A Study of Flow Alignment Instability During Rectilinear Oscillatory Shear of Nematics

Rectilinear oscillatory shearing of nematic liquid crystals in thin large-area (10 cm × 8 cm × 10 μm) cells has been studied in the range 20 to 100 Hz for two uniform initial alignments, one planar and parallel to the direction of oscillation and the other homeotropic. In the former case, for certain positive values of the viscosity coefficient α₃ a flow-alignment instability occurs above a critical amplitude A_c for the displacement of the movable upper plate, leading to formation of regular cm-wide bands of alternating 2 π and -2 π twist, running parallel to the alignment direction. The width of the bands decreases with decreasing temperature and increasing frequency, the total number present depending on the size of the cell. In homeotropic alignment, similar bands of lower twist form at an angle to the direction of oscillation, in this case both for certain positive values of α₃ and, in addition, for α₃ slightly negative. A theoretical expression for A _c in the cases with α₃ > O has been derived and substantially confirmed experimentally. The theory further predicts the existence of a remarkable “stability gap” where, although the expression for A_c is a minimum, there is no instability and formation of bands should not occur. By exploiting the divergence of α₃ at a smectic A phase, theory and experiment have been compared over a wide range of α₃ values, and the stability gap confirmed experimentally for several materials. In the case of “weak” smectic phases (i. e., showing actual or incipient re-entrant behavior) the expected divergence of A_c does not occur until significantly inside the smectic phase range. Extrapolation of 1/A² _c against temperature can be used to locate accurately in the temperature at which α₃ is zero. A variety of materials has been studied, revealing that α₃ may be positive in a rather wider range of materials than had previously been realized, and that when α₃ is positive it usually remains so over most or all of the nematic range.     

Low-temperature heat capacity of some alkali metal tungstates

Heat capacity measurements have been made on the two triclinic tungstates, Li_0.2WO₃ and Na_0.33WO₃ from 1 to 60 K. In addition to the normal Debye term the data show a large contribution which can be fit to a single Einstein mode associated with the oscillation of the alkali ions in the holes formed by the corner bonding of six WO₆ octahedra. The Einstein characteristic temperatures obtained are 71 ± 2 and 78 ± 2 K for Li_0.2WO₃ and Na_0.33WO₃, respectively. The results are compared with those reported earlier for the hexagonal tungsten bronzes.     

Low emittance, high brilliance relativistic electron beams from a laser-plasma accelerator

Progress in laser wakefield accelerators indicates their suitability as a driver of compact free-electron lasers (FELs). High brightness is defined by the normalized transverse emittance, which should be less than 1π mm mrad for an x-ray FEL. We report high-resolution measurements of the emittance of 125 MeV, monoenergetic beams from a wakefield accelerator. An emittance as low as 1.1±0.1π mm mrad is measured using a pepper-pot mask. This sets an upper limit on the emittance, which is comparable with conventional linear accelerators. A peak transverse brightness of 5×101? A m?1 rad?1 makes it suitable for compact XUV FELs.     

Regional brain activity after prolonged cholinergic enhancement in early Alzheimer's disease

This functional magnetic resonance imaging (fMRI) study examined changes in brain activation after prolonged (20 weeks) and stabilized treatment with the cholinesterase inhibitor galantamine in a small group of patients with very mild Alzheimer's disease (AD). Two cognitive activation paradigms were chosen: one requiring semantic association and the other relying on attention and requiring target detection. A group of age- and education-matched healthy controls was also scanned for comparison. A modest (but not statistically significant) improvement in behavioral scores after treatment was observed in both fMRI tasks. There were brain activation increases in the semantic association task after treatment, and the differences in brain activation present in the comparison of AD patients' baseline images with those of controls were not detectable after treatment. In the target detection task, regions that were activated in the elderly controls but not in the baseline images of the AD group also showed significant activation after treatment. Overall, however, the increases were modest and might reflect the heterogeneity of clinical response to treatment in this small group. Future pharmacological fMRI studies should include clinical response as a factor in the analysis of cholinergic enhancement effects in AD patients.     

Oxidation state of tungsten in the NaxWO3bronzes

X-ray photoemission spectra of W4f electrons in vacuum-cleaved cubic Na_xWO₃ covering the entire composition range give no evidence for the existence of multiple W valences states in the bulk. Strong oxidation effects are observed in air exposed surfaces.     

Dihedral quartic approximations and series for π

Imaginary quadratic fields with class groups that have C(4) as a subgroup are analyzed in depth, and the units of associated dihedral quartic fields are thereby evaluated using Epstein zeta functions. Emphasis is placed on extreme examples such as Q(√?3502) which is probably the last case having an even discriminant and C(4) × C(4) as its class group. These extreme examples lead to very remarkable approximations and series for π such as

$\text{π=}\text{6}\text{3502}\text{log}\text{(2π)+7.37×10}{}^{\mathrm{?82}}$

where u is the product of four, rather simple, quartic units. The approximations and series relate to Baker's theory of linear forms in logarithms and to certain modular identities. Related topics are discussed briefly.     

Assessment of MIST Alert,a commercial qualitative assay for detection of paralytic shellfish poisoning toxins in bivalve molluscs

A recently developed commercial rapid test kit (MIST Alert) was assessed for determination of the presence of paralytic shellfish poisoning (PSP) toxins in shellfish. Several commercially important shellfish species obtained from the UK shellfish toxin monitoring program, containing a range of total PSP toxicities as determined by the mouse bioassay (MBA), were tested. The kit detected toxin in all samples containing the European Community tolerance level of 80 microg saxitoxin (STX) equivalents/100 g shellfish flesh as determined by the MBA. With one exception, the kit detected toxin in all samples that contained >40 microg STX equivalents/100 g according to the MBA. Among samples in which the MBA did not detect toxin, the kit disagreed in 25% of the tests, although further analysis by liquid chromatography (LC) and MBA of some samples confirmed the presence of toxins. These results suggest that MIST Alert may be suitable as an initial screen for PSP toxins as part of routine monitoring programs, thereby greatly reducing the number of MBAs. Trials were also performed by nonscientific personnel to evaluate the ease of use and interpretation of results obtained by MIST Alert. The results indicated that the kits could be readily used and accurately interpreted by individuals with no technical or scientific background.     

On forming the Romberg table

Romberg-type extrapolation is commonly used in many areas of numerical computation. An algorithm is presented for forming the Romberg table for general step-length sequence and general powers in the asymptotic expansion. It is then shown that parameters of the algorithm can be used to gain an a priori bound on propagation of rounding errors in the table.     

Topical Astodrimer Sodium,a Non-Toxic Polyanionic Dendrimer,Demonstrates Antiviral Activity in an Experimental Ocular Adenovirus Infection Model

There is no approved antiviral therapy for adenovirus (HAdV) ocular infections. Astodrimer sodium (SPL7013) is a polyanionic dendrimer with antiviral activity. The current study evaluated the ocular tolerability and anti-adenoviral efficacy of topical SPL7013 in rabbit ocular models. In a tolerability study, rabbits were treated with 3% SPL7013, vehicle, or 0.5% cidofovir. Their eyes were graded using the Draize scale. In antiviral efficacy studies, HAdV5 inoculated eyes were treated with 3% SPL7013, vehicle, or 0.5% cidofovir. Eyes were cultured for the virus on days 0, 1, 3, 4, 5, 7, 9, 11, and 14. Viral titers were determined. There were no differences in Draize scores between 3% SPL7013 and vehicle on any day. Cidofovir produced significantly higher Draize scores on day 12 than SPL7013 and vehicle. The 3% SPL7013 and 0.5% cidofovir significantly reduced daily viral titers and positive cultures per total compared with vehicle on several different days. The 3% SPL7013 and 0.5% cidofovir significantly reduced the duration of HAdV5 shedding compared to vehicle. The 3% SPL7013 demonstrated significantly more antiviral activity compared with vehicle in the Ad5/NZW rabbit ocular model. The 3% SPL7013 induced “minimal” to “practically non-irritating” Draize scores in the ocular tolerability study. Further development of astodrimer sodium as a topical antiviral therapy for adenoviral ocular infections is indicated.