Thermotropic liquid crystals exhibit strongly temperature-dependent positional correlations among molecules in the nematic phase above the transition to a smectic-A or C phase. However, even in the ...absence of a lower temperature smectic phase, nematics composed of reduced symmetry molecules (bent-core mesogens, for example) reveal small angle X-ray scattering (SAXS) patterns with features similar to those associated with pretransitional smectic order. We report on a quantitative analysis and comparison of two-dimensional SAXS data on a bent-core nematic compound (lacking a smectic phase) and a rod-like nematic (exhibiting a nematic to smectic-C transition). This analysis demonstrates that a nanostructure based on distinct, smectic-C ordered molecular clusters provides a more accurate account of the diffraction data from the bent-core nematic than the standard models used to describe fluctuating smectic order in rod-like nematics. The average dimensions of the clusters are weakly temperature dependent, while the interlayer spacing and layer tilt angle with respect to the average molecular long axis are temperature independent. Our results may help to explain interesting and unusual macroscopic properties recently discovered in the fluid phases of bent-core molecules.
A comparative analysis of 2D SAXS data on bent-core nematic (BCN) and rod-like nematic (RN) liquid crystals demonstrates that the short-range structure of the BCN is based on distinct, smectic-C ordered molecular clusters, while the standard model of fluctuating (nonequilibrium) smectic order best describes the RN.
We report dynamic light scattering measurements of the orientational (Frank) elastic constants and associated viscosities among a homologous series of a liquid crystalline dimer, trimer, and tetramer ...exhibiting a uniaxial nematic (N) to twist-bend nematic (N
) phase transition. The elastic constants for director splay (K
), twist (K
) and bend (K
) exhibit the relations K
> K
> K
and K
/K
> 2 over the bulk of the N phase. Their behavior near the N-N
transition shows dependency on the parity of the number (n) of the rigid mesomorphic units in the flexible n-mers. Namely, the bend constant K
in the dimer and tetramer turns upward and starts increasing close to the transition, following a monotonic decrease through most of the N phases. In contrast, K
for the trimer flattens off just above the transition and shows no pretransitional enhancement. The twist constant K
increases pretransitionally in both even and odd n-mers, but more weakly so in the trimer, while K
increases steadily on cooling without evidence of pretransitional behavior in any n-mer. The viscosities associated with pure splay, twist-dominated twist-bend, and pure bend fluctuations in the N phase are comparable in magnitude to those of rod-like monomers. All three viscosities increase with decreasing temperature, but the bend viscosity in particular grows sharply near the N-N
transition. The N-N
pretransitional behavior is shown to be in qualitative agreement with the predictions of a coarse-grained theory, which models the N
phase as a "pseudo-layered" structure with the symmetry (but not the mass density wave) of a smectic-A* phase.
Flexoelectricity is a coupling between orientational deformation and electric polarization. We present a direct method for measuring the flexoelectric coefficients of nematic liquid crystals (NLCs) ...via the electric current produced by periodic mechanical flexing of the NLC's bounding surfaces. This method is suitable for measuring the response of bent-core liquid crystals, which are expected to demonstrate a much larger flexoelectric effect than traditional, calamitic liquid crystals. Our results reveal that not only is the bend flexoelectric coefficient of bent-core NLCs gigantic (more than 3 orders of magnitude larger than in calamitics) but also it is much larger than would be expected from microscopic models based on molecular geometry. Thus, bent-core nematic materials can form the basis of a technological breakthrough for conversion between mechanical and electrical energy.
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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 1 Pa 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
∼ 2 kPa; this value is discussed within the context of a simple theoretical model based upon a coarse-grained elastic free energy.
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.
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Using a magnetic Frederiks transition technique, we measure the temperature and concentration dependences of splay K1, twist K2, and bend K3 elastic constants for the lyotropic chromonic liquid ...crystal sunset yellow formed through noncovalent reversible aggregation of organic molecules in water. K1 and K3 are comparable to each other and are an order of magnitude higher than K2. At higher concentrations and lower temperatures, K1 and the ratios K1/K3 and K1/K2 increase, which is attributed to elongation of self-assembled lyotropic chromonic liquid crystal aggregates, a feature not found in conventional thermotropic and lyotropic liquid crystals formed by covalently bound units of a fixed length.
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The threshold voltage for electro-hydrodynamic convection (EHC) in nematic liquid crystals in the low-frequency convective regime is examined. The addition of dopant species is known to strongly ...effect the frequency response of this regime. In this work, we investigate the effect of adding optically anisotropic species on the low-frequency threshold, and, demonstrate that certain dichroic dyes have a marked effect. We have measured electrical transport properties of suitably doped liquid crystal mixtures in order to make a comparison with theoretical predictions based upon linear stability considerations. Our results show satisfactory agreement with these predictions. Our findings indicate future directions for new applications of dynamic scattering mode technologies with more accessible operating parameters previously known.
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We demonstrate subnanometer resolution cryo-TEM imaging of smectic layers in the smectic and nematic phases of two bent-core liquid crystals. Our results show perfect periodicity over several hundred ...layers in the smectic phase and also provide the first direct evidence of smectic clusters on length scales of 30-50 nm in a nematic liquid crystal. The results are corroborated with small angle x-ray scattering measurements. The observation of smectic clusters in the nematic phase is of special interest in bent-core liquid crystals, where the smectic clusters are stable over wide temperature ranges, in contrast to the well-known pretransitional "cybotactic" clusters that appear only in the vicinity of a bulk smectic phase. The means to characterize and manipulate this nanoscale molecular order could open up completely new liquid crystal-based technologies.
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We have determined the nematic-isotropic transition temperature as a function of an applied magnetic field in three different thermotropic liquid crystalline dimers. These molecules are comprised of ...two rigid calamitic moieties joined end to end by flexible spacers with odd numbers of methylene groups. They show an unprecedented magnetic field enhancement of nematic order in that the transition temperature is increased by up to 15 K when subjected to a 22 T magnetic field. The increase is conjectured to be caused by a magnetic-field-induced decrease of the average bend angle in the aliphatic spacers connecting the rigid mesogenic units of the dimers.
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The nematic twist-bend (TB) phase, exhibited by certain achiral thermotropic liquid crystalline (LC) dimers, features a nanometer-scale, heliconical rotation of the average molecular long axis ...(director) with equally probable left- and right-handed domains. On meso to macroscopic scales, the TB phase may be considered as a stack of equivalent slabs or "pseudo-layers", each one helical pitch in thickness. The long wavelength fluctuation modes should then be analogous to those of a smectic-A phase, and in particular the hydrodynamic mode combining "layer" compression and bending ought to be characterized by an effective layer compression elastic constant
B
eff
and average director splay constant
K
eff
1
. The magnitude of
K
eff
1
is expected to be similar to the splay constant of an ordinary nematic LC, but due to the absence of a true mass density wave,
B
eff
could differ substantially from the typical value of ∼10
6
Pa in a conventional smectic-A. Here we report the results of a dynamic light scattering study, which confirms the "pseudo-layer" structure of the TB phase with
B
eff
in the range 10
3
-10
4
Pa. We show additionally that the temperature dependence of
B
eff
at the TB to nematic transition is accurately described by a coarse-grained free energy density, which is based on a Landau-deGennes expansion in terms of a heli-polar order parameter that characterizes the TB state and is linearly coupled to bend distortion of the director.
The value and temperature dependence of the "pseudo-layer" compression modulus are reported for a twist-bend nematic liquid crystal.
Milton, et al.
Opt. Express
22
,
8035
(
2014
)
10.1364/OE.22.008035
have constructed a model electronic liquid crystal contact lens for the correction of presbyopia. This paper constructs a ...theoretical model for this lens. Good agreement between theory and experimental data is achieved, although the indications are that the precise parameters of the lens differ slightly from those prescribed by the designers of the lens. We discuss the temperature dependence of the optical power, the sensitivity of the device to manufacturing process and the properties of such lenses containing a number of different known liquid crystals. The model can be used for engineering optimization of the existing prototype.
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