Phase Separation in the Advective Cahn–Hilliard Equation

The Cahn–Hilliard equation is a classic model of phase separation in binary mixtures that exhibits spontaneous coarsening of the phases. We study the Cahn–Hilliard equation with an imposed advection term in order to model the stirring and eventual mixing of the phases. The main result is that if the imposed advection is sufficiently mixing, then no phase separation occurs, and the solution instead converges exponentially to a homogeneous mixed state. The mixing effectiveness of the imposed drift is quantified in terms of the dissipation time of the associated advection–hyperdiffusion equation, and we produce examples of velocity fields with a small dissipation time. We also study the relationship between this quantity and the dissipation time of the standard advection–diffusion equation.

     

Charge transport in polythiophene:fullerene:nanotube bulk heterojunction photovoltaic devices investigated by impedance spectroscopy

P3HT:PCBM bulk heterojunction devices incorporating SWNTs, which are predominantly metallic in character, have been analyzed using impedance spectroscopy to understand the effect of SWNTs on their charge carrier transport properties. SWNTs reduce the effective lifetime of injected charge carriers. Frequency dependence of capacitance and conductance of P3HT:PCBM devices show monotonic variations without any clear peak positions. Simulations of the complex admittance of the P3HT:PCBM devices under trap free space charge limited current within the framework of Scher–Montrol theory are used to qualitatively show that such characteristics are a signature of charge transport which is highly dispersive in nature. The position of peak τ_peak in the imaginary part of impedance Im(Z), which is essentially same as the first transition frequency of Cole–Cole plot, has a direct relation with the effective dc mobility of charge carriers which varies with dispersion parameters. Using the dc mobility values and the voltage variation of peak frequency of Im(Z), the ratio of τ_dc to τ_peak has been calculated. The magnitude of this ratio is indicative of the degree of dispersiveness in transport. It has been shown that, SWNTs at low concentrations tend to reduce the dispersiveness in charge transport.     

Chern–Simons classes for a superconnection

In this note we define the Chern–Simons classes of a flat superconnection, D+L$D+L$, on a complex Z/2Z$\mathbb{Z}/2\mathbb{Z}$-graded vector bundle E$E$ on a manifold such that D$D$ preserves the grading and L$L$ is an odd endomorphism of E$E$. As an application, we obtain a definition of Chern–Simons classes of a (not necessarily flat) morphism between flat vector bundles on a smooth manifold. An application of Reznikov's theorem shows the triviality of these classes when the manifold is a compact Kähler manifold or a smooth complex quasi-projective variety in degrees >1$>1$.     

Unraveling the aflatoxin-FAPY conundrum: structural basis for differential replicative processing of isomeric forms of the formamidopyrimidine-type DNA adduct of aflatoxin B1

Aflatoxin B1 (AFB) epoxide forms an unstable N7 guanine adduct in DNA. The adduct undergoes base-catalyzed ring opening to give a highly persistent formamidopyrimidine (FAPY) adduct which exists as a mixture of forms. Acid hydrolysis of the FAPY adduct gives the FAPY base which exists in two separable but interconvertible forms that have been assigned by various workers as functional, positional, or conformational isomers. Recently, this structural question became important when one of the two major FAPY species in DNA was found to be potently mutagenic and the other a block to replication [Smela, M. E.; Hamm, M. L.; Henderson, P. T.; Harris, C. M.; Harris, T. M.; Essigmann, J. M. Proc. Natl. Acad. Sci. U.S.A. 2002, 99, 6655-6660]. NMR studies carried out on the AFB-FAPY bases and deoxynucleoside 3',5'-dibutyrates now establish that the separable FAPY bases and nucleosides are diastereomeric N5 formyl derivatives involving axial asymmetry around the congested pyrimidine C5-N5 bond. Anomerization of the protected beta-deoxyriboside was not observed, but in the absence of acyl protection, both anomerization and furanosyl --> pyranosyl ring expansion occurred. In oligodeoxynucleotides, two equilibrating FAPY species, separable by HPLC, are assigned as anomers. The form normally present in duplex DNA is the mutagenic species. It has previously been assigned as the beta anomer by NMR (Mao, H.; Deng, Z. W.; Wang, F.; Harris, T. M.; Stone, M. P. Biochemistry 1998, 37, 4374-4387). In single-stranded environments the dominant species is the beta anomer; it is a block to replication.     

A cationic tetrahedral chromophore for amplified DNA detection

The tetrahedral cationic chromophore, tetrakis [4-(9,9-bis(6′-(N,N,N-trimethylammonium)hexyl)-2-fluorenyl)phenyl]methane (1) shows better fluorescence resonance energy transfer (FRET) to the fluorescein (Fl) attached to the 5′-terminus of double-stranded DNA (dsDNA-Fl) as compared to the linear oligomers 2 and 3 and also provides efficient DNA hybridization detection.     

Effects of target-masker contextual similarity on the multimasker penalty in a three-talker diotic listening task

In many multitalker listening tasks, the degradation in performance that occurs when the number of interfering talkers increases from one to two is much larger than would be predicted from the corresponding decrease in the signal-to-noise ratio (SNR). In this experiment, a variety of contextually-relevant speech maskers, contextually-irrelevant speech maskers and non-speech maskers were used to examine the impact that the characteristics of the interfering sound sources have on the magnitude of this "multimasker penalty." The results show that a significant multimasker penalty only occurred in cases where two specific conditions were met: 1) the stimulus contained at least one contextually-relevant masker that could be confused with the target; and 2) the signal-to-noise ratio of the target relative to the combined masker stimulus was less than 0 dB. Remarkably, in cases where one masker was contextually relevant, the specific characteristics of the second masker had virtually no impact on the size of the multimasker penalty. Indeed, when the results were corrected for random guessing, there was essentially no difference in performance between conditions with three contextually-relevant talkers and those with two contextually-relevant talkers and one irrelevant talker. The results of a second experiment suggest that the listeners are generally able to hear keywords spoken by all three talkers even in situations where the multimasker penalty occurs, implying that the primary cause of the penalty is a degradation in the listener's ability to use prosodic cues and voice characteristics to link together words spoken at different points in the target phrase.     

Rate dependence of human visual cortical response due to brief stimulation: an event-related fMRI study

The human brain response to a wide range of visual stimulus rates presented over a prolonged time period has been investigated by various neuroimaging techniques. However, to date, no imaging study has been performed to study the dynamic human brain response to various stimulus rates when presented in a short time. This report describes activation in the human brain due to brief visual stimulus presentation (1 s) for stimulus rates varying from 1 to 20 Hz using event-related functional MRI (fMRI). Our results show that the amplitude of the fMRI response increases with the stimulus frequency and plateaus at 6 Hz. This finding differs slightly from the results of previous blocked task paradigm experiments (with a longer time of stimulus presentation), in which the response peaks at approximately 8 Hz and then decreases. Our results are in close agreement with previously published psychophysical studies, suggesting that the fMRI signal in this experiment is indicative of cortical activity related to visual processing.     

Electromechanical response of (3–0, 3–1) particulate,fibrous, and porous piezoelectric composites with anisotropic constituents: A model based on the homogenization method

An analytical framework based on the homogenization method has been developed to predict the effective electromechanical properties of periodic, particulate and porous, piezoelectric composites with anisotropic constituents. Expressions are provided for the effective moduli tensors of n-phase composites based on the respective strain and electric field concentration tensors. By taking into account the shape and distribution of the inclusion and by invoking a simple numerical procedure, solutions for the electromechanical properties of a general anisotropic inclusion in an anisotropic matrix are obtained. While analytical forms are provided for predicting the electroelastic moduli of composites with spherical and cylindrical inclusions, numerical evaluation of integrals over the composite microstructure is required in order to obtain the corresponding expressions for a general ellipsoidal particle in a piezoelectric matrix. The electroelastic moduli of piezoelectric composites predicted by the analytical model developed in the present study demonstrate excellent agreement with results obtained from three-dimensional finite-element models for several piezoelectric systems that exhibit varying degrees of elastic anisotropy.     

Flotation-spectrophotometric method for the determination of mercury using iodide and brilliant green

This paper describes a sensitive spectrophotometric method for the determination of mercury, based on the flotation of a mercury iodide-brilliant green complex with cyclohexane at pH 5 and dissolution of the complex in acetone. The absorbance of the complex is measured at 625 nm. Beer's law is obeyed from 4–500 g/l of mercury. The method is highly sensitive ( = 5.96 × 10⁵ 1 mol^–1 cm^–1) and selective, as it is free from interferences by almost all cations and anions in the presence of EDTA. Furthermore, a two-level orthogonal array design as a chemometric method has been used for optimization of the various parameters involved. The method has been successfully applied to paper industry effluents.