Session M28: Liquid Crystals II

Stable nematic droplets with handles

We use a simple method to generate nematic liquid crystal droplets with handles. The method relies on the viscous forces exerted by a flowing continuous phase above its yield stress over a liquid crystal which is extruded from an injection needle; the resultant jet is forced to close into a torus, due to the imposed rotation, and is stable against surface tension instabilities, due to the elasticity of the outer phase. We find that the ground state of these nematic liquid crystal toroidal droplets is defect free and exhibits twist, irrespective of the aspect ratio of the torus. By including the saddle-splay contribution to the elastic free energy density, we find that this state indeed corresponds to the lowest energy state. For droplets with additional handles, we find there are two surface defects or boojums per additional handle.   

Nanosecond electrooptics of nematic liquid crystals: induced orientational order and quenching of director fluctuations

We demonstrate a fast (1-100 ns) electrooptic response of a thermotropic nematic liquid crystal in a geometry when a strong electric field (\textgreater\ 10$^{\mathrm{8}}$~V/m) does not realign the director and influences only the orientational order and the spectrum of director fluctuations.   

Impact of Photo-Induced Surface Adsorption of Azo-Dyes on the Liquid Crystal Anchoring Conditions

Using optical techniques, we measured the anchoring conditions of azo-dye doped nematic liquid crystals on rubbed polyimide surfaces. Linearly polarized light induces the formation of a second easy axis on the polymer surface oriented toward the polarization direction of the pump laser beam. This additional easy axis is the result of photo-induced adsorption of the \textit{cis} isomer of the azo dye. An effective easy axis is the weighted average of the original easy axis and this new easy axis.   

Surface Nano pattering for aligning Chromonic liquid crystals

We present results on planar alignment of several Chromonic Liquid Crystals. We use a high aspect ratio nano pattern of electrically conductive ITO, which was fabricated by employing a new patterning technique that relies on a secondary sputtering phenomenon (SSP). This method is particularly useful in the case of aligning Chromonics which are considerably harder to align in comparison with conventional thermotropics. Berreman's theory was employed to study the alignment of the Liquid Crystals as a function of the anchoring energy which depend on the dimension of the ITO patterns.   

Differential Dynamic Microscopy for measuring viscoelastic ratios of Chromonic Liquid Crystals

Differential Dynamic Microscopy(DDM) enables one to access the scattering information from a sample by Fourier analyzing the real space images obtained from a light microscope. Thermal fluctuations of the director about the mean position allows one to study the viscoelastic properties of the nematic. Normally such measurements of the viscoelastic constants require time consuming and sensitive light scattering experiments. DDM enables us to extract the same data just by analyzing a real space movie a few seconds long using a high speed camera. We present results of viscoelastic measurements of Chromonic liquid crystal Sunset yellow using DDM measurements.   

Phase and Topological Behavior of Lyotropic Chromonic Liquid Crystals in Double Emulsions

Lyotropic chromonic liquid crystals, assembled by non-covalent interactions, have fascinating temperature- and concentration-dependent phase behavior. Using water-oil-water double emulsions, we are able control the inner droplet chromonic phase concentration by osmosis through the oil phase. We then study the configurations of the chromonic liquid crystal phases in droplets by varying the oil types, oil soluble surfactants, and inner droplet diameter. We employ polarization microscopy to observe resulting nematic and columnar phases of Sunset Yellow FCF, and we deduce the liquid crystal configuration of both phases within the droplets. Simulations based on Jones matrices confirm droplet appearance, and preliminary observations of chromonic liquid crystal shells in oil-water-oil double emulsions are reported.   

Homeotropic alignment of the lyotropic chromonic liquid crystal Sunset Yellow FCF using pi-pi stacking chemical interactions

We report on the homeotropic alignment of the lyotropic chromonic liquid crystal, Sunset Yellow FCF (SSY), using pi-pi stacking interactions between the SSY molecules and (1) thin parylene films or (2) a graphene monolayer. The nematic and columnar phases of SSY molecules arise via self-assembly in water into stacks through non-covalent attractions between the SSY molecules. Interestingly, we find that the same non-covalent interactions between SSY molecules and a parylene or graphene alignment layer lead to homeotropic anchoring of these stacks. The nematic phase of SSY is introduced between two glass substrates coated with parylene films or graphene monolayers, and homeotropic alignment of SSY is confirmed by polarized optical microscopy and conoscopy. Additionally, we observe and can explain the stripe domains that occur during cooling of the sample in this cell, and we consider possible novel applications for homeotropically aligned chromonic liquid crystals.   

Kinetics of Assembly and Dis-assembly of Structures Forming a Chromonic Liquid Crystal at Low Concentrations

The molecules of the near-IR absorbing dye IR-806 spontaneously assemble in water at very low concentrations, forming a chromonic liquid crystal phase at room temperature when the concentration is above 0.5 wt\%. The assembly process proceeds in two steps and results in a complex structure that orientationally orders in a liquid crystal phase. The kinetics of the assembly and dis-assembly of these complex structures can be followed through absorption measurements by rapidly mixing the initial sample with either a small fraction of salt solution (assembly) or a large fraction of water (dis-assembly). The kinetics of dis-assembly is exponential while the kinetics of assembly is non-exponential, both with rate constants depending on the starting and ending conditions, but falling in the 0.1-1.0 s$^{-1}$ range. While past equilibrium absorption measurements on IR-806 offer evidence for a threshold concentration for the assembly of these complex structures, the kinetics experiments show with certainty the existence of such a threshold. Similar experiments on Benzopurpurin 4B, another dye that forms a chromonic liquid crystal at low concentrations, reveal kinetics that are slower by two orders of magnitude and a threshold concentration for the assembly of complex structures.   

Free energy power expansion for orientationally ordered phases: energy and entropy

We propose a new approach for description of orientational phase transitions that utilizes the following specific features of the orientational energy $E$ and entropy $S$: (a) $S$ possesses an additional symmetry in comparison with $E$, being invariant under rotation of the molecular frame; and (b) $E$ contributes only to the second order terms because the pair molecular interaction is dominant. The approach is based on minimization of the scaled orientational free energy $\bar{F}=F/T=E/T-S$ instead of $F$ because $\bar{F}$ obeys the standard assumption of the Landau theory that only the second order terms are temperature dependent. We apply the approach to build a model for nematic phases in materials with non-polar parallelepiped-type molecules with symmetry$\ D_{2h}$. The presented model introduces complex OPs, generalizes the Landau-de Gennes (LdeG) theory and predicts the existence of a biaxial nematic phase for the forth order expansion of $\bar{F}$.   

Surface topography and rotational symmetry breaking

The surface electroclinic effect, which is a rotation of the molecular director in the substrate plane proportional to an electric field applied normal to the substrate, requires both a chiral environment and $C_{2}$ (or lower) rotational symmetry about the field. The two symmetries typically are created in tandem by manipulating the surface topography, a process that conflates their effects. Here we use a pair of rubbed polymer-coated substrates in a twist geometry to obtain our main result, viz., that the strengths of two symmetries, in this case the rub-induced breaking of $C_{\infty }$ rotational symmetry and chiral symmetry, can be separated and quantified. Experimentally we observe that the strength of the reduced rotational symmetry arising from the rub-induced scratches, which is proportional to the electroclinic response, scales linearly with the induced topographical rms roughness and increases with increasing rubbing strength of the polymer. Our results also suggest that the azimuthal anchoring strength coefficient is relatively insensitive to the strength of the rubbing.   

Probing Viscoelasticity of Cholesteric Liquid Crystals in a Twisting Cell

Viscoelastic properties of liquid crystals are typically studied either using Poiseuille flow, which can be produced by a pressure gradient in a capillary tube,\footnote{R. J. Atkin, ``Poiseuille Flow of Liquid Crystals of the Nematic Type, ARCHIVE FOR RATIONAL MECHANICS AND ANALYSIS, \textbf{38},~224-240 (1970)} or Couette flow, which can be generated by a shear between concentric cylinders.\footnote{CLADIS, P. E., {\&} S. TORZA, ``Stability of nematic liquid crystals in Couette flow''. \textit{Phys. Rev. Lett.} \textbf{35}, 1283-1286 (1975).} We use a different method in which we twist the liquid crystal sandwiched between two cylindrical glass plates, one of which can rotate about its center, the other of which is fixed. When the cell is twisted, there is a force proportional to the twist angle and the twist elastic constant, and inversely proportional to the pitch and sample thickness, normal to the substrates due to the change in pitch in the cholesteric liquid crystal (CLC). Measuring this force on various CLCs with known pitch we could obtain the twist elastic constants. In addition to the equilibrium force, we observed a transient force during the rotation, which is related to the flow of the material, thus allowing us to determine the Leslie viscosity component $\alpha_{1}$, which typically cannot be assessed by other methods. We expect this apparatus to be a useful tool to study the visco-elastic properties of liquid crystals.   

Chiral hierarchal self-assembly in Langmuir monolayers of diacetylenic lipids

A Langmuir monolayer made of chiral lipid molecules forms a hierarchal structure when compressed in the intermediate temperature range below the chain melting temperature. These structures are captured via Brewster angle microscopy. When the liquid monolayer is compressed, an optically anisotropic condensed phase nucleates in the form of long, thin claws. These claws pack closely to form stripes. This appears to be a new mechanism for forming stripes within Langmuir monolayers. In the lower temperature range these stripes arrange into spirals within overall circular domains, while near the chain melting transition the stripes arrange into target-structure. We attributed this transition to a change in boundary conditions at the core of the largest-scale circular domains.