Bulletin of the American Physical Society
APS March Meeting 2010
Volume 55, Number 2
Monday–Friday, March 15–19, 2010; Portland, Oregon
Session H13: Liquid Crystals: Nanoparticles and Surfaces |
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Sponsoring Units: DFD Chair: Robin Selinger, Kent State University Room: B112 |
Tuesday, March 16, 2010 8:00AM - 8:12AM |
H13.00001: Maier-Saupe-type theory of ferroelectric nanoparticles in nematic liquid crystals Jonathan Selinger, Lena Lopatina Several experiments have reported that ferroelectric nanoparticles have drastic effects on nematic liquid crystals--increasing the isotropic-nematic transition temperature by about 5 K, and greatly increasing the sensitivity to applied electric fields. In a recent paper [1], we modeled these effects through a Landau theory, based on coupled orientational order parameters for the liquid crystal and the nanoparticles. This model has one important limitation: Like all Landau theories, it involves an expansion of the free energy in powers of the order parameters, and hence it overestimates the order parameters that occur in the low-temperature phase. For that reason, we now develop a new Maier-Saupe-type model, which explicitly shows the low-temperature saturation of the order parameters. This model reduces to the Landau theory in the limit of high temperature or weak coupling, but shows different behavior in the opposite limit. We compare these calculations with experimental results on ferroelectric nanoparticles in liquid crystals. \\[4pt] [1] L. M. Lopatina and J. V. Selinger, Phys. Rev. Lett. 102, 197802 (2009). [Preview Abstract] |
Tuesday, March 16, 2010 8:12AM - 8:24AM |
H13.00002: Study of Phase Transitions in Barium Titanate - Liquid Crystal Nano-colloidal Suspensions Krishna Sigdel, Germano Iannacchione A high-resolution ac-calorimetric study of the isotropic to nematic (\textit{I}-\textit{N}) and the nematic to smectic-\textit{A} (\textit{N}-Sm\textit{A}) phase transitions in nano-colloidal suspensions of Barium Titanate (BaTiO$_{3}$) in the liquid crystal (LC) octylcyanobiphenyl (8CB) as a function of BaTiO$_{3}$ concentration is reported. Heating and cooling scans were performed for all samples ($0.1 $ - $1.4$~wt\% of BaTiO$_{3}$ and pure 8CB) over a wide temperature range well above and below the two transitions. Both the \textit{I}-\textit{N} and the \textit{N}- Sm\textit{A} transitions evolve in character and have their transition temperatures shift lower as the wt\% of BaTiO$_{3}$ increases. Increasing hysteresis with increasing concentration is also observed at the \textit{I}- \textit{N} transition between heating and cooling scans within the two- phase \textit{I}+\textit{N} coexistence region. These results will be contrasted with other LC colloidal dispersion systems. [Preview Abstract] |
Tuesday, March 16, 2010 8:24AM - 8:36AM |
H13.00003: Optical control of nano-particles and colloidal architectures Hector Mireles, Angel Martinez, Ivan Smalyukh Controlled structural assembly of micro- and nano-sized particles is essential for many technologies, ranging from optical metamaterials to photovoltaic devices. We demonstrate a low-intensity ``opto-elastic'' method of manipulating micron- and submicron- sized particles suspended within a nematic liquid crystal. By optically controlling a surface monolayer of an azobenzene based dye, we manipulate the surface boundary conditions and liquid crystal director in the sample bulk to form domains of uniform alignment. Large elastic deformations are thus generated at the interface between adjacent domains with different director orientations. By exploiting elastic forces acting on the particles near the misalignment walls, we manipulate micron and sub-micron-sized particles such as melamine resin polymer microspheres and silver nano-rods. This method of ``opto-elastic'' manipulation requires light intensities three orders of magnitude smaller than in the case of laser tweezers utilizing optical gradient forces and enables massively-parallel manipulation of multiple particles on the scales of millimeters and centimeters. [Preview Abstract] |
Tuesday, March 16, 2010 8:36AM - 8:48AM |
H13.00004: Interaction of a bi-molecular liquid crystal film with functionalized nanoparticles Jefferson W. Taylor, L.J. Martinez-Miranda We investigate the properties of a bi-molecular film of liquid crystal close to a magnetic nanoparticle (CoFe) with a functionalization compound (MHDA) with the atomic force microscope (AFM). We seek to investigate if the functionalization compound has an effect on the ordering of the liquid crystal in the vicinity of the nanoparticle. Studies in bulk liquid crystals have shown that the functionalization compound influences how the liquid crystal reorganize [1]. The results of this investigation will be compared to the results of work done on phospholipids in close contact with uncovered silica nanoparticles [2]. Preliminary studies of the liquid crystal in contact with the nanoparticles show that it behaves similarly to the way the phospholipids behave. More functionalization compounds are studied in order to establish whether it behaves differently depending on the functionalization compound. \\[4pt] [1] L. J. Mart\'{\i}nez-Miranda, L. K. Kurihara, J. Appl. Phys \underline {105}, 084305 (2009). \\[0pt] [2] Yuri Roiter, Maryna Ornatska, Aravind R. Rammohan, Jitendra Balakrishnan, David R. Heine, and Sergiy Minko, Langmuir, \underline {25}, 6287-6299 (2009). [Preview Abstract] |
Tuesday, March 16, 2010 8:48AM - 9:00AM |
H13.00005: The Role of Liquid Crystal Order in Liquid Crystal-Zno Nanoparticle Photovoltaics Luz J. Martinez-Miranda, Kaitlin M. Traister, Lourdes Salamanca-Riba, Iriselies Melendez-Rodriguez We investigate the role order plays in the transfer of charges in ZnO nanoparticle-8CB Liquid crystal system for photovoltaic applications. It is known that liquid crystals will align the nanoparticles in rows. We have found that in addition the nanoparticle helps align the liquid crystal when mixed in the appropriate percentage mixture in the liquid crystal. In this system, the liquid crystal acts as the hole conductor whereas the nanoparticle or nanowire acts as the electron conductor. We have used InSnO (ITO) electrodes to measure the electrical current. We have changed the percentage weight of ZnO in 8CB from 1.18{\%} to 40{\%}. We have observed that in this system a 30{\%} weight of ZnO in the liquid crystal octylcyanobiphenyl (8CB) will 1. Help to further align the liquid crystal; 2. Produce the largest change in V$_{oc}$ with respect to the V$_{oc}$ of the nanoparticle; 3. Increase the current generated. [Preview Abstract] |
Tuesday, March 16, 2010 9:00AM - 9:12AM |
H13.00006: Electro-optical properties of CdSe quantum dots dispersed in a chiral nematic liquid crystal J. Kirchhoff, R.H. Inman, S. Ghosh, L.S. Hirst The electro-optical properties of quantum dots can be significantly altered if they are assembled in close proximity to each other.~ The partial ordering of liquid crystal molecules can be utilized to form directed quantum dot assemblies.~ Typically, this results in a red shift in the emission spectrum of the dots as the induced order leads to enhanced dipolar interactions, resulting in electronically coupled states.~ Spherical cadmium selenide quantum dots of different diameters are dispersed in various concentrations in a chiral nematic liquid crystal phase.~ The quantum dots are seen to aggregate, the sizes of the aggregates depend on the size, concentration, and mixing time of the dots, and the aggregates in turn form defects in the liquid crystal texture.~ The effect of the dots on the electro-optical response of the liquid crystal is studied, with a decrease in a threshold electric field transition attributed to the quantum dots.~ Quantum dots with emission peaks ranging from 490 nm to 640 nm were studied using polarized optical microscopy, fluorescence microscopy, and scanning microscopy photoluminescence measurements. [Preview Abstract] |
Tuesday, March 16, 2010 9:12AM - 9:24AM |
H13.00007: The Study Of Charge Carrier Transport On The Calamitic Liquid Crystals `` 5, 5'-Di-(Alkyl-Pyridin-Yl) - 2' Bithiophenes'' Naresh Shakya, Chandra Pokhrel, Brett Ellman, Yulia Getmanenko , Robert Twieg The hole and electron mobilities in both types of calamitic liquid crystals C9 [5,5'-Di-(5-n-nonyl-pyridin-2-yl)-2,2'-bithiophenes] and C10 [5,5'-Di-(5-n-decyl-pyridin-2-yl)-2,2'-bithiophenes] were studied. The charge carrier mobilities were strongly electric field dependent. The mobilities decreased continuously with increase in the electric field up to a certain value, after which it became constant. Both types of charge carrier mobilities are independent of the temperature over our temperature range. The qualitative feature of our results could be tentatively explained by the Monte--Carlo modeling proposed by H Bassler. However, the results require further study for better understanding. [Preview Abstract] |
Tuesday, March 16, 2010 9:24AM - 9:36AM |
H13.00008: Bent-core alignment monolayers Wilder Iglesias, Antal Jakli, Elizabeth K. Mann Langmuir Films have been source of high expectations not just because of the opportunity that they provide to study low-dimensional fluids, but also because of the practical and potential applications that comes from the possibility to transfer these thin films into another surface, through Langmuir-Blodgett (LB) and Langmuir-Schaefer (LS) techniques. We use these transference techniques to deposit a monolayer of a bent-core liquid crystal molecule (Z2B) into different substrates, to use later as alignment layer in a liquid crystal cell, where the direction and degree of the alignment is changed by increasing and decreasing the packing of the molecules in the monolayer (dipping the substrate on a Langmuir Trough with different surface pressures). [Preview Abstract] |
Tuesday, March 16, 2010 9:36AM - 9:48AM |
H13.00009: Island Diffusion in Freely Suspended Smectic A Films: Crossover From 2D to 3D Behavior Zoom Nguyen, Markus Atkinson, Cheol Park, Joseph Maclennan, Matthew Glaser, Noel Clark Measuring the diffusion constant has been an important tool in studying the hydrodynamics of two-dimensional (2D) systems. The well-known Saffman equations predict how the diffusion of an inclusion in a 2D fluid film depends on the inclusion's size. For inclusions with radius R large compared to the characteristic Saffman length $l_S$, the fluid bounding the two dimensional system needs to be considered, rendering it effectively a 3D problem, and the diffusion constant varies as 1/R. In the 2D limit (if $R << l_S$), it varies as ln(1/R). We present here a clean two-dimensional system using freely suspended smectic A liquid crystal films that allows us to test this behavior in both regimes. By tracking the self-diffusion of single islands (thicker, circular domains embedded in the films), we are able to calculate the diffussion constant. The sensitivity of the measurement also allows us to detect the hydrodynamic interactions between islands. [Preview Abstract] |
Tuesday, March 16, 2010 9:48AM - 10:00AM |
H13.00010: Title: Liquid crystal cells with ``dirty" substrates Quan Zhang, Leo Radzihovsky We explore liquid crystal order in a cell with a ``dirty'' substrate imposing a random surface pinning. Modeling such systems by a random-field xy-model with surface heterogeneity, we find that orientational order in the three-dimensional system is marginally unstable to such surface pinning. We compute the Larkin length scale, and the corresponding surface and bulk distortions. On longer scales we calculate correlation functions using the functional renormalization group and matching methods, finding a universal logarithmic and double-logarithmic roughness in two and three dimensions, respectively. For a finite thickness cell, we explore the interplay of homogeneous-heterogeneous substrate pair and detail crossovers as a function of disorder strength and cell thickness. [Preview Abstract] |
Tuesday, March 16, 2010 10:00AM - 10:12AM |
H13.00011: Thickness dependent phase behavior of AFLC liquid crystal films Cheng-Cher Huang, LiDong Pan, Shun Wang, Chain-Shu Hsu Free standing films of a liquid crystal compound with simple surface enhanced order were studied. The resultant phase diagram demonstrates that (1) the short helical pitch smectic-C*$_{alpha}$ phase disappears below a film thickness of 10 layers, and (2) the temperature window of a distorted 4 layer smectic-C*$_{d4}$ phase increases dramatically upon decreasing film thickness. The experimental findings were attributed to the reduced dimensionality and enhanced surface effects in thin films. The results of the smectic-C*$_{alpha}$ phase are consistent with what have been reported for helically ordered magnetic thin films, with a noticeable difference due to the opposite effect of the surface on ordering in the two systems. [Preview Abstract] |
Tuesday, March 16, 2010 10:12AM - 10:24AM |
H13.00012: Colloidal crystal-liquid Interface: Anisotropy, Free energy, and Structure Duc Nguyen, Triet Dang, Bart Weber, Peter Schall Investigation of the crystal--liquid interface is of central importance for understanding crystal growth, and nucleation. This interface is the most difficult to study experimentally because it is buried between two condensed phases. We use micron-size colloidal particles as models to visualize atomic processes of crystal growth. Because of their larger length and time scales, by optical microscopy, we can observe and even track the motion of the individual particles in three dimensions and time. This provides an excellent opportunity to study the crystal-liquid interface on the ``atomic'' level. We determine all thermodynamic properties of the system: pressure, the chemical potential, and the free energy density. We use interface fluctuations to determine the interfacial tension and its dependence on the crystal orientation, important parameters of crystal growth. Remarkably, the anisotropy of the interfacial tension that we find is very similar to that measured for real metallic crystals [Preview Abstract] |
Tuesday, March 16, 2010 10:24AM - 10:36AM |
H13.00013: Cooperative Ordering at Liquid Crystal Interfaces and its Role in Orientational Memory David Patrick Orientational memory in interfacial liquid crystal films occurs when cells heated above the isotropic transition temperature return to their initial ordered texture upon cooling. First observed over 80 years ago, the origins of orientational memory, which is sometimes called the surface memory effect, remain poorly understood. In this study, films of the thermotropic liquid crystal 4'-octyl-4-cyanobiphenyl on graphite were studied by scanning tunneling and polarizing optical microscopy. Strong orientational memory was observed despite relatively weak molecule-surface interactions of the kind previously thought to be responsible for this effect. By preparing cells in a uniformly oriented initial reference state and separately measuring bulk and surface order parameters as systems were thermally disordered, cooperative interactions were found to play an important role, leading to the recovery of long-range order that neither the bulk nor surface layers alone retained. When the surface and bulk layers were partially decoupled using a magnetic field, orientational memory in the surface layer almost disappeared. The findings provide a new interpretation of the origins of orientational memory in liquid crystal films and underscore the potentially important role of cooperativity in bulk $\leftrightarrow $ interfacial liquid crystal interactions. [Preview Abstract] |
Tuesday, March 16, 2010 10:36AM - 10:48AM |
H13.00014: Effect of Alignment on the Nematic to Isotropic Phase Transitions of 8CB Dipti Sharma This study reports the effect of alignment on the nematic to isotropic phase transition of bulk octylcyanobiphenyl liquid crystal. This effect reveals a reduced time and temperature lag with possible solutions to the time response and backlight bleed drawbacks of liquid crystal displays (LCDs). The aligned octylcyanobiphenyl shows a quicker and early occurrence of the nematic transition with less deviation from thermal rates than the unaligned octylcyanobiphenyl using calorimetry technique. Smaller enthalpy of activation indicates less energy requirement and makes the aligned octylcyanobiphenyl suitable for LCDs. The results are discussed in terms of the formation of aligned domains of octylcyanobiphenyl molecules under the force of magnetic field. [Preview Abstract] |
Tuesday, March 16, 2010 10:48AM - 11:00AM |
H13.00015: Study on the surface enhanced ordering effect of liquid crystal films LiDong Pan, Shun Wang, Chain-Shu Hsu, Cheng-Cher Huang Null Transmission Ellipsometer was employed to study the surface enhanced ordering effect in one smectic liquid crystal. In the SmA temperature window of free standing films with thickness around 15 molecular layers, we studied the response of the surface arrangement as a function of temperature and transverse electric field. From the results we obtained the interaction between the two surfaces as a function of temperature and film thickness. The effective range of surface interaction is found to be around 20 molecular layers. [Preview Abstract] |
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