Bulletin of the American Physical Society
2015 Annual Spring Meeting of the APS Ohio-Region Section
Volume 60, Number 3
Friday–Saturday, March 27–28, 2015; Kent, Ohio
Session B4: Biophysics/Soft Matter I |
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Chair: Michael Fisch, Kent State University Room: KSU Student Center 316 |
Friday, March 27, 2015 3:00PM - 3:15PM |
B4.00001: Fluctuation modes in the twist bend nematic phase of certain liquid crystal mixtures studied by dynamic light scattering Zeinab Parsouzi A.Sh, Volodymyr Borshch, Pavan Challa Kumar, Oleg Lavrentovich, James T. Gleeson, Antal Jakli, Samuel Sprunt The ``twist-bend nematic''($N_{TB}$) is a recently discovered phase of liquid crystals. Its ground state features a heliconical molecular arrangement in which the nematic director precesses uniformly about a fixed axis, at a finite angle to this axis. The helicoid has a nanoscale pitch. We present the dynamic light scattering studies in two different mixtures of a dimeric material (M2), which exhibits nematic and $N_{TB}$ phases. In the nematic phase, two fluctuation modes are observed: one is the usual hydrodynamic, uniaxial director mode, while the second is clearly non-hydrodynamic and corresponds to fluctuations of biaxial order. The non-hydrodynamic mode is also observed in the $N_{TB}$ phase, with a significantly higher and strongly temperature-dependent relaxation rate. Scattering from the twist-bend director mode disappears in the $N_{TB}$ phase, while splay fluctuations still contribute. We discuss our results in terms of a theoretical model based on a combination of two elements: (1)a coarse-grained elastic free energy (in which the $N_{TB}$ is modeled a smectic-like, pseudo-layered structure) and (2)a recent theory [1] describing local polar order in the $N_{TB}$ phase.\\[4pt] [1] Phys.Rev.E 87,052503(2013) [Preview Abstract] |
Friday, March 27, 2015 3:15PM - 3:30PM |
B4.00002: Defect Induced Polar Structures in Nematic Liquid Crystals Shokir Pardaev, Antal Jakli, James Gleeson, Brett Ellman, Samuel Sprunt We use angle-resolved second harmonic light scattering as a tool to probe polar structures due to defects in a series of liquid crystal samples, including rod-like and bent-core molecules forming the standard uniaxial nematic phase, and dimers that exhibit the exotic twist-bend phase. We describe models to explain the spatial distribution and polarization of second harmonic scattered light, based on electric polarization induced by deformations of the molecular orientation associated with specific topological defect structures. We thank O. Parri at Merck Chemicals Ltd., Southampton, UK for providing the studied material for us [Preview Abstract] |
Friday, March 27, 2015 3:30PM - 3:45PM |
B4.00003: Flow Properties of a Twist-bend Nematic Liquid Crystal Seyyed Muhammad Salili, Chanjoong Kim, Samuel Sprunt, James Gleeson, Owain Parri, Antal Jakli 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 $1Pa$ the apparent viscosity of $N_{tb} $ is $1000$ times larger than in the nematic phase. At stress above $10Pa$ 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 $B\sim 2kPa$; this value is discussed within the context of a simple theoretical model based upon a coarse-grained elastic free energy. [Preview Abstract] |
Friday, March 27, 2015 3:45PM - 4:00PM |
B4.00004: Domain Walls Mimicking Nematic Biaxiality in the Oxadiazole Bent-Core Liquid Crystal C7 Young-Ki Kim, Greta Cukrov, Jie Xiang, Sung-Tae Shin, Oleg D. Lavrentovich We investigate the origin of ``secondary disclinations'' that were recently described as new evidence of a biaxial nematic phase in an oxadiazole bent-core thermotropic liquid crystal C7. With an assortment of optical techniques such as polarizing optical microscopy, LC PolScope, and fluorescence confocal polarizing microscopy, we demonstrate that the secondary disclinations represent non-singular domain walls formed in an uniaxial nematic during the surface anchoring transition, in which surface orientation of the director changes from tangential (parallel to the bounding plates) to tilted. Each domain wall separates two regions with the director tilted in opposite azimuthal directions. At the center of the domain wall, the director remains parallel to the bonding plates. Furthermore, we verify that the thickness-dependent anchoring transition of C7 is associated with a local electric field caused by ionic impurities in the material, and examine how the thermal degradation of material affects the surface alignment. The study shows that C7 exhibits only a uniaxial nematic phase. [Preview Abstract] |
Friday, March 27, 2015 4:00PM - 4:15PM |
B4.00005: Living liquid crystals Shuang Zhou, Andrey Sokolov, Oleg Lavrentovich, Igor Aronson Bio-mechanical hybrids are an emerging class of engineered composite soft materials. By transducing energy stored in the environment to drive systematic movements, they can move and reconfigure their structure and properties in response to external stimuli. This functionality is critical for a variety of applications, from bioinspired micromachines and sensors to self-assembled microrobots. Here, by combining two seemingly incompatible concepts, living swimming bacteria and inanimate but orientationally ordered lyotropic liquid crystal, we conceive a fundamentally new class of matter - living liquid crystals (LLCs). The coupling between the activity-triggered flows and director reorientations results in a wealth of phenomena, including: (a) a characteristic length to describe the coupling between the orientation of LLC and the bacterial motion, (b) periodic stripe instabilities of the director in surface-anchored LLCs, (c) director pattern evolution into an array of disclinations with positive and negative topological charges as the surface anchoring is weakened or when the bacterial activity is enhanced. Our study provides an insight in understanding hierarchy of spatial scales in other active matter systems, as well as providing basis for devices with new functionalities. [Preview Abstract] |
Friday, March 27, 2015 4:15PM - 4:30PM |
B4.00006: Investigations of Liquid Crystal Alignment Layers Prepared by Mechanical Buffing and Exposure to Polarized Blue Light Lewis Sharpnack, Ibrahim Abdulhalim, Satyendra Kumar Proper liquid crystal (LC) surface alignment remains critical to their technological applications in electrooptical and photonic devices. Researchers have been on a quest to find an alternative to the empirically perfected method involving mechanical buffing of a polymer film deposited on a substrate. Previous investigations of organic alignment films, mechanically buffed or exposed to polarized UV light to affect alignment, have shown that in all cases the air to alignment layer interface acquires roughness anisotropy irrespective of the method of their preparation. A new generation of alignment materials, polarized blue light exposed chalcogenide glass films and that do not require the use of contact methods (e.g., buffing) has been developed. The surface morphology and roughness anisotropy of these films has been investigated with x-ray reflectivity, SEM, and AFM techniques. We will present preliminary findings for several new alignment films along with the conventional PI films. [Preview Abstract] |
Friday, March 27, 2015 4:30PM - 4:45PM |
B4.00007: Liquid crystal-enabled electro-osmosis through spatially separated charges in photo-patterned surface alignment Chenhui Peng, Yubing Guo, Sergij Shiyanovskii, Qihuo Wei, Oleg Lavrentovich Electrically-controlled dynamics of fluids and particles at microscales is a fascinating area of research with applications ranging from microfluidics and sensing to sorting of biomolecules. We demonstrate that anisotropic conductivity of liquid crystals in combination with photopatterned surface alignment enables highly efficient electro-osmosis (LCEO) rooted in space charging of regions with distorted orientations. LCEO velocities grow with the square of the field, which allows one to use an AC field to drive steady flows and to avoid electrode damage. By controlling the director patterns, one can dramatically change the nature of LCEO flows, for example, trigger a pumping effect in dipolar configuration and reverse the flow direction in quadrupolar patterns. Ionic currents in liquid crystals that have been traditionally considered as an undesirable feature in displays, offer a broad platform for versatile applications such as liquid crystal enabled electrokinetics, micropumping and mixing. [Preview Abstract] |
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