Abstract: Cholesteric liquid crystals are known to exhibit a circular-polarization selective Bragg reflection band over a wavelength region determined by their reflective index and pitch. In a planar device where the molecules are aligned unidirectionally at the substrate surface, cholesteric liquid crystals act as flat dielectric mirrors, reflecting light at an angle fulfilling the law of reflection. Here, it is shown that by introducing a random structure inspired by the Morpho didius butterfly into the distribution of the helix phase, the reflected light phase becomes randomized, leading to a diffuse reflection that spans over an angle greater than ±30°. The reflection maintains its circular polarization selectivity, making these materials unique compared to the butterfly itself and other man-made structures mimicking the butterfly's structure.
Pub.: 27 Feb '17, Pinned: 05 Jul '17
Abstract: Peculiarities of the defect modes of cholesteric liquid crystals (CLCs) with an isotropic/anisotropic defect inside are investigated. The influence of the defect layer thickness and its anisotropy of refraction, the influence of the system thickness and of the defect layer position in the system, as well as the influence of the dielectric borders on the defect modes is investigated. It is shown that it is possible to change reflection at the defect modes in wide intervals and change the defect mode wavelength, by tuning the defect location and its thickness. Such a system possesses transmission asymmetry. Also, the CLC system thickness and the refraction coefficient of the medium bordering the CLC layer on its both sides have essential influence on the reflection at the defect mode and on the reflection frequency.
Pub.: 10 Mar '17, Pinned: 05 Jul '17
Abstract: Most theoretical descriptions of lyotropic cholesteric liquid crystals to date focus on homogeneous systems in which the rod concentration, as opposed to the rod orientation, is uniform. In this work, we build upon the Onsager-Straley theory for twisted nematics and study the effect of weak concentration gradients, generated by some external potential, on the cholesteric twist. We apply our theory to chiral nematics of nanohelices in which the supramolecular helix sense is known to spontaneously change sign upon variation of particle concentration, passing through a so-called compensation point at which the mesoscopic twist vanishes. We show that the imposed field offers exquisite control of the handedness and magnitude of the helicoidal director field, even at weak field strengths. Within the same framework we also quantify the director fluctuation spectrum and find evidence for a correlation length diverging at the compensation point.
Pub.: 13 May '17, Pinned: 05 Jul '17
Abstract: Liquid crystals play an important role in biology because the combination of order and mobility is a basic requirement for self-organisation and structure formation in living systems. Cholesteric liquid crystals are omnipresent in living matter under both in vivo and in vitro conditions and address the major types of molecules essential to life. In the animal and plant kingdoms, the cholesteric structure is a recurring design, suggesting a convergent evolution to an optimised left-handed helix. Herein, we review the recent advances in the cholesteric organisation of DNA, chromatin, chitin, cellulose, collagen, viruses, silk and cholesterol ester deposition in atherosclerosis. Cholesteric structures can be found in bacteriophages, archaea, eukaryotes, bacterial nucleoids, chromosomes of unicellular algae, sperm nuclei of many vertebrates, cuticles of crustaceans and insects, bone, tendon, cornea, fish scales and scutes, cuttlebone and squid pens, plant cell walls, virus suspensions, silk produced by spiders and silkworms, and arterial wall lesions. This article specifically aims at describing the consequences of the cholesteric geometry in living matter, which are far from being fully defined and understood, and discusses various perspectives. The roles and functions of biological cholesteric liquid crystals include maximisation of packing efficiency, morphogenesis, mechanical stability, optical information, radiation protection and evolution pressure.
Pub.: 08 Jun '17, Pinned: 05 Jul '17
Abstract: A new optical method is proposed to investigate the reflectance of structurally coloured objects, such as Morpho butterfly wing scales and cholesteric liquid crystals. Using a reflected-light microscope and a digital single-lens reflex (DSLR) camera, we have successfully measured the two-dimensional reflection pattern of individual wing scales of Morpho butterflies. We demonstrate that this method enables us to measure the bidirectional reflectance distribution function (BRDF). The scattering image observed in the back focal plane of the objective is projected onto the camera sensor by inserting a Bertrand lens in the optical path of the microscope. With monochromatic light illumination, we quantify the angle-dependent reflectance spectra from the wing scales of Morpho rhetenor by retrieving the raw signal from the digital camera sensor. We also demonstrate that the polarization-dependent reflection of individual wing scales is readily observed using this method, using the individual wing scales of Morpho cypris. In an effort to show the generality of the method, we used a chiral nematic fluid to illustrate the angle-dependent reflectance as seen by this method.
Pub.: 21 Jun '17, Pinned: 05 Jul '17
Abstract: Using tools and concepts from contact topology we show that non-vanishing twist implies conservation of the layer structure in cholesteric liquid crystals. This leads to a number of additional topological invariants for cholesteric textures, such as layer numbers, that are not captured by traditional descriptions, characterises the nature and size of the chiral energy barriers between metastable configurations, and gives a geometric characterisation of cholesteric dynamics in any context, including active systems, those in confined geometries or under the influence of an external field.
Pub.: 29 Jun '17, Pinned: 05 Jul '17
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