Ring-Pattern Dynamics in Smectic-C* and Smectic-C_A* Freely Suspended Liquid Crystal Films

Ring patterns of concentric 2pi-solitons in molecular orientation, form in freely suspended chiral smectic-C films in response to an in-plane rotating electric field. We present measurements of the zero-field relaxation of ring patterns and of the driven dynamics of ring formation under conditions of synchronous winding, and a simple model which enables their quantitative description in low polarization DOBAMBC. In smectic C_A* TFMHPOBC we observe an odd-even layer number effect, with odd number layer films exhibiting order of magnitude slower relaxation rates than even layer films. We show that this rate difference is due to much larger spontaneous polarization in odd number layer films.Comment: 4 RevTeX pgs, 4 eps figures, submitted to Phys. Rev. Let

Impact of permeability evolution in igneous sills on hydrothermal flow and hydrocarbon transport in volcanic sedimentary basins

Sills emplaced in organic-rich sedimentary rocks trigger the generation and migration of hydrocarbons in volcanic sedimentary basins. Based on seismic and geological observations, numerical modeling studies of hydrothermal flow around sills show that thermogenic methane is channeled below the intrusion towards its tip, where hydrothermal vents nucleate and transport methane to the surface. However, these models typically assume impermeable sills and ignore potential effects of permeability evolution in cooling sills, e.g., due to fracturing. Here, we combine a geological field study of a volcanic basin (Neuquén Basin, Argentina) with a hybrid finite-element–finite-volume method (FEM–FVM) of numerical modeling of hydrothermal flow around a sill, including hydrocarbon generation and transport. Our field observations show widespread veins within sills composed of graphitized bitumen and cooling joints filled with solid bitumen or fluidized shale. Raman spectroscopy indicates graphitization at temperatures between 350 and 500 ∘C, suggesting fluid flow within the intrusions during cooling. This finding motivates our modeling setup, which investigates flow patterns around and through intrusions that become porous and permeable upon solidification. The results show three flow phases affecting the transport of hydrocarbons generated in the contact aureole: (1) contact-parallel flow toward the sill tip prior to solidification, (2) upon complete solidification, sudden vertical “flushing” of overpressured hydrocarbon-rich fluids from the lower contact aureole towards and into the hot sill along its entire length, and (3) stabilization of hydrocarbon distribution and fading hydrothermal flow. In low-permeability host rocks, hydraulic fracturing facilitates flow and hydrocarbon migration toward the sill by temporarily elevating porosity and permeability. Up to 7.5 % of the generated methane is exposed to temperatures &gt;400 ∘C in the simulations and may thus be permanently stored as graphite in or near the sill. Porosity and permeability creation within cooling sills may impact hydrothermal flow, hydrocarbon transport, and venting in volcanic basins, as it considerably alters the fluid pressure configuration, provides vertical flow paths, and helps to dissipate overpressure below the sills.</p

Distinct degassing pulses during magma invasion in the stratified Karoo Basin – New insights from hydrothermal fluid flow modelling

Magma emplacement in organic‐rich sedimentary basins is a main driver of past environmental crises. Using a 2D numerical model, we investigate the process of thermal cracking in contact aureoles of cooling sills and subsequent transport and emission of thermogenic methane by hydrothermal fluids. Our model includes a Mohr‐Coulomb failure criterion to initiate hydrofracturing and a dynamic porosity/permeability. We investigate the Karoo Basin, taking into account host‐rock material properties from borehole data, realistic total organic carbon content, and different sill geometries. Consistent with geological observations, we find that thermal plumes quickly rise at the edges of saucer‐shaped sills, guided along vertically fractured high permeability pathways. Contrastingly, less focused and slower plumes rise from the edges and the central part of flat‐lying sills. Using a novel upscaling method based on sill‐to‐sediment ratio we find that degassing of the Karoo Basin occurred in two distinct phases during magma invasion. Rapid degassing triggered by sills emplaced within the top 1.5 km emitted ~1.6·103 Gt of thermogenic methane, while thermal plumes originating from deeper sills, carrying a 12‐times greater mass of methane, may not reach the surface. We suggest that these large quantities of methane could be re‐mobilized by the heat provided by neighboring sills. We conclude that the Karoo LIP may have emitted as much as ~22.3·103 Gt of thermogenic methane in the half million years of magmatic activity, with emissions up to 3 Gt/year. This quantity of methane and the emission rates can explain the negative δ13C excursion of the Toarcian environmental crisis. Key Points Sill geometry and emplacement depth as well as intruded host rock type are the main factors controlling methane mobilization and degassing Dehydration‐related porosity increase and pore‐pressure‐induced hydrofracturing are important mechanisms for a quick transport of methane from sill to the surface The Karoo Basin may have degassed ~22.3·103 Gt of thermogenic methane in the half million years of magmatic activit

Experimental investigations of a new TGBc mesophase

We report on optical and structural X-ray studies on a smectic C twist grain boundary phase (TGBC) of a pure liquid crystalline material. It is shown that this TGBC phase, which exists over a large range of temperature, is definitely different from previously reported and predicted TGBC phases. The two main experimental features are: (i) the observation in planar geometry of an optical texture exhibiting a square grid pattern, and (ii) the detection of a broad Bragg ring in reciprocal space instead of one (TGBA) or two (TGBC) sharp rings. We suggest a few possible tracks that can be explored to understand the complex structure of this new phase

Bending and shaping: cubics, calamitics and columnars

The mesomorphism of a series of complexes of Pd(II), Pt(II) and Ag(I) is discussed and systematic structural variations are highlighted which lead to an appreciation of important factors determining the liquid-crystalline polymorphism of these complexes. Models are proposed for cubic phase formation and the occurrence of an unusual lamellar phase in-between a SmC and a columnar phase is discussed

Columnar and smectic ordering in a TGBA_{\rm A} phase

Optical textures characteristic of cholesteric and columnar structures were simultaneously detected for the first time in the same mesophase of a liquid crystal. However, X-ray diffraction measurements show that the mesophase under consideration is smectic A in nature. We show here that the compound presents, indeed, a twist grain boundary (TGB) phase within a large temperature range, and we propose an explanation of the supramolecular organization in this phase

Nitronyl nitroxide and imino nitroxide mono- and biradicals in Langmuir and Langmuir-Blodgett films

In an effort to prepare high-spin-ordered layers, several conjugated molecules bearing one nitronyl nitroxide radical (NN) or two imino nitroxide magnetically coupled radicals (IN) have been synthesized. The monoradical and the biradical molecules proved to be amphiphilic enough to form Langmuir films on a water surface. For the two monoradicals, the molecular area extrapolated at zero-surface pressure in the final part of the isotherms is nearly the same (A0 ∼ 0.55 nm2 molecule-1) and close to what may be expected for such molecular shapes. For the longest rigid-rod biradical, bis imino nitroxide pentamer, 5p(bisIN), the pressure-area isotherm shows a low compressibility and Brewster angle microscopy has revealed that the film is solidlike, the molecules having a tendency to aggregate, forming plates on the water surface. The bis imino nitroxide trimer, 3p(bisIN), has a larger final molecular area A0 ∼ 0.80 nm2 molecule-1, the molecules being, by then, oriented perpendicular to the surface as deduced from grazing incidence X-ray analysis (GIXA) and surface potential measurements. After deposition of monolayers on various hydrophilic substrates by the Langmuir-Blodgett (LB) technique, the biradical 3p(bisIN) happens to be the most interesting molecule since good transfer ratios (TR) are easily obtained and electron paramagnetic resonance (EPR) measurements indicate paramagnetic properties. In addition, atomic-force microscopy (AFM) imaging of the films shows defects in the form of elevated zones. The AFM reveals that these zones are partially made of molecules piling up, their heights being multiples of single-molecule lengths. These elevated domains occur in different shapes, either randomly distributed circular zones or aligned, coalesced domains lying along preferred directions. Y-type multilayers up to nine layers were also transferred on hydrophilic glass, with a somewhat poorer TR