Flow properties of a twist-bend nematic liquid crystal

Abstract

We present the first shear alignment studies and rheological measurements in the twist-bend nematic (N tb ) liquid crystal phase of odd numbered flexible dimer molecules. It is found that the N tb phase is strongly shear-thinning. At shear stresses below 1Pa the apparent viscosity of N tb is 1000 times larger than in the nematic phase. At stress above 10 Pa the N tb viscosity drops by two orders of magnitude and the material exhibits Newtonian fluid behavior. This is consistent with the heliconic axis becoming normal to the shear plane via shear-induced alignment. From measurements of the dynamic modulus we estimate the compression modulus of the pseudo-layers to be B~2kPa; this value is discussed within the context of a simple theoretical model based upon a coarse-grained elastic free energy.

Anomalous Increase in Nematic-Isotropic Transition Temperature in Dimer Molecules Induced by a Magnetic Field

“…Lastly, the three compounds we have studied show no indications of other varieties of ordering (or fluctuations thereof), such as positional clustering [17,23,25] or biaxiality, which have been proposed to explain otherwise inexplicably large ΔT NI . [12,13] Most theories relating molecular morphology to the onset of the N-I transition derive from Onsager [35]; however, approaches based only on excluded volume effects are athermal and cannot provide insight to transition temperatures.…”

“…The three compounds exhibit the recently discovered twist-bend nematic (N tb ) liquid crystal phase, in which the director follows an ambidextrous conical helix with a remarkably short pitch in the 10 nm range. [18] [19] Thus the N tb phase can be considered as a nanoscale pseudolayered structure [20][21][22], which explains the smectic like textures [20][21][22][23][24] and viscoelastic properties [25] In our experiments we load each compound in a 10 10 mm planar glass cell whose inner surfaces are treated with a unidirectional rubbed polyimide PI2555 (HD Micro Systems) that promotes molecular alignment parallel to the substrates (homogeneous planar alignment) and along the rubbing direction; the distance d between substrates was 5 μm. The filled liquid crystal cell is then inserted into a Teflon-insulated temperature-controlled oven (temperature stability 0.05ºC) with optical access.…”

Second harmonic light scattering induced by defects in the twist-bend nematic phase of liquid crystal dimers

“…This concept has recently been applied to the N TB phase to describe smectic-like elastic properties [6,17] and fluctuation modes [17], strong shear thinning behavior [24], and optical stripe and focal textures [25]. Starting from a theoretically proposed free energy density couplingn and P [9], we show that "pseudo-layer" deformations associated with PFCs naturally lead to a distortion of the helical field P that breaks centrosymmetry at length scales comparable to (or greater than) optical wavelengths.…”

Investigating the Cusp between the nano- and macro-sciences in supermolecular liquid-crystalline twist-bend nematogens

“…X-ray diffraction shows a lack of lamellar organisation in the N TB phase, whereas by optical microscopy it exhibits apparent long-range order. 19 The nematic phase is typically fluid, whereas the viscosity of the twist-bend nematic is significantly higher, 21 and Brownian motion visible that is visible in the nematic phase of bimesogens is appreciably reduced upon entering the N TB phase. 19,20 It is possible to draw freestanding films of the N TB phase, which is something that cannot be done for calamitic nematic liquid crystals.…”