Bulletin of the American Physical Society
APS March Meeting 2020
Volume 65, Number 1
Monday–Friday, March 2–6, 2020; Denver, Colorado
Session M29: Liquid Crystals II: Phases and Transitions |
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Sponsoring Units: DSOFT DPOLY Chair: Vianney Gimenez-Pinto, Lincoln University, Missouri Room: 501 |
Wednesday, March 4, 2020 11:15AM - 11:27AM |
M29.00001: Theory of the splay nematic phase Michely Rosseto, Jonathan Selinger Recent experiments have reported a novel splay nematic phase, which has alternating domains of positive and negative splay [1]. To model this phase, researchers have considered a 1D splay modulation of the director field, accompanied by a 1D modulation of polar order. When the flexoelectric coupling between splay and polar order becomes sufficiently strong, the uniform nematic state becomes unstable to the formation of a modulated phase [1,2]. Here, we re-examine this theory in terms of a new approach to liquid crystal elasticity [3], which shows that pure splay deformation is double-splay rather than planar single-splay. Following that reasoning, we propose a structure with a 2D splay modulation of the director field, accompanied by a 2D modulation of polar order, and show that the 2D structure generally has a lower free energy than the 1D structure. We also investigate a generalized flexoelectric coupling between director deformations and tetrahedral order, but still find a lower free energy for the 2D modulation. |
Wednesday, March 4, 2020 11:27AM - 11:39AM |
M29.00002: Twisted topological defect lines in achiral nematic liquid crystal confined in glass capillaries Zhaofei Zheng, Jose X Velez, Daniel Beller, Francesca Serra Chirality occupies a remarkable space in biology and soft matter. In liquid crystals, chirality at the molecular scale propagates to the macroscale in non-trivial ways and can emerge even from achiral components. Here we report that common nonchiral thermotropic liquid crystals confined in square capillary tubes with homeotropic anchoring can show a spontaneous twist during the phase transition from smectic-A to nematic. The chiral structure is transient, but it can be stabilized for a few hours by accurate control of the temperature. We hypothesize that this chiral structure is related to the different rate of change of the twist and bend elastic constants at the phase transition. However, also the shape of the confining capillary plays a key role. In fact, this effect is clearly seen in capillaries, with square cross-section: the edges of the capillary, on the one hand, stabilize disclination lines in the nematic phase, and on the other hand they induce the formation of many defects in the smectic phase. |
Wednesday, March 4, 2020 11:39AM - 11:51AM |
M29.00003: Phase Transition Triggering in a Liquid Crystal Droplet Jake Shechter, Noe Atzin Canas, Rui Zhang, Ali Mozaffari, Benjamin A Strain, Linda M Oster, Manisha Chahar, Juan De Pablo, S. Thai Thayumanavan, Jennifer L Ross Liquid crystals (LCs) are a class of molecules that form a variety of phases easily influenced by external interactions. Of particular interest are rod-like LC molecules confined to a spherical geometry, which have a competition between interfacial tension and elastic deformations. The phase of the liquid crystal inside a droplet can be controlled using surfactants, influencing the boundary conditions, in an oil-in-water emulsion. Here, we test the effects of the surfactant sodium dodecyl sulfate (SDS) on the phase of polydisperse LC droplets in an aqueous background examining different diameter sizes and SDS concentrations. We trigger phase transitions by the increase or decrease of SDS concentration while observing an individual LC droplets held in place using an optical tweezer. The dynamic phase transitions are compared to previous molecular dynamic simulations that predicted the intermediate states of the LC inside the droplets. We observe a hysteresis in the SDS concentration that induces the phase transition from radial to bipolar and back as well as a fluctuation in state when the phase transition starts. |
Wednesday, March 4, 2020 11:51AM - 12:03PM |
M29.00004: Nanoparticle Network embedded polymer films formed using liquid crystal transition templating. Alauna Wheeler, Linda S. Hirst Our lab recently demonstrated that nanoparticles, well-dispersed in the isotropic liquid crystal (LC) phase, will self-assemble into hollow structures when the LC solvent undergoes cooling through the isotropic-to-nematic (I-N) phase transition. The form and size of the nanoparticle structures can be controlled by varying the nanoparticle concentration and the cooling rate (1). Thus far this process has only been accomplished using 5CB (4’-pentyl-4-Biphenylcarbonitrile) as the LC solvent. In this study we demonstrate that hollow networks of self-assembled nanoparticles can be formed in different nematic LC mixtures, compatible with polymerization. The cooling rate across the I-N phase transition can be used to control the size and density of the network structures. We embed these nanoparticle structures in a thin film by using a polymerizable LC mixture in the formation process and characterize the properties of the film. |
Wednesday, March 4, 2020 12:03PM - 12:15PM |
M29.00005: One pot synthesis of hollow colloidal gel DEVIKA GIREESAN SUDHA, Jocelyn Ochoa Hernandez, Linda S. Hirst Liquid crystal (LC) phase transitions can drive the assembly of dispersed nanoparticles. Hollow microstructures of various morphologies can be created, on cooling through the nematic-isotropic transition temperature (TNI) at different temperature quenches. At high quench rates single compartment hollow capsules are observed. These colloidal capsules frustrate orientational order in the liquid crystal leading to topological defects. These defects stabilize the capsules into a mechanically robust gel. The capsule sizes can be controlled by varying the quench depth. Here we present structural analysis using confocal and fluorescence imaging of these gels for a range of capsule sizes. We also observed continuous tubular network morphologies at lower quench rates. These networks under birefringence show the presence of defect lines and loops. Such one pot synthesis technique leads to the rapid self-assembly of liquid crystal nanocomposites, that may find applications in controlled release, sensing and catalysis. |
Wednesday, March 4, 2020 12:15PM - 12:27PM |
M29.00006: Intertwined spontaneous helix formation in achiral bent-core liquid crystals Vikina Martinez, Adam Green, Cheol Park, Matthew Glaser, Joseph E MacLennan, Noel Anthony Clark Inspired by several reports of spontaneous helix formation in bent-core smectics, we have investigated the smectic phases of a series of bent-core mesogens with 4-cyanoresorcinol bisbenzoate cores. In PAL30, the homolog with n=14 alkyl tails, we observed an incommensurate, bent-core phase with a single helix incommensurate with the smectic layering [1], a structure broadly analogous to the phase of rod-shaped molecules. In the PAL29, the n=16 homolog, we have confirmed, using both resonant and non-resonant x-ray diffraction, as well as electro-optical methods, the existence of a phase with intertwined helices, similar to the structure recently reported by Salamonczyk and co-workers [2]. Establishing the existence of the intertwined double-helical phase and the single-helical phase in such closely related homologs is an important step in understanding the competing interactions that lead to such exotic phase structures. |
Wednesday, March 4, 2020 12:27PM - 12:39PM |
M29.00007: Transformations Between Different Self-Organized Defect Arrays at the Nematic-Smectic A Phase Transition Daniel Beller, Ahram Suh, Min-Jun Gim, Dong Ki Yoon We model transformations in self-organized liquid crystal defect arrays in hybrid-aligned thin films at the nematic-smectic A phase transition. The smectic A phase forms a packing of focal conic domains, while the nematic phase features a square lattice of boojum surface-defects at larger spacing. Heating from smectic A to nematic transforms the focal conic domain packing into a dense array of boojums, which we study through Landau-de Gennes numerical modeling and a connection to graph theory. The reverse transformation upon cooling proceeds through a very different morphological sequence, beginning with a stripe instability which we examine analytically and numerically. The focal conic domain packings that emerge retain a geometric memory of the nematic boojum configuration. |
Wednesday, March 4, 2020 12:39PM - 12:51PM |
M29.00008: Investigation of Thermal Properties of Itraconazole at Ultra-Fast Rates John Zapata-Hincapie, Madhusudhan Reddy Pallaka, Sindee L Simon
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Wednesday, March 4, 2020 12:51PM - 1:03PM |
M29.00009: Coalescence Dynamics of Fluids in Two Dimensions: Merging of Islands in Freely-Suspended Smectic Films Duong Nguyen, Kirsten Harth, Aaron Goldfain, Cheol Park, Joseph E MacLennan, Matthew Glaser, Noel Anthony Clark Smectic liquid crystal films a few molecular layers thick that are freely suspended in air are used as a model system to study the coalescence of fluids in two dimensions. High-speed video microscopy is used to study the coalescence of islands, thicker, disk-shaped regions of the film bounded by edge dislocations. The early time growth of the bridge connecting the merging islands is essentially linear, while at long times the relaxation dynamics are exponential. The observations are compared with Hopper’s classical hydrodynamic model of the coalescence of two infinitely long, fluid cylinders [Journal of the American Ceramic Society 76, 2947 (1993)] and with models appropriate to quasi-two dimensional fluids developed by Mann [ Phys. Rev. E 51, 5708 (1995)] and Camley [Biophysical Journal 99, L44 (2010)] to explain their observations of domain relaxation in Langmuir films. |
Wednesday, March 4, 2020 1:03PM - 1:15PM |
M29.00010: Phase behavior and elastic response of liquid crystal mixtures in atomistic models Jiale Shi, Hythem Sidky, Jonathan K. Whitmer In practical applications, liquid crystal devices tend to utilize mixtures of liquid crystal molecules. Liquid crystals mixtures can be formulated to have a broad nematic phase range, elastic responses and dielectric anisotropies which are distinct compared to the components. Elasticity, in particular, is important in determining equilibrium morphologies and material responses. Despite this importance, limited knowledge exists about the connections between microscopic composition and macroscopic elastic response in liquid crystal blends. Here, we apply advanced sampling methods in atomistic models to study both the phase behavior and elastic response of liquid crystal mixture. Our simulation work can help understand the elasticity of liquid crystals mixtures and help to rationally design new liquid crystal mixtures. |
Wednesday, March 4, 2020 1:15PM - 1:27PM |
M29.00011: Macroscopic sheets of negative order parameter Liquid Crystal Elastomer with high dissipation and auxetic properties Devesh Mistry, Ross Volpe, Nicholas Traugutt, Christopher Yakacki Negative Order Parameter (NOP) Liquid Crystal Elastomers (LCEs) offer a route to new LCE actuation modes and the study of previously impossible LC systems.[1-3] |
Wednesday, March 4, 2020 1:27PM - 1:39PM |
M29.00012: Oligomeric odd–even effect in liquid crystals. Rony Saha, Greta Babakhanova, Zeinab Parsouzi A.Sh, Mojtaba Rajabi, Prabesh Gyawali, Chris Welch, Georg H. Mehl, James Gleeson, Oleg D Lavrentovich, Samuel N Sprunt, Antal Istvan Jakli In organic materials, they are usually associated with the number of methylene groups in aliphatic chains. In this work, we unveil multiple signatures of a new odd–even effect in liquid crystals that occurs at the larger scale of molecular moieties that by themselves express liquid crystalline behavior. We demonstrate that oligomeric liquid crystals, with n= 1–4 number of rigid mesogenic segments connected by flexible aliphatic chains with an odd number of methylene groups, produce an odd–even effect in optical anisotropy and the bend elastic constant of the liquid crystal oligomer. This effect is different from the usual odd–even effects with respect to the parity of carbon atoms in an aliphatic chain and can be understood in term of the average molecular shape and the associations between n-mers based on the packing of these shapes. We also show that, in spite of the fact that there is no long-range electron density modulation, careful analysis of synchrotron SAXS results can provide important information about the molecular associations in the N and NTB phases that other techniques cannot access. |
Wednesday, March 4, 2020 1:39PM - 1:51PM |
M29.00013: Relative orientation of defects in smectic-C films Xingzhou Tang, Jonathan Selinger In a 2D liquid crystal, each topological defect has a topological charge and a characteristic orientation, and hence can be regarded as an oriented particle. Theories predict that the trajectories of annihilating defects depend on their relative orientation [1,2]. Recently, these predictions have been tested in experiments on smectic-C films [3]. Those experiments find curved trajectories that are similar to the predictions, but the detailed relationship between the defect orientations and the far-field director field is different. To understand this difference, we improve the previous theories by adding the effects of elastic anisotropy and nonequilibrium dynamics. We find that the theory with elastic anisotropy gives good agreement with the experiment, and hence can describe the effects of defect orientation. |
Wednesday, March 4, 2020 1:51PM - 2:03PM |
M29.00014: Tying knots in liquid crystals Jung-Shen Tai, Ivan I Smalyukh The topological nature of knots in fields has fascinated physicists and mathematicians. Experimentally they had been found only as transient features or required complex boundary conditions to exist until recently [1]. Here we present energetically stable, micrometer-sized knots in the chiral background of liquid crystals. These knots are spatially localized and diffusing in all directions. They self-assemble into crystalline lattices with open and closed structures, resembling colloidal particles and atomic nuclei. These knots are robust and topologically distinct from the host medium and can be morphed and reconfigured by weak stimuli under conditions such as those in displays. We use a combination of energy-minimizing numerical modeling and optical imaging to uncover the internal structure and topology of individual knots and the various hierarchical crystalline organizations they form. We further explore the effects of elastic anisotropy and external fields on the stability of these knots. |
Wednesday, March 4, 2020 2:03PM - 2:15PM |
M29.00015: Computational model of a Maier-Saupe field theory for nematic liquid crystals Cody Schimming, Jorge Vinals We develop a computational model based on a field theory extension of the Maier-Saupe molecular model of a uniaxial, nematic liquid crystal. A tensor order parameter is defined as the second moment of an orientational probability distribution, leading to a free energy that is not convex within the isotropic-nematic coexistence region, and that goes to infinity if the eigenvalues of order parameter become non-physical. An elastic free energy is added which is an expansion in spatial gradients of the order parameter up to third order, allowing for parametric control over the anisotropy and elasticity of the liquid crystal. Computations of the spatial profile of the order parameter are presented for an isotropic-nematic interface in one dimension, a nematic defect in two dimensions, and a tactoid in two dimensions. We compare the simulation results to those given by the Landau de-Gennes free energy. |
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