Bulletin of the American Physical Society
2024 APS March Meeting
Monday–Friday, March 4–8, 2024; Minneapolis & Virtual
Session B35: Liquid Crystals II |
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Sponsoring Units: DSOFT Chair: Shuang Zhou, University of Massachusetts Amherst Room: 103A |
Monday, March 4, 2024 11:30AM - 11:42AM |
B35.00001: Mesoscale Modeling of Phase Separation in Structured Fluids Gabriel I Vega-Bellido, Robert A Riggleman Phase separation of polymeric fluids yields structures used in a variety of engineering applications from gas separations to bicontinuous structures with complimentary phases. Often, the constituent phases are amorphous with no large-scale internal structure, for which the dynamics of phase separation are well established. However, when one of the phases is ordered, such as in a liquid crystal, the kinetics of phase separation and predictions of the structures that form are comparatively poorly understood, especially at mesoscale length scales due to compuitational constraints. We present a computationally efficient coarse-grained molecular model for liquid crystals capable of forming both nematic and smectic phases. This model allows for the study of intermediate out-of-equilibrium states as the system undergoes phase separation. By varying the volume fraction, the phase formed by the liquid crystal, and its anchoring conditions with respect to the polymer, we observe a variety of structures during the phase separation process. We studied the coarsening dynamics of the systems via structure factor analysis and characterized their interfacial geometry by quantifying the curvature of the liquid crystal-polymer interface, thereby highlighting the emergence of various novel features compared to phase separation in isotropic fluids. We hope the improved understanding of these structured fluid systems will lead to novel materials featuring tailored interfaces for use in membrane technologies. |
Monday, March 4, 2024 11:42AM - 11:54AM |
B35.00002: Coalescence of Sessile Radial Nematic Droplets Charlotte Slaughter, Sophie Ettinger, Yihao Chen, Zhe Feng, Rui Zhang, Peter J Collings, Arjun G Yodh We report on experiments that demonstrate coalescence of two “largely” radial nematic liquid crystal (NLC) droplets with homeotropic anchoring. Previously, extensive theoretical work has suggested that coalescence of two NLC radial droplets is prevented by the energy barrier for formation of a topological defect between the two droplets [1, 2]. Our experimental work suggests that droplet wetting and deformation lowers this energy barrier. We describe the spontaneous formation of a Q = −1 defect at the neck between two sessile droplets, and the subsequent annihilation of defects to generate a single radial hedgehog droplet with a Q = +1 defect. Using polarized optical microscopy (side and top views), we are able to observe droplet merging and defect and director evolution, as well as characterize how the drop contact angle influences likelihood of coalescence. Simulations were also employed to build connections with the POM images and director configurations, as well as to calculate the change in energy due to distortion of the director field, which suggests that the coalescence energy barrier is lowered. |
Monday, March 4, 2024 11:54AM - 12:06PM |
B35.00003: Externally Controllable Nematic Flow Induced by Active Surface Anchorings Seyed Seyednejad, Miha Ravnik Usually, active matters exhibit some sort of nematic order which constantly is distorted due to internal activity of the system. But can this also happen in reverse? Can nematic order distortions cause stable activity in intrinsically passive systems? Well, there is a phenomenon called backflow effect. This means that elastic distortions can cause flux in nematic liquid crystals (NLC). However, to achieve a net flux, the distortions must be induced in a non-reciprocal manner. One of the most effective ways to induce the elastic distortions into the bulk of the NLC is to change the boundary conditions in time and space, which we call active surface anchoring. Our simulations show that if the surface anchoring on the surface non- reciprocally changes in time, e.g., like a wave, there will be a back-and-forth flow with a nonzero time average, as in a heartbeat. In experiments, this can be achieved by grafting photosensitive elastomers onto the surface. Such elastomers reversibly change their folding by UV and visible light, leading the surface anchoring switch between planar and homeotropic. |
Monday, March 4, 2024 12:06PM - 12:18PM |
B35.00004: Hierarchical Helical Structure of Cholesteric Liquid Crystals Enabled Localized Optical/Mechanical Control with Nanoscale Precision Tejal R Pawale, Mesonma Anwasi, David A Czaplewski, Ralu Divan, Xiao Li Chirality is a geometric property that naturally occurs at all levels in biological systems and has unique ability to introduce functionalities such as selective reflection of light, mechanical adaption, and resistance against external stresses.Numerous efforts have been made to mimic such morphologies and their properties in synthetic materials. Chiral nematic phase of Liquid Crystals is known to form elegant and distinct structures across length scales resembling the chiral structure omnipresent in living systems such as the exoskeletons of beetle, plant tissues and others. In this work, we relied on chemically nano-patterned substrate with varying anchoring energy to self-assemble uniform lying down helices which directs the formation of hierarchical helical structure at room temperature. The twisted rope like nature of this morphology was investigated using different parameters to gain control over the structure and its properties while polymerization was used to generate a soft template. The study provides a potential avenue to design and fabricate miniaturized or wearable devices, which requires delicate control of optical and mechanical properties with nanoscale precision. |
Monday, March 4, 2024 12:18PM - 12:30PM |
B35.00005: Chirality And Shape In Liquid Crystal Tactoids Francisco M Navarro, Chaitanya S Joshi, Timothy J Atherton Droplets or tactoids of anisotropic fluids such as liquid crystals may adopt non-spherical equilibrium shapes when immersed in an isotropic host. This is possible due to a competition between the isotropic and anisotropic components of surface tension as well as elasticity. The presence of chirality greatly enriches the space of permissible shapes because of the possibility of geometric frustration: In bulk, chiral liquid crystals adopt a periodic helical configuration; while in a confined geometry this preferred periodic structure may be incompatible with the confinement. We numerically investigate how the chiral order of the liquid crystal may be imprinted on the droplet shape by elucidating the solution space of chiral liquid crystals as a function of material parameters. We find both commensurate and frustrated solutions. Prospects for experimental realization will also be discussed. |
Monday, March 4, 2024 12:30PM - 12:42PM |
B35.00006: Nanowire structure of liquid crystals due to a rough interface Luz J Martinez-Miranda, Mario F Borunda We investigate in this talk the effect of the buried interface has on the structure liquid crystal small molecule, which facilitates charge transfer. The liquid crystals have the property of self alignment that distinguishes them from other organic molecules. The rough interface has been prepared such that the liquid crystal aligns parallel to the surface of the interface. The parallel alignment includes the imperfections or openings in the rough interface that have depths that vary between 10 and 1000nm. The imperfections of this interface are similar to cylindrical tubes or containers. There are many ways that the liquid crystal can go parallel inside the imperfections, but there is always a way that costs less energy to the system. This can be determined through theoretical work. Once we have such orientation, we add the liquid crystals parallel to the interface and because they self-align, the liquid crystals that are in the next layer away from the interface will align following the molecules parallel to the interface. They will fill eventually the opening or imperfection up to its center, where there will be a defect. When the electrons and the holes separate at the interface they will have a way to move through the overlap of the orbitals in this structure of parallel molecules, and eventually reach the electrodes and add to the efficiency. We discuss also how grazing angle X-ray scattering can be used together with other characterization techniques to give us information about the alignment of the liquid crystals. |
Monday, March 4, 2024 12:42PM - 12:54PM |
B35.00007: Directional Swimming of B. subtilis Bacteria Near the Surface of Ferroelectric Nematic Liquid Crystal Films Mahesha J Kodithuwakku Arachchige, Zakaria Siddiquee, Hend M Baza, Alex O Adaka, Robert J. Twieg, Oleg D Lavrentovich, ANTAL I JAKLI Controlling the motion of bacteria by volume interactions in lyotropic chromonic nematic liquid crystal environments was studied widely in the past decade 1. The goal of this study was to find out whether the motion of bacteria can be guided by surface interactions. For this we chose a recently discovered ferroelectric nematic liquid crystal (FNLC) which is glassy at room temperature 2,3. We put a drop of bacteria in terrific broth on a film of aligned FNLC and found that bacteria tend to align preferably opposite to the polarization direction of the FNLC. Interestingly, that direction of the polarization of FNLC can be set by applying electric fields or by using surface alignments prior to applying bacteria. This effect can be explained by the highly charged surface of bacteria and the potassium ion concentration gradient in the medium induced by the depolarization field of the underlying FNLC film. |
Monday, March 4, 2024 12:54PM - 1:06PM |
B35.00008: Hyperspectral Tunable Filters via Polymer-Stabilized Oblique Helicoidal Cholesteric Liquid Crystals Jun-Hyung Im, Yeongseon Choi, Eunsu Cho, Daeseop Choi, YoungKi Kim Spectral filters play an important role in sorting specific wavelength information from incident light. However, conventional filters, such as pigment-based color filters, acousto-optic filters, and Lyot filters, have limitations in terms of poor color purity, low tunability, and low transmittance [1,2]. The experimental discovery of oblique helicoidal cholesteric (ChOH) liquid crystals (LCs), with their periodic pseudo-layers, realizes highly tunable Bragg reflectors across a wide spectral range in response to electric field (E) and temperature [3,4]. |
Monday, March 4, 2024 1:06PM - 1:18PM |
B35.00009: Cholesteric Liquid Crystal Control Through Microfluidic Flow Manipulation Tadej Emersic, Kushal Bagchi, Aiden Jensen, Paul F Nealey, Juan J De Pablo Materials capable of undergoing color changes in response to stimuli are of interest for various applications, including sensing, display technology, and camouflage. Cholesteric liquid crystals represent a unique class of soft materials due to their self-assembled helical structure, which is sensitive to various external stimuli and known for its selective light reflection. However, their response to pressure-driven flow in microfluidic channels has largely remained experimentally unexplored. Here, we investigate a cholesteric system with a helical pitch comparable to the wavelength of visible light, enabling it to exhibit structural coloration. We demonstrate that fluid flow can effectively align the helical axis of cholesterics within confined geometries. Before the application of flow, the color of the cholesteric phase remains independent of temperature. However, after flow alignment, a blue shift is observed as the temperature increases. Microfluidic flow creates cholesteric textures with helical pitches longer than those observed in the stationary chiral phase. We observe that flow creates stable-colored bands, which maintain their stability for months. The flow-structure relationships revealed by our study have potential relevance for applications such as additive manufacturing of liquid crystals. |
Monday, March 4, 2024 1:18PM - 1:30PM |
B35.00010: Interactions in nematic liquid crystals: from activity to deformability Thomas G Chandler, Saverio E Spagnolie Fluid anisotropy, or direction-dependent response to deformation, can be observed in biofluids like mucus or, at a larger scale, self-aligning swarms of active bacteria. A model fluid used to investigate such environments is a nematic liquid crystal. Large colloidal particles undergo shape-dependent interactions when immersed in these complex environments, whilst deformable bodies (like red blood cells) tend to be stretched, offering a passive means of measuring cell material properties. Adding to the complexity are microorganisms that propel themselves through these environments, giving rise to active stresses. In this talk, we will use complex variables to analytically solve for the interaction between bodies immersed in liquid crystalline environments. This approach allows for the solution of a wide range of problems, opening the door to studying the role of body geometry, liquid crystal anchoring conditions, and deformability. Shape-dependent forces between bodies, local tractions, and active stresses will also be discussed. |
Monday, March 4, 2024 1:30PM - 1:42PM |
B35.00011: Low Voltage Switchable Chiral Ferroelectric Nematic (NF*) Liquid Crystal Smart Window Rohan O Dharmarathna, Kelum Perera, Md Sakhawat Hossain Himel, Alex O Adaka, Tamas Kosa, ANTAL I JAKLI There is a high demand for smart glasses that can switch between transparent and opaque states in smart privacy windows and display applications. Polymer stabilized liquid crystal (PSLC) and polymer dispersed liquid crystal (PDLC) based smart windows are available in the market, but they require high voltages. Ferroelectric nematic (NF) liquid crystals with polar fluid structures have been discovered recently . In this research project we developed a low voltage switchable chiral ferroelectric nematic (NF*) liquid crystal smart window. For that we optimized a PSLC mixture, that switches from transparent to opaque (reverse-mode) at an AC voltage less than 10 V. In the transparent state we measured 80 ~ 85 % transmission and in opaque state we measured 20 ~ 16 % transmission for 550 nm. The switching time was around 50 ms. For application purposes we are studying cells with flexible, ITO coated PET (Polyethylene terephthalate) substrates as well. |
Monday, March 4, 2024 1:42PM - 1:54PM |
B35.00012: Blue phase liquid crystals in a Grandjean–Cano cell Kushal Bagchi, Tadej Emersic, José A Martínez-González, Juan J De Pablo, Paul F Nealey Blue phase liquid crystals are soft photonic crystals that are of interest for sensing and display applications. In recent years, considerable progress has been made in forming blue phase single crystals on chemical patterns. Blue phase single crystals are appealing for technological applications owing to their optical homogeneity. To study single crystal formation when the thickness is incommensurate with blue phase lattice size we prepare cells with a thickness gradient or a “Grandjean–Cano cell”. We observe formation of stripes in the direction perpendicular to the thickness gradient in blue phase single crystals. The stripes occur at a thickness interval significantly smaller than the lattice size. To understand the origin of these mysterious stripes we perform Landau-de Gennes based continuum simulations. Based on experimental and theoretical findings we speculate that the spacing between stripes is related to the size of double twist cylinders in the blue phase. |
Monday, March 4, 2024 1:54PM - 2:06PM |
B35.00013: Electro-caloric Effect of Liquid Crystals with Direct Isotropic - Ferroelectric Nematic transition. Alex O Adaka, Parikshit Guragain, Robert J. Twieg, ANTAL I JAKLI The study of electro-caloric effects (ECEs) in solid state materials such as ferroelectric ceramics and ferroelectric polymers have great impact in developing cooling systems. Recently, dielectric fluids at Isotropic – Nematic [1] and Isotropic – Nematic – Ferroelectric Nematic [2] transitions have been studied and found to have large (up to 3K / (V/μm)) ECEs with simpler structure compared to the solid-state materials. Here, we report studies on systems with direct Isotropic-Ferroelectric Nematic transition. With an enthalpy of 4.7J/g and electric field of 1.1V/μm we can induce a 2K change in the transition temperature in spite of the fact that the material is fairly ionic. Studies are underway to increase the effect by reducing the ionic content. |
Monday, March 4, 2024 2:06PM - 2:18PM |
B35.00014: Landau Theory of Barocaloric Plastic Crystals Gian G Guzman-Verri, Roberto Marín-Delgado, Xavier Moya We present a simple Landau phenomenology for plastic-to-crystal phase transitions and use the resulting model to calculate barocaloric effects in plastic crystals that are driven by hydrostatic pressure. The essential ingredients of the model are (i) a multipole-moment order parameter that describes the orientational ordering of the constituent molecules, (ii) coupling between such order parameter and elastic strains, and (iii) the thermal expansion of the solid. The model captures main features of plastic-to-crystal phase transitions, namely large volume and entropy changes at the transition, and strong dependence of the transition temperature with pressure. Using solid C60 under 0.60 GPa as case example, we show that calculated peak isothermal entropy changes of about 58 kJK-1kg-1 and peak adiabatic entropy changes of about 23K agree well with experimental values. |
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