Session R05: Liquid Crystals I

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Recently, Popov et al. [1] showed that a flat film of liquid crystal 4-cyano-4’-pentylbiphenyl (5CB) mixed with chiral dopant S811 suspended in sub-millimeter size grids could spontaneously form biconvex microlenses when they were fully immersed in water. Here we present the optical properties of microlenses array by addition of chiral nanoparticles. The helical twisting power of the chiral ligand-capped nanoparticles is about 400 µm^-1, greater than that of the strongest molecular chiral dopants [2]. By studying the optical properties of these microlens arrays, we show that it is possible to calculate the helical twisting power of the chiral ligand-capped nanoparticles using only minimal amount of materials compared with conventional methods such as Grandjean-Cano wedge or contact cell.

References
[1] P. Popov, L. W. Honaker, M. Mirheydari, E. K. Mann, and A. Jákli, Chiral Nematic Liquid Crystal Microlenses, Sci. Rep. 7, 1603 (2017).
[2] K. Perera, A. Nemati, E. Mann, T. Hegmann, and A. I. Jákli, Optical Properties of Nematic Microlenses Doped with Chiral Nanoparticles, Proc. SPIE 11472, Liquid Crystals XXIV, 114720O (20 August 2020)   

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This research shows the first evidence of using polarized light to manipulate nanoscale heliconical structure of a twist bend nematic liquid crystal dimer containing both sulfur atoms and an azo linkage by tender X-ray scattering. The scattering pattern reveals two different heliconical pitch values of aligned and unaligned domains under polarized light. In addition to the bulk alignment, the value of the heliconical pitch can be tuned in two timescales by UV-violet light and recovered with a temperature dependent memory. As expected, an azo linkage group liquid crystal material with nematic (N) to nematic twist bend (NTB) transition highly sensitive to UV irradiation due to the formation of cis-isomers, which can be strongly influence in pitch. This is the first time we not only quantitatively verify this expectation, but we also show that there are two different pitch values. We also show that the pitch variation happens in two time scales and has a temperature dependent memory. In addition, we show evidence of the polarized UV-Violet light leads to a bulk metastable alignment of the heliconical axis of the NTB phase.   

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Controlling the assembly of colloidal particles in liquid crystal (LC) has recently been of both theoretical and experimental interest. The aggregation behavior of colloidal particles in LC can lead to the formation of new aggregate morphologies and fractal structures. Here we report the one pot synthesis of colloidal aggregates in nematic liquid crystal brought about by the isotropic to nematic phase transition. These aggregates are highly anisotropic on short length scales, with web-like morphologies composed of capsules organized into chains. Using fluorescence microscopy to observe capsules constructed of quantum dots, we perform temperature quenches and the capsules are formed within one second. Shortly after formation, the capsules spontaneously assemble into chains, which subsequently organize into clusters. As the number density of capsules increases with quench depth, these clusters form a gel-like morphology consisting of densely packed chains of capsules. Over time, the gels undergo aging and become more compact, which we quantify using a pair correlation function, g(r). Velocity analysis reveals that ageing effects are combination of Frank elasticity relaxation, and defect line breakage/shrinkage.   

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In the presence of hybrid anchoring conditions, smectic-A (SmA) LCs form focal conic domain (FCD) defects to preserve equal spacing of their layers. Motivated by recent experimental findings on the assembly of FCDs at undulated surfaces, we computationally study the evolution of LC topological defects in the phase transition from the SmA to the nematic (N) phase. We examine how FCDs of nonzero eccentricity and spatially varying orientation transform into patterns of disclination lines which then relax. Using Landau-de Gennes free energy minimization, we determine the presence of metastable defect states at surfaces with 1D and 2D height undulations.   

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Colloidal nanoplates form liquid crystal phases at high concentrations driven by the Onsager excluded volume effect. For ferromagnetic nanoplates, where magnetic dipole-dipole interactions are present, a ferromagnetic nematic phase has been observed [Nat Comm, 7: 10394, 2016]. Here, we study liquid crystal phase behavior of colloidal mixtures of ferromagnetic barium hexaferrite and non-magnetic, exfoliated monolayer zirconium phosphate nanoplates. A series of phases, including a paramagnetic isotropic phase, a paramagnetic nematic, a paramagnetic glassy phase, a reentrant isotropic phase, and a ferromagnetic nematic are observed. All these phases show a strong response to applied magnetic fields. Synchrotron X-ray diffraction used to probe the order, with and without applied fields, shows evidence of local heterogeneous structures in the paramagnetic nematic phase.   

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We have carried out atomistic simulation studies of the recently described ferroelectric nematic (N_F) phase [1] in an effort to identify the key features of molecular interactions and correlations responsible for polar order in N_F materials. Comparative simulations of polar and nonpolar states of the N_F mesogen RM734 reveal distinct polarity-dependent intermolecular correlations and characteristic polar and antipolar pair association motifs, and demonstrate that the polar state of RM734 is stabilized by short-range electrostatic interactions. This electrostatic stabilization mechanism is absent in closely related compounds that lack the N_F phase, indicating a sensitive dependence of polar ordering tendency on molecular shape and charge distribution. These results imply that polar order in the N_F phase results from a subtle interplay of specific short-range electrostatic and steric interactions.
[1] “First-Principles Demonstration of Ferroelectricity in a Thermotropic Nematic Liquid Crystal: Spontaneous Polar Domain Formation and Spectacular Electro-Optics”, X. Chen et al. PNAS 2020
Author for Correspondence: matthew.glaser@colorado.edu   

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Liquid crystals have drawn interest in the past decade due to their ability to trap colloidal objects in topological defects and direct their assembly into specific patterns. Recently, R. Preusse, et. al. demonstrated the ability to assemble these defects into pre-defined patterns based on surface topography. Here, we expand on this work, dispersing gold nanoparticles into smectic liquid crystal and observing the structures formed. We demonstrate the ability to trap and spontaneously assemble these gold nanoparticles into the defect lines formed by the surface topography. In this way, we show that, by engineering precise surface topographies, we can direct the assembly of nanoparticles into complex structures using liquid crystals.   

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The assembly of topological defects in liquid crystals has drawn significant interest in the last decade due to their ability to trap colloidal objects and direct their arrangements. They have also brought about a high impact in modern technologies, in particular in optics, e.g., microlens arrays, soft lithography templates, and optically selective masks. Here we study the formation of defects in smectic A liquid crystal with hybrid texture at undulated surfaces. We investigate the role of surface topography on the organization of focal conic domains (FCDs) in smectic films. We demonstrate new methods for assembling FCDs and disclinations into hierarchical structures. This work paves the way for creating new procedures to control the assembly of functional nanomaterials into tunable assemblies that may find relevance in the field of energy technology.   

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Surface anchoring is the key mechanism to control the alignment of liquid crystals. Experimental determination of surface anchoring strength is important for the proper control of alignment. Here we characterize the strength of azimuthal surface anchoring produced by the recently developed photoalignment technique based on plasmonic metamsaks [1]. The measurements use photopatterned arrays of topological point defects of strength +1 and -1 in a nematic liquid crystal. The integer-strength defects split into pairs of defects of half-integer strength with a lower elastic energy cost. The separation distance between the split pair allows one to determine the anchoring extrapolation length by balancing elastic and anchoring forces [2]. The strength of the azimuthal anchoring is directly proportional to the UV exposure time during the photoalignment of the azobenzene layer.
[1] Y. Guo, M. Jiang, C. Peng, K. Sun, O. Yaroshchuk, O. Lavrentovich, and Q. H. Wei, Adv. Mater. 28, 2353 (2016)
[2] T. Turiv, J. Krieger, G. Babakhanova, H. Yu, S. V. Shiyanovskii, Q.-H. Wei, M.-H. Kim, and O. D. Lavrentovich, 651, 1 (2020)   

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Liquid crystalline elastomers have demonstrated a rich variety of actuation behavior ranging from in-plane contraction/elongation to out-of-plane twisting, bending, curling, popping-up cones, and other complex morphologies. Analytical techniques have successfully determined custom-made 3-D liquid crystal director configurations that encode the actuation of a thin LCE film to an arbitrary target morphology. Here, we investigate the shape change of a custom-made automobile-shaped actuator. Via fully 3-D finite element elastodynamics simulations of a sample with in-plane geometry and 3-D director microstructure given by custom design, we model the actuator’s shape change and analyze the effect of sample thickness in its stimulus-responsive morphing and final morphology. We found that the sample thickness averages out details of the automobile-morphology while maintaining its overall target shape. In addition, our simulations show a pop-up sequence in the initial morphing of the sample to external stimulus. Similar pop-up morphing behavior has been observed in experiments of thick LCE samples.   

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The nanostructure of two novel sulfur containing dimer materials have been investigated
experimentally by hard and by resonant tender X-ray scattering techniques. We find that below the nematic (N) to NTB transition the positional correlations drop significantly, while the bond-order correlations increase from zero to about 60 nm. The temperature dependences of the heliconical pitch show stronger variation neat the N-NTB transition that the CBnCB-type dimers. The heliconical pitch is observable even in the upper 3-4°C range of the underlying smectic phase. The coexistence of the smectic order and the heliconical order indicates a SmCTB -type phase where the rigid units of the dimers are tilted with respect to the layer normal to allow for the bending of the dimers, but the tilt direction rotates following the heliconical order.   

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Under external stress, a pinned dislocation segment in a crystalline solid can deform and continuously generate dislocation loops, known as a Frank-Read source. We investigate an analogous mechanism of defect multiplication for disclination lines in nematic liquid crystals. We model a liquid crystal cell containing a disclination half loop pinned to one substrate via patterned surface anchoring. External stress is imposed by gradually rotating the uniform planar anchoring orientation on the opposite substrate. As stress increases, the half loop stretches until a threshold stress is reached and a disclination loop is emitted, leaving the original pinned half loop intact such that the process can repeat. To understand this finding, we develop a simple theory based on the free energy of the system, and explain the behavior of this nematic Frank-Read source by the balance of the Peach-Koehler force and the tension of the disclination line. We compare our findings to related experiments [1] and discuss implications for disclination nucleation mechanisms in both passive and active nematic liquid crystals. [1] J. Angelo et al, DOI: 10.1080/15421406.2017.1287486.   

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Recent experiments have demonstrated the ability for the nematic liquid crystal phase transitions to create nanoparticle structures. Control over the exact dimensions of the resulting structures is limited when these structures are produced in a bulk liquid crystal domain. To explore how to produce these structures with specificity, we dispersed quantum-dots into single isotropic 5CB droplets cooled into nematic phase to create a single capsule. We were able to show positive correlation between droplet and capsule size with a Pearson’s R value of 0.75. We then used Landau-De Gennes numerical modeling of liquid crystals and the phase transitions to predict the expected defect patterning in the surrounding liquid crystal due to anchoring conditions of the structures predicted by previous experimental results. We observed three different final configurations that were determined by the ratio of the system size to the anchoring extrapolation length.   

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Small droplets of paraffin oil, both neat and doped with barium hexaferrite (BF) colloidal particles, were deposited on the surface of freely suspended smectic films. Droplet chains are observed both on uniform films and along layer steps, with the droplet spacing depending on the frequency of an applied electric field. When doped with a very small amount of BF colloidal particles, the droplets also respond to applied magnetic fields, which induce translational motion, island formation, and changes in the size of the droplets. The aggregation and chaining behavior induced by an electric field are significantly altered by the presence of a magnetic field. The interactions between droplets and their dependence on the presence of electric and magnetic fields could be indicative of dipolar interactions between the droplets. We will present the behavior of collective systems of these isotropic oil and ferromagnetic inclusions on 2D films, including their equilibrium spatial organization and interactions, and their shape evolution as a function of applied electric and magnetic field in both smectic A and smectic C phase films   

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We study a nanocomposite consisting of a ferroelectric liquid crystal and a magnetic nanoparticle, of 2.5 to 3.5 nanometers in size. We use the ferroic properties exhibited by the nanocomposite, by increasing the applied electric field and observing the ordering of the magnetization. We study the 10.8% wt of nanoparticles in the liquid crystal. We observe the formation of droplets of about 5 – 10 microns in diameter. The concentration was chosen because it seems to act as a lubricant to the liquid crystal, but it is a larger concentration. Because of this larger concentration we observe the formation of clusters that tend to be contained inside the droplets. We estimate the value of the magnetic field in a 7micron droplet and associate it to the number of droplets aligned as a function of electric field, which is about 0.7T. As the electric field increases, we observe a change in the ordering inside the droplets and the distribution of the clusters inside them. We show how the change in the order of the droplets as the electric field increases contributes to the magnetic field. We present the evolution of the ordering of the droplet as the electric field is increased.