Short Separation Research Articles

Fission of Entangled Spins: An Electronic Structure Perspective

Electronic structure aspects of singlet fission process are discussed. Correlated adiabatic wave functions of the bright singlet and dark multiexciton states of tetracene and pentacene dimers are analyzed in terms of their character (excitonic, charge-resonance, multiexciton). At short interfragment separation (3.5–4 A), both multiexcitonic and singly excited singlet states have noticeable charge-resonance contributions that fall off quickly at longer distances. Nonadiabatic couplings between the states are discussed. The limitations of diabatic framework in the context of singlet fission are explained. Based on the Cauchy–Schwarz inequality, we propose using the norm of one-particle transition density matrix, ∥γ∥, as a proxy for couplings. The analysis of ∥γ∥ and state characters reveals that the couplings between the multiexciton and singly excited states depend strongly on the weights of charge-resonance configurations in these states. To characterize energetics relevant to triplets separation step, we...

Determination of the structure of liquids: an asymptotic approach

Accurate determination of a liquid structure, especially at high temperatures, remains challenging, as reflected in the scatter between different measurements. The experimental challenge is compounded by the process of the numerical transformation from the structure factor to the radial distribution function. The resulting uncertainty is often greater than that required to resolve issues associated with changes in the short-range order of the liquid, such as the existence of liquid–liquid phase transitions or correlations between thermophysical properties and structure. In the present contribution it is demonstrated for liquid bismuth as a model system that the structure factor can be obtained to high accuracy, by comparing several independent measurements in different setups. A simple method is proposed for improving the accuracy of the radial distribution functions, based on the extension of the finite range of momentum transfer,q, in the measured data by analytical asymptotic expressions. A unified mathematical formalism for the asymptotic dependence of the structure factor is developed and the asymptotic form of the Percus–Yevick hard-sphere solution is obtained as a special limiting case. The multiple expressions in the literature are shown to reflect uncertainty in the nature of the repulsive interatomic interaction at short separation distances. Applying this asymptotic method, it is shown that it enables access to details of the fine structure of the liquid and its temperature dependence.

Effective potentials for representing polymers in melts as chains of interacting soft particles

This paper outlines the derivation of an analytical pair potential in a coarse grained description of polymer melts where each chain is represented as a collection of soft spheres. Each particle is located at the center of mass of a polymer subchain, while the polymer is divided into an arbitrary number of identical chain subsections, each comprised of a large number of monomers. It is demonstrated that the soft effective pair potentials acting between these center-of-mass sites is described by a soft repulsive region at separation distances less than the average size of each coarse grained unit and a long repulsive tail, with a small attractive component. The attractive component is located at a length scale beyond the size of the coarse grained unit and its form varies with the level of interpenetration between the coarse-grained units. Consistent with numerically derived potentials, it is found that the short range features of the potential dominate the liquid structure, while the long-tail features dominate the virial-route thermodynamics of the system. It follows that the accurate determination of the effective potential in both short and large separation distances is relevant for ensuring structural and thermodynamic consistency in the coarse-grained description of the macromolecular liquid. It is further shown that due to the sensitivity of thermodynamic properties to the large-scale features of the potential, which are irrelevant to the reproducibility of structural correlations, the determination of thermodynamically accurate potentials by numerical optimization of structure alone is not a reliable strategy in the high-density regime for high levels of coarse-graining.

Rapid speciation analysis of mercury in seawater and marine fish by cation exchange chromatography hyphenated with inductively coupled plasma mass spectrometry

In this work, a hybrid method for the rapid speciation of mercury compounds by cation exchange chromatographic separation and inductively coupled plasma mass spectrometry (ICP-MS) detection is reported. Effective separation of inorganic mercury (Hg2+), methylmercury (MeHg), ethylmercury (EtHg) and phenylmercury (PhHg) within 2–2.5min was achieved on two consecutive 12.5-mm strong cation exchange guard columns with 2.0mM l-cysteine or thiourea (pH 2.0) as the mobile phase. This separation met the requirements of green analytical chemistry such as the prevention of toxic waste, safer HPLC mobile phases, and short separation times to reduce operating costs. The detection limits for Hg2+, MeHg, EtHg and PhHg were 0.019, 0.027, 0.031 and 0.022μgL−1, each, and the repeatabilities of peak height and peak area (5.0μgL−1 for each Hg species) were all lower than 3%. Contents of Hg species and total mercury in certified reference materials of seawater (GBW(E) 080042) and fish tissue (GBW 10029) were in good accordance with the certified values, and satisfactory recoveries (96–102% for GBW(E) 080042 and 94–101% for GBW 10029) validated the developed method. The developed method was applied for the speciation of mercury in five seawater sample and five marine fish samples. The concentrations of mercury species in all analyzed fish samples did not exceed the permissible levels of the National Standard of China.

Quantum reflection by the Casimir–Polder potential: a three-parameter model

We investigate the quantum reflection governed by the Casimir–Polder potential which scales as −1/r3 at short separation and as −1/r4 at long separation. Using the available analytical solutions for the −1/r4 potential and quantum defect theory, we develop a three-parameter model for quantum reflection. The three parameters characterizing the potential are essentially constant near the threshold. The application scope of the model is examined. For a potential dominated by the −1/r4 behaviour, which is the case in many realistic atom–surface systems, our model gives accurate results over the energy range in which the quantum reflection is significant. Moreover, this parameterized model allows one to fit the experimental data efficiently.

Aggressive and reproductive behaviors following neonatal maternal separation in male Sprague-Dawley rats

To assess effects of maternal separation on later aggressive and reproductive behaviors, male Sprague-Dawley pups were separated from their mothers every other day for 8 h (long-term separation; LTS), 4 h (short term separation; STS), or 0 h (no separation; NS) from post-natal day 2 through 20. Maternal behaviors, including lactation performance, nest building, grouping, and hovering over pups were measured during the separation period. At maturity, aggressive behavior of male subjects was assessed. Subjects were also mated to novel females and reproductive outcomes measured. It was hypothesized that male pups periodically separated from their mothers would demonstrate increased aggression and decreased reproductive success as adults. Contrary to expectations, maternal separation did not affect later adult aggressive behavior or reproductive outcomes. Additional studies examining the role of the neonatal environment in shaping subsequent male behavior are needed.

Electronic excitation and structural relaxation of the adenine dinucleotide in gas phase and solution

The excited states and potential surfaces of the adenine dinucleotide are analyzed in gas phase and in solution using a correlated ab initio methodology in a QM/MM framework. In agreement with previous studies, a rather flat S1 surface with a number of minima of different character is found. Specifically, our results suggest that exciplexes with remarkably short intermolecular separation down to ~2.0 Å are formed. A detailed analysis shows that due to strong orbital interactions their character differs significantly from any states present in the Franck-Condon region. The lowest S1 energy minimum is a ππ* exciplex with only a small amount of charge transfer. It possesses appreciable oscillator strength with a polarization almost perpendicular to the planes of the two adenine molecules.

MICRO-SCALE METHOD FOR SEPARATION AND QUANTIFICATION OF ATENOLOL AND HYDROCHLOROTHIAZIDE BY SEQUENTIAL INJECTION CHROMATOGRAPHY

A microscale stopped-flow separation manifold, which is termed sequential injection chromatograph (SIC), was exploited for separating and quantifying atenolol and hydrochlorothiazide. A short C18 separation column (4.6 × 25 mm) with a monolithic structure was used to offer a rapid and sufficient separation. Microscale fiber optic spectrometric devices were coupled with the SIC manifold for UV detection at 220 nm. The proposed SIC method was optimized, validated, and applied to both formulations single and binary mixture. The mobile phase composition (MPC) was optimized using the 23 full-factorial design approach while other conditions were optimized using the univariate approach. The optimum MPC was 40 µmol/L phosphate:methanol:acetonitrile (85:7.5:7.5, v/v/v) at pH 3.0. Other optimum conditions were 40 µL/s for flow rate and 40 µL for sample volume. Satisfactory separation with a resolution of 1.6 and numbers of theoretical plates of more than 630 was achieved. The interesting features of the proposed SIC method are the microliter-scale of the consumed volume of mobile phase (3.50 mL) and high sample frequency (23 samples/hr).

Study of Stall Development Around an Airfoil by Means of High Fidelity Large Eddy Simulation

Large Eddy Simulation of the bubble bursting process over a NACA-0012 airfoil at \(Re_{c}=10^{5}\) indicates that the flow at a fixed angle of attack below the critical stall value exhibits a short (with respect to Gaster’s criteria, Gaster, Number CP-4 in AGARD, 1966) Laminar Separation Bubble (LSB) at the leading edge of the airfoil. The airfoil is smoothly pitched-up through the static stall angle to reproduce the bursting process of the short LSB that initiates a leading edge stall typical of low Reynolds number airfoil. The temporal evolution of characteristic length scales is monitored during the transient flow. Particular attention is paid to the characteristic time involved during the growth and bursting of the LSB. A recent empirical bursting criterion is used to analyse the LES results.

Monte Carlo Simulations of Parallel Charged Platelets as an Approach to Tactoid Formation in Clay

The free energy of interaction between parallel charged platelets with divalent counterions has been calculated using Monte Carlo simulations to investigate the electrostatic effects on aggregation. The platelets are primarily intended to represent clay particles. With divalent counterions, the free energy for two platelets or two tactoids (clusters of parallel platelets) shows a minimum at a short separation due to the attraction caused by ion-ion correlations. In a salt-free system, the free energy of interaction has a long-range repulsive tail beyond the minimum. The repulsion increases for tactoids with larger aggregation numbers, whereas the depth of the free-energy minimum is gradually reduced. For large enough aggregation numbers, the repulsion is dominating and the minimum is no longer a global free-energy minimum. This is an effect of the depletion of counterions free in solution (outside tactoids) as counterions and platelets aggregate into tactoids and the resulting redistribution of counterions in the system changes the effective interactions between platelets and tactoids. The difference in tactoid-tactoid interactions as a function of aggregation number can be removed by adding enough salt to mask the depletion. Adding salt also reduces the repulsive tail of the free energy of interaction and enhances the minimum. No dependence on the aggregation number suggests that an isodesmic model with a monotonically decaying distribution of aggregation numbers can be used to describe a clay system. This may help to explain the experimental observations of low average numbers of platelets in tactoids, although factors not included in the simulation model may also play an important role.

Synthesis and Characterization of Fluorescence Resonance Energy Transfer-Based Nanoprobes by Coating CdTe QDs with Rhodamine B in Gelatin Nanoparticles

A FRET-based nanoprobe consisting of CdTe quantum dot as donor and rhodamine B as acceptor embedded in gelatin nanoparticles was constructed and the photophysics of the system was characterized. The FRET-based sensors were led to at fixed donor/acceptor distance by synthesizing via a two-step desolvation way. This approach allowed for short acceptor-donor separation and thus for high energy transfer efficiencies. The morphology of the nanoprobes was presented spherically and relatively uniform with a diameter of about 100 nm, which was characterized by transmission electron microscope. The luminescent property was proved to be stable in vitro and in living cells, indicating that the nanoprobes can be expected to be promising candidates for biological imaging studies.

DEVELOPMENT AND VALIDATION OF RAPID STABILITY INDICATING HPLC-DETERMINATIONS OF ANTIEPILEPTIC DRUGS PHENOBARBITAL IN SUPPOSITORIES AND PHENYTOIN IN CAPSULES AS WELL AS IN URINE SAMPLE

Two simple, rapid, accurate, and stability indicating HPLC methods are described for quantitation of Phenobarbital and Phenytoin in bulk powders, dosage forms, or urine. Chromatographic quantitation of Phenobarbital was developed on Chromolith® Performance RP-18e column, using isocratic binary mobile phase of MeOH and H2O (38: 62, V/V) at flow rate of 3 mLmin−1. Determination of Phenytoin was achieved using a conventional RP C-18, applying isocratic binary mobile phase of ACN and H2O (25: 75, V/V) at flow rate of 1 mL min−1. The elution times of Phenobarbital and Phenytoin are 1.397 ± 0.039 and 5.604 ± 0.013 min, respectively. Each method was validated for system suitability, linearity, precision, limits of detection and quantitation, specificity, stability, and robustness. Stability tests were done through exposure of the analyte solutions to four different stress conditions: reflux with 1 N HCl, reflux with 1 N NaOH, reflux with 30% H2O2, and exposure to UV radiation. Limits of detection and quantitation were 0.125 and 0.250 µgmL−1 as well as 0.250 and 0.500 µgmL−1 for Phenobarbital and Phenytoin, respectively. Due to the short separation time of Phenobarbital, the method was applied for dissolution study in presence of beta-cyclodextrin. The proposed methods can be used for routine samples or stability studies.

Cyclonic separation process intensification oil removal based on microbubble flotation

The cyclonic-static microbubble flotation column has dual effects including the cyclonic separation and floatation separation with the characteristics of the small lower limit of the effective separation size, short separation time, large handling capacity, and low operation cost. It shows significant advantages in the oily wastewater treatment field, especially the polymer flooding oily wastewater treatment aspect. In this paper, the cyclonic separation function mechanism of the cyclonic-static microbubble flotation column was studied, the impact of the parameters including the feeding rate, aeration rate, circulating pressure, and underflow split ratio on the cyclonic separation efficiency was investigated, and the cyclonic separation efficiency model was established as well. In addition, by applying the Doppler Laser Velocimeter (LDV) and Fluent simulation software, the test and simulation to the single-phase flow velocity field of the cyclonic separation section of the cyclonic-static microbubble flotation column were carried out, and the velocity distribution rule of the cyclonic separation section was analyzed under the single-phase flow conditions.

Kinematic and dynamic collision statistics of cloud droplets from high-resolution simulations

We study the dynamic and kinematic collision statistics of cloud droplets for a range of flow Taylor microscale Reynolds numbers (up to 500), using a highly scalable hybrid direct numerical simulation approach. Accurate results of radial relative velocity (RRV) and radial distribution function (RDF) at contact have been obtained by taking advantage of their power-law scaling at short separation distances. Three specific but inter-related questions have been addressed in a systematic manner for geometric collisions of same-size droplets (of radius from 10 to 60 μm) in a typical cloud turbulence (dissipation rate at 400 cm2 s−3). Firstly, both deterministic and stochastic forcing schemes were employed to test the sensitivity of the simulation results on the large-scale driving mechanism. We found that, in general, the results are quantitatively similar, with the deterministic forcing giving a slightly larger RDF and collision kernel. This difference, however, is negligible for droplets of radius less than 30 μm. Secondly, we have shown that the dependence of pair statistics on the flow Reynolds number Rλ or larger scale fluid motion is of secondary importance, with a tendency for this effect to saturate at high enough Rλ leading to Rλ-independent results. Both DNS results and theoretical arguments show that the saturation happens at a smaller Rλ for smaller droplets. Finally, since most previous studies of turbulent collision of inertial particles concerned non-sedimenting particles, we have specifically addressed the role of gravity on collision statistics, by simultaneously simulating collision statistics with and without gravity. It is shown that the collision statistics is not affected by gravity when a < ac, where the critical droplet radius ac is found to be around 30 μm for the RRV, and around 20 μm for the RDF. For larger droplets, gravity alters the particle–eddy interaction time and significantly reduces the RRV. The effect of gravity on the RDF is rather complex: gravity reduces the RDF for intermediate-sized droplets but enhances the RDF for larger droplets. In addition, we have studied the scaling exponents of both RDF and RRV, and found that gravity modifies the RDF scaling exponents for both intermediate and large particles, in a manner very similar to the effect of gravity on the RDF at contact. Gravity is shown to cause the scaling exponents for RDF and RRV to level off for large droplets, in contrast to diminishing exponents for non-sedimenting particles.

Further improvement in reducing superficial contamination in NIRS using double short separation measurements

Near-Infrared Spectroscopy (NIRS) allows the recovery of the evoked hemodynamic response to brain activation. In adult human populations, the NIRS signal is strongly contaminated by systemic interference occurring in the superficial layers of the head. An approach to overcome this difficulty is to use additional NIRS measurements with short optode separations to measure the systemic hemodynamic fluctuations occurring in the superficial layers. These measurements can then be used as regressors in the post-experiment analysis to remove the systemic contamination and isolate the brain signal. In our previous work, we showed that the systemic interference measured in NIRS is heterogeneous across the surface of the scalp. As a consequence, the short separation measurement used in the regression procedure must be located close to the standard NIRS channel from which the evoked hemodynamic response of the brain is to be recovered. Here, we demonstrate that using two short separation measurements, one at the source optode and one at the detector optode, further increases the performance of the short separation regression method compared to using a single short separation measurement. While a single short separation channel produces an average reduction in noise of 33% for HbO, using a short separation channel at both source and detector reduces noise by 59% compared to the standard method using a general linear model (GLM) without short separation. For HbR, noise reduction of 3% is achieved using a single short separation and this number goes to 47% when two short separations are used. Our work emphasizes the importance of integrating short separation measurements both at the source and at the detector optode of the standard channels from which the hemodynamic response is to be recovered. While the implementation of short separation sources presents some difficulties experimentally, the improvement in noise reduction is significant enough to justify the practical challenges.

Early Life Stress and Post-Weaning High Fat Diet Alter Tyrosine Hydroxylase Regulation and AT1 Receptor Expression in the Adrenal Gland in a Sex Dependent Manner

Previous studies have shown that early life stress induced by maternal separation or non-handling can lead to behavioural deficits in rats and that these deficits can be alleviated by providing palatable cafeteria high-fat diet (HFD). In these studies we investigated the effects of maternal separation or non-handling and HFD on tyrosine hydroxylase (TH) protein and TH phosphorylation at Ser40 (pSer40TH) and the expression of angiotensin II receptor type 1 (AT1R) protein in the adrenal gland as markers of sympatho-adrenomedullary activation. After littering, Sprague-Dawley rats were assigned to short maternal separation, S15 (15 min), prolonged maternal separation, S180 (180 min) daily from postnatal days 2-14 or were non-handled (NH) until weaning. Siblings were exposed to HFD or chow from day 21 until 19 weeks when adrenals were harvested. Maternal separation and non-handling had no effects on adrenal TH protein in both sexes. We found an effect of HFD only in the females; HFD significantly increased TH levels in NH rats and pSer40TH in S180 rats (relative to corresponding chow-fed groups), but had no effect on AT1R expression in any group. In contrast, in male rats HFD had no effect on TH protein levels, but significantly increased pSer40TH across all treatment groups. There was no effect of HFD on AT1R expression in male rats; however, maternal separation (for 15 or 180 min) caused significant increases in AT1R expression (relative to NH group regardless of diet). This is the first study to report that early life stress and diet modulate TH protein, pSer40TH and AT1R protein levels in the adrenal gland in a sex dependent manner. These results are interpreted in respect to the potential adverse effects that these changes in the adrenal gland may have in males and females in adult life.

Sensitivity of delta-P1 approximation model to the reduced scattering parameter

For spatial-resolved diffuse reflectance, the sensitivity expression to the reduced scattering parameter μs' is derived and numerically analyzed in the two-point-source delta-P1 approximation of transport theory, and the influences of the scattering parameter μs' on the delta-P1 approximation reflectance, hybrid diffuse approximation reflectance, and diffusion approximation reflectance are also compared. The results show that the optical properties of biological tissue can be better described by the delta-P1 approxiamate model in relatively high scattering and absorbing medium for a short souce-detector separation; the optimum probe spacing ρopt is obtained and approximately equal to 2.7-4 transport mean free path for the reduced albedo a'&gt;0.83, at which a variable reduced scattering coefficient has the smallest effect on the measurement of optical absorption in the turbid medium and ρopt reduces with the scattering parameter μs' increasing. The study is of great significance for optimizing the probe geometry and measuring the optical parameters of biological tissues.

Window Exchange Umbrella Sampling to Enhance Conformational Sampling and Quantify Energetics in Transmembrane Helix Assembly

The method of window exchange umbrella sampling molecular dynamics (WEUSMD) with a pre-optimized parameter set was recently used to obtain the most probable conformations and the energetics of transmembrane (TM) helix assembly of a generic TM sequence. When applied to glycophorin A TM domain (GpA-TM) using the window potentials along the helix-helix distance, however, tight interfacial packing of GpA-TM resulted in insufficient conformational sampling at short helix-helix separation. To address this sampling issue, we approached in two directions: (i) Extension of WEUSMD into two dimensions with the restraint potentials along the rHH and crossing angle (Ω) to bypass barriers for hidden variables; (ii) Design of another scheme for the window spacing rather than a uniform one to control the average acceptance probability (Pa) between neighboring windows systematically. The two-dimensional WEUSMD results demonstrate that the incomplete sampling in the one-dimensional WEUSMD arises from high barriers along the crossing angle between the GpA-TM helices. In principle, the multi-dimensional WEUSMD is suitable for modeling TM helix assembly. However, for three or higher dimensions, this approach becomes prohibitively intensive. To avoid this issue, one may consider the latter approach, the variable window spacing for WEUSMD, where the (highest) restraint force constant for the window at the shortest rHH geometrically decreases with rHH and the first passage time optimized parameter set is estimated by using an analytic approximation of Pa in general case (different window force constants for the exchange pairs). To demonstrate the improved sampling power of WEUSMD that in turn provides trustworthy potential of mean force, we applied WEUSMD with variable window spacing for the assembly of GpA-TM, and indeed obtained the improved sampling power compared to that with uniform window spacing.

Asymptotic description of incipient separation bubble bursting

AbstractThe appearance of short laminar separation bubbles in high Reynolds number (Re) wall bounded flows due to appropriate adverse pressure gradient conditions is usually associated with minor effects on global flow properties (e.g. lift force). However, localized reverse flow regions are known to react very sensitively to perturbations and in further consequence may trigger the laminar‐turbulent transition process or even cause global separation. The present investigation of marginally separated boundary layer flows is based on an asymptotic approach Re → ∞. Special emphasis is placed on solutions of the corresponding model equations which blow up within finite time indicating the ejection of a vortical structure and the emergence of shorter spatio‐temporal scales reminiscent of the early transition scenario (‘ bubble bursting’ ). Within the framework of marginal separation theory, an alternative adjoint operator method is used to formulate evolution equations governing the viscous‐inviscid interaction process in leading and higher order correction required for the study of later stages of the flow development. Their blow up structure specifies the initial condition of and the match to the subsequent triple deck stage. (© 2012 Wiley‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Cauchy Problems and Breakdown in the Theory of Marginally Separated Flows

AbstractFlow separation can be viewed as one of the trigger events of laminar‐turbulent boundary layer transition. Thus, intensive investigations have been carried out as to when and how laminar boundary layers break down. In the special case of marginal separation, i.e. the formation of short laminar separation bubbles, high Reynolds number asymptotic theory yields integro‐differential equations governing the essential flow behavior. We will present further considerations of associated initial value problems (IVPs) in planar and three‐dimensional flow cases regarding the general ill‐posedness. As for regularization methods, higher order asymptotic terms comprising the streamline curvature in the boundary layer region are taken into account rendering the hence modified problem well‐posed, which can be seen from the dispersion relation and numerical solutions of the full problem. Even though the regularized IVPs admit limiting steady states in their time evolution, the phenomenon of finite time blow‐up, which can be interpreted as “bubble bursting”, is still present for certain initial conditions. The inevitable breakdown of the asymptotic description leads to the emergence of a new asymptotic structure of shorter spatio‐temporal scales associated with the unique self‐similar blow‐up structure. (© 2012 Wiley‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)