Session J51: Liquid Crystals: Smectic, Ferroelectric, Nanocomposites and DNA

Two-Dimensional Microfluidics: Hydrodynamic Interactions in Ultra-Thin Smectic Liquid Crystal Films

Hydrodynamics is important in nature and has a wide range of applications in science and industry. Most studies of fluid dynamics have been carried out in 3D systems, but there is an increasing interest in the hydrodynamics in confined geometries. Smectics can form ultra-thin, stable fluid films of uniform but readily variable thickness, a structure which enables the quantitative study of 2D hydrodynamics. Hydrodynamic interactions in 2D extend much further across the fluid than in 3D, and all the dynamics is confined to a well-defined plane, facilitating clean, high-contrast and high-resolution experiments. Here, we explore the hydrodynamic interactions of disk-like smectic islands with other islands and with a straight film boundary acting as a 1D wall. High speed video microscopy confirms that the translational diffusion of an island is anisotropic in the vicinity of another island or the wall, and this anisotropy persists even at large separations many times the island radius.   

Two-Dimensional Microfluidics: hydrodynamics of drops and interfaces in flowing smectic liquid crystal channels

The quantization of film thickness in freely suspended fluid smectic liquid crystal film enables the study of the hydrodynamics of drops and interfaces in 2D. We report microfluidic experiments, in which we observe the hydrodynamics of 2D drops flowing in channels. Using high-speed video microscopy, we track the shape of 2D drops and interfaces, visualizing the deterministic lateral displacement-based separation and pinched flow separation phenomena previously observed only in 3D. Finally, we demonstrate techniques for 2D drop generation and sorting, which will be used for 2D microfluidic applications.   

Smectic liquid crystal cells with a ``dirty'' substrate

I will describe our recent studies of smectic liquid crystal cells with a ``dirty'' substrate. Acting as quenched disorder, such substrate heterogeneity destabilizes long-range smectic order on the surface and in the bulk for arbitrarily weak randomness. We analyze the statistics of the corresponding distortions, their decay into the bulk, topological defects and the role of nonlinear smectic elasticity. We will discuss our predictions in the context of recent experiments on ferroelectric smectic-C liquid crystals.   

Structural Reorganization of 4 - Cyano - 4' - octyloxybiphenyl (8OCB) revealed by Fast Scanning Calorimetry

4-Cyano-4'-octyloxybiphenyl (8OCB) is a liquid crystal with a few crystalline polymorphic modifications, of which the square plate form is the most elusive. Square plate form was reported to be only solution grown at low temperature and transformed to metastable parallelepiped form immediately even at -20 OC. With the chip calorimeter, we got the smectic glass of 8OCB when it was quenched from the melt with cooling rate of 20, 000 K/s. In the subsequent reheating with rates ranged from 2,000 K/s to 7,000 K/s, we could find two melting peaks located at 310K and 320K, respectively. Under faster heating, the peak at 310K became dominating, while the peak at 320K weakening. At heating rate of 8000 K/s, there was only melting peak of 310K. If further increased the heating rate, the melting peak at 310K would become smaller again because the crystal growth was suppressed until basically invisible at heating rate of about 20,000 K/s. This work shows that the square plate form is the dominating form when grown from the smectic glass, but it starts transforming to the parallelepiped form at heating slower than 8000 K/s. At heating slower than 1000 K/s, the transformation is completed and there is no chance to capture the square plate form.   

A Biaxial Banana Liquid Crystal Phase with Short-range Layer Ordering

W623, a single-tail, bent-core molecule with a polar termination on one end and a siloxane-terminated tail on the other, exhibits a ferroelectric, orthorhombic, fluid smectic liquid crystal phase, the SmAP$_{F}$. Powder x-ray diffraction (XRD) measurements reveal an exotic structural transition on cooling from the SmAP$_{F}$ to a SmX phase, in which resolution-limited fluid smectic layering reflections give way to four much broader peaks, indicating short-range layer ordering. This behavior points to the kind of internal frustration that gives rise to our recently discovered helical nanofilament phases. We have performed two-dimensional XRD on aligned samples and discovered that one of the four peaks is from the in-plane order. Freeze-fracture transmission electron microscope (FFTEM) measurements confirm that there is two-dimensional short-range order in the SmX phase, with one periodicity in the layer plane and another normal to the layers. The in-plane periodicity can be measured directly from the packing of fibrils to be about 8 nm, consistent with the in-plane x-ray reflection peak. We will present depolarized transmission light microscopy, high-resolution XRD, and FFTEM studies of pure W623, and of mixtures of W623 with the calamitic liquid crystal 8CB.   

A Bent -- Shape Leaning Smectic Liquid Crystal Material

Liquid crystals of bent-shape molecules theoretically can form four types of fluid smectic layer structures: (a) A polar smectic phase (called SmAP) where both molecular plane and the line connecting the end of average molecules (director) are perpendicular to the layer normal; (b) A double tilted chiral polar structure; (c) A single tilted phase (SmCP) where only the molecular plane is tilted with respect to the layer normal. The fourth possibility, where the director is tilted with respect to the layer normal, the ``leaning'' SmLP phase, has never been verified experimentally. Here we present the first bent-core material that forms a SmLP structure, thus proving the reality of all theoretically predicted bent-core smectic phases.   

Effective Conductivity due to Continuous Polarization Reorientation in Fluid Ferroelectrics

In crystal ferroelectrics, the macroscopic polarization density \textbf{P} is stabilized to a set of discrete orientations by the underlying lattice, and ferroelectricity characterized by field-induced switching of \textbf{P} between these stable states. Fluid ferroelectrics exhibit \textbf{P} with no energy barriers to its reorientation. As a result, \textbf{P} can respond to applied electric field in a continuous fashion. We show here that, due to the reorientation of \textbf{P}, an otherwise insulating fluid ferroelectric behaves electrically as a resistive medium, with conductivity in the semiconducting range. Measurements of cell dynamics are reported for the SmAP$_{F}$ material W623, a bent-core liquid crystal (LC) with large macroscopic polarization that we find to exhibit nearly ideal field-induced block polarization reorientation. We have investigated theoretically the dynamic behavior of block polarization in the SmAP$_{F}$ phase, finding that a reorienting LC polarization block behaves electrically as a resistor. Experimental studies of W623 confirm this behavior, revealing the low resistance of the block-reorienting LC and the corresponding characteristic flat-topped step in the current response.   

Microscopic origins of first-order SmA-SmC phase behavior and electro optics in de Vries smectic liquid crystals

Many de Vries liquid crystals exhibit a first-order SmA-SmC phase transition. The original de Vries hollow cone model, in which molecules in the SmA phase are tilted with respect to the layer normal but are uniformly distributed in azimuthal orientation, $\phi$, has been used successfully to model many properties of de Vries materials, but the microscopic origins of first-order behavior remain obscure. We describe a microscopic mechanism for first-order behavior in de Vries smectics based on the hollow cone model, embodied in a generalized planar spin model where effective interactions between tilted molecules in the smectic layer planes are represented by a nearest-neighbor pair potential, $u(\phi_{ij})$, with a minimum of variable width around $\phi_{ij}=\phi_i-\phi_j=0$. Using mean-field theory and Monte Carlo simulation, we find that the SmA-SmC transition is second order for a relatively broad minimum in the potential, and becomes first-order as the minimum narrows. This reflects the expected behavior due to excluded volume interactions in a tilted smectic, in which increasing cone angle leads to a more steeply varying effective azimuthal potential. This model naturally explains the observed first-order behavior in the framework of a hollow cone model.   

Morphology and Dynamics of Liquid Crystalline Molecules Confined in Nano-pores

Broadband dielectric and Infrared spectroscopy are combined to study the molecular dynamics of the liquid crystalline compounds belong isothiocyanatobiphenyl homologous series (abbreviated as nBT) confined in pores of mean diameters from 4 nm to 10.5 nm. In bulk, the studied substances show only one liquid crystalline phase: the SmE phase with orthorhombic arrangement within the molecular layers. In contrast to well-known bulk dielectric properties of nBTs, confinement leads to modification of the molecular dynamics. Two relaxation processes are detected. The slower process corresponds to molecular reorientation around short axis and it is faster in pores than in bulk. The second process is attributed to a librational motion of the molecules close to the walls. Both processes exhibit an Arrhenius-type temperature dependence. Detailed analysis of the temperature dependent infrared spectra indicates the different impact of confinement on the rigid and flexible molecular units of nBTs. Transition Moment Orientational Analysis is employed to explore the orientational order of molecules in pores.   

Temperature Dependence of Smectic Liquid Crystals Mixed With Magnetic Nanoparticles

We investigate the properties of bulk liquid crystal mixed with a magnetic nanoparticle (CoFe) as a function of temperature. We compare our results to those of a heat capacity measurement of Cordoyiannis et al.\footnote{George Cordoyiannis, Lynn K. Kurihara, Luz J. Martinez-Miranda, Christ Glorieux, and Jan Thoen, Phys. Rev. E \textbf{79}, 011702 (2009)} and compare the way the smectic as a function of temperature the way the nematic behaves. We study how the liquid crystal reorganizes in the presence of the functionalized nanoparticles as a function of temperature and compare it to how it behaves at room temperature.\footnote{L. J. Mart\'inez-Miranda, and Lynn Kurihara, J. Appl. Phys, \textbf{105}, p. 084305 (2009).} The X-rays give rise to three or four peaks whose evolution in temperature varies depending on their origin. In particular the second peak does not seem to vary much with temperature, and can be associated with the first several molecular layers attached to the nanoparticles.   

The Role of ZnO Particle Size, Shape and Concentration on Liquid Crystal Order and Current-Voltage Properties for Potential Photovoltaic Applications

We investigate the role order plays in the transfer of charges in ZnO nanoparticle - octylcyanobiphenyl (8CB) liquid crystal system for photovoltaic applications as well as the role the nominally 7x5x5nm$^{3}$ or 20x5x5nm$^{3}$ ZnO nanoparticles play in improving that order. Our results for the 5nm nanoparticles show an improvement in the alignment of the liquid crystal with increasing weight percentage of ZnO nanoparticles$^{1}$. Our results for the 7x5x5 nm$^{3}$ sample show that the current is larger than the current obtained for the 5 nm samples. We find that order is improved for concentrations close to 35{\%} wt ZnO for both the 7x5x5 nm$^{3}$ and 20x5x5 nm$^{3}$. We have analyzed the X-ray scans for both the 7x5x5 and the 20x5x5 nm$^{3}$ samples. The signal corresponding to the liquid crystal aligned parallel to the substrate is much smaller than the peak corresponding to the liquid crystal aligned approximately at 70\r{ } with respect to the substrate for the 7x5x5 nm$^{3}$ sample whereas this same peak is comparable or more intense for the 20x5x5 nm$^{3}$ sample. 1. L. J. Mart\'{\i}nez-Miranda, Kaitlin M. Traister, Iriselies Mel\'{e}ndez-Rodr\'{\i}guez, and Lourdes Salamanca-Riba, Appl. Phys. Letts, \underline {97,} 223301 (2010).   

ZnO Nanowire Arrays with Liquid Crystals for Photovoltaic Applications

Liquid crystals are small monodisperse molecules with high mobilities and are easy and cheap to process. In addition, some of their phases exhibit molecular orientation that can provide a path for the electrons, or holes, to move from one electrode to the other. We have added a smectic A liquid crystal (8CB) to ZnO nanowire arrays of different diameters and have observed a photovoltaic effect as a function of the concentration of ZnO in the liquid crystal. The nanowire arrays are covered with 8CB liquid crystal for hole conduction. We have observed an increase in the light absorption of the PV cells as a function of wavelength of the light for the ZnO nanowire cells. We present a detailed study of the structure of the system.   

Ligation of Complementary Oligomers in Liquid Crystals of nanoDNA

The chromonic stacking mode of short oligomeric DNA upon forming liquid crystalline phases presents an intriguing possibility for liquid crystal autocatalysis, the promotion, by LC ordering, of chemical synthesis that stabilizes LC ordering. In such a scenario the concentration and physical organization of ligation reactants and the fluidity of the liquid crystal phase promotes the appropriate chemical ligation of oligomers. Because it is a mode of elongation free of other catalysts, this offers a tantalizing means of oligonucleotide self-elongation that might have implications in prebiotic life. We present here work toward elucidating possible catalytic enhancement by liquid crystalline phase formation. Ligation approaches based on water soluble carbodiimide base activation and photopolymerization will be presented.   

Liquid Crystal Ordering of Random DNA Oligomers

Concentrated solutions of DNA oligomers (6 to 20 base pairs) organize into chiral nematic (NEM) and columnar (COL) liquid crystal (LC) phases. When the oligomer duplexes are mixed with single strands, LC phase formation proceeds through macroscopic phase separation, as a consequence of the combination of various self-assembly processes including strand pairing, reversible linear aggregation, demixing and LC ordering. We extended our investigation to the case of LC ordering in oligonucleotides whose sequences are partially or entirely randomly chosen, and we observed LC phases even in entirely random 20mers, corresponding to a family of 4$^{20} \quad \approx $ 10$^{12}$ different sequences. We have tracked the origin of this behaviour: random sequences pair into generally defected duplexes, a large fraction of them terminating with stretches of unpaired bases (overhangs); overhangs promote linear aggregation of duplexes, with a mean strength depending on the overhang length; LC formation is accompanied by a phase separation where the duplexes with longer overhangs aggregate to form COL LC domains that coexist with an isotropic fluid rich in duplexes whose structure cannot aggregate.