Presence Of Dispersion Research Articles

Cancellation of stimulated-Raman-scattering cross talk in wavelength-division-multiplexed optical communication systems by series or parallel techniques

We compare techniques using two fibers connected either in series or in parallel for cancelling cross talk that is due to stimulated Raman scattering in wavelength-division-multiplexed systems. Theory shows that such cross talk can be completely eliminated by a series arrangement in lossy dispersionless fibers by means of midway amplification and spectral inversion. In dispersive fibers, cross-talk reduction of the order of the Raman gain G can be obtained by adding dispersion compensation and alternating dispersion. Alternatively, a similar cross-talk reduction can be obtained by use of parallel transmission of signals on different sets of wavelengths and differential detection; in the presence of dispersion, the dispersion for the different sets of wavelengths must be opposite. The parallel approach appears to be more practical.

Rheological behaviour of stabilised aqueous alumina dispersions in presence of hydroxyethyl cellulose

The flocculation behaviour of sterically stabilised aqueous α-Al 2O 3 dispersions in presence of hydroxyethyl cellulose (HEC) of various molecular weights has been studied by means of rheology. Steady state shear stress-shear rate measurements as well as oscillatory measurements suggest the existence of a critical flocculation polymer volume fraction (CFF) Φ p + above which the rheological properties of the dispersions deviate from those observed when the dispersion contains no added polymer. The ceramic powder had an average diameter of 0.4 μm and was stabilised in the dispersion by the polyelectrolyte ammonium polymethylmethacrylate. Both Φ p (polymer volume fraction) and Φ S (solids volume fraction) had a positive effect on the storage modulus G′. A Sisko type equation of the form η = η ∞ + K γ n − 1 was found to closely fit the shear stress-shear rate data. The value of K, which can be seen as an effective yield stress of the suspension, was also found to increase with Φ p and Φ S The exponent n was found to strongly depend on Φ S and it was essentially insensitive to Φ p above the critical flocculation fraction. This value decreased with increasing volume fraction. The polymer molecular weight was found to affect positively both G′ and K although the effect on K was more pronounced. A depletion flocculation mechanism, whereby the HEC, which is non adsorbing on the alumina particles (Liang W., Tadros, Th.F. and Luckham, P., Rheological properties of concentrated sterically stabilized latex dispersions in the presence of hydroxyethyl cellulose. J. Colloid Interface Sci., 1993, 160, 183–189), is depleted during a particle collision, giving rise to an osmotic

Selective oxidation of n-butane to maleic anhydride with vanadium phosphorus catalysts prepared by comminution in the presence of dispersants

A method of preparation of high surface area vanadium phosphorus catalysts for the partial oxidation of n-butane is described that is based on a comminution procedure. The ball milling of catalysts precursors in the presence of dispersants is shown to decrease significantly the mean crystallite size of the VOHPO 4.0.5H 2O from 5 × 10 −6m to ca. 3.5 × 10 −8m. A range of dispersants and solvents is described and the use of cyclohexane as solvent and the use of a dispersant based on poly-12-hydroxystearic acid is discussed in detail. These catalyst precursors give final catalysts with high surface area following activation (ca. 40 m 2 g −1 versus ca. 10 m 2 g −1 in the absence of the comminution). These catalysts are found to be particularly active for use under fuel rich reaction conditions butane the higher explosion limit when high maleic anhydride yields can be obtained. The effect of the addition of promoters is also discussed. In general addition of low levels of La, Ce, Cu and Mo all increase the activity of maleic anhydride formation without any significant effect on selectivity when fuel lean conditions are used butane lower explosion limit .04 However, the effect on activity is much less pronounced when fuel rich conditions are used and in this case the addition of these compounds leads to lower maleic anhydride selectivity.

The magnetotelluric response over 2D media with resistivity frequency dispersion1

AbstractWe investigate the magnetotelluric response of two‐dimensional bodies, characterized by the presence of low‐frequency dispersion phenomena of the electrical parameters. The Cole‐Cole dispersion model is assumed to represent the frequency dependence of the ‘impedivity’ complex function, defined as the inverse of Stoyer's ‘admittivity’ complex parameter. To simulate real geological situations, we consider three structural models, representing a sedimentary basin, a geothermal system and a magma chamber, assumed to be partially or totally dispersive. From a detailed study of the frequency and space behaviours of the magnetotelluric parameters, taking known non‐dispersive results as reference, we outline the main peculiarities of the local distortion effects, caused by the presence of dispersion in the target media. Finally, we discuss the interpretative errors which can be made by neglecting the dispersion phenomena. The apparent dispersion function, which was defined in a previous paper to describe similar effects in the one‐dimensional case, is again used as a reliable indicator of location, shape and spatial extent of the dispersive bodies. The general result of this study is a marked improvement in the resolution power of the magnetotelluric method.

Bright and dark simultons in second-harmonic generation

A symbiotic pair of dark (second-harmonic) and bright (fundamental frequency) solitons sustain each other and travel locked in second-harmonic generation dominated by group-velocity difference or spatial walk-off. The conditions of existence of the soliton pair are determined, and its stability in the presence of dispersion is discussed.

Influence of dispersants on trophic transfer of petroleum hydrocarbons in a marine food chain

The objective of this research was to determine the impact of dispersing agents on petroleum hydrocarbon (PH) bioavailability and trophic transfer in primary levels of a marine food chain. Uptake, bioaccumulation, and metabolic transformation of a model PH, [ 14C]naphthalene, were measured and compared for Prudhoe Bay Crude Oil (PBCO) dispersed with Corexit ® 9527 (DO) and undispersed preparations of the water-accommodated fraction (WAF) of PBCO. The model food chain consisted of Isochrysis galbana, a primary producer, and Brachionus plicatilis, a primary consumer. Fractionation and identification of metabolites was done by HPLC co-chromatography, and quantitation was done by liquid scintillation counting (LSC). Results show that uptake of naphthalene increases somewhat ( P < 0.01) in the presence of dispersant in algae and a slight ( P < 0.001) increase in uptake was observed in rotifers via trophic transfer.

Dark solitons in dispersive quadratic media

We analyze dark solitons supported by second-order parametric interactions between the fundamental and second harmonics in the presence of dispersion and group-velocity mismatch. We reveal various types of solitary waves, including a family of two-wave dark solitons propagating on a modulationally stable background, dark solitons with nonmonotonic tails, and bound states of dark solitons.

Solitary waves due to χ^(2): χ^(2) cascading

Solitary waves in materials with a cascaded χ(2):χ(2) nonlinearity are investigated, and the implications of the robustness hypothesis for these solitary waves are discussed. Both temporal and spatial solitary waves are studied. First, the basic equations that describe the χ(2):χ(2) nonlinearity in the presence of dispersion or diffraction are derived in the plane-wave approximation, and we show that these equations reduce to the nonlinear Schrodinger equation in the limit of large phase mismatch and can be considered a Hamiltonian deformation of the nonlinear Schrodinger equation. We then proceed to a comprehensive description of all the solitary-wave solutions of the basic equations that can be expressed as a simple sum of a constant term, a term proportional to a power of the hyperbolic secant, and a term proportional to a power of the hyperbolic secant multiplied by the hyperbolic tangent. This formulation includes all the previously known solitary-wave solutions and some exotic new ones as well. Our solutions are derived in the presence of an arbitrary group-velocity difference between the two harmonics, but a transformation that relates our solutions to zero-velocity solutions is derived. We find that all the solitary-wave solutions are zero-parameter and one-parameter families, as opposed to nonlinear-Schrodinger-equation solitons, which are a two-parameter family of solutions. Finally, we discuss the prediction of the robustness hypothesis that there should be a two-parameter family of solutions with solitonlike behavior, and we discuss the experimental requirements for observation of solitonlike behavior.

Transport of bacteria in porous media: I. An experimental investigation

The convective transport of concentrated suspension of bacteria in porous media is of interest for several processes such as microbial enhanced oil recovery and in situ bioremediation. The parameters which affect the transport of the bacterium Bacillus licheniformis JF-2, a candidate microorganism for microbial enhanced oil recovery, were investigated experimentally in sandpacks. Bacteria retention and permeability reduction occurred primarily in the first few centimeters upon entering the porous medium. In downstream sections of the sandpack, the permeability reduction was low, even in cases in which high cell concentrations (10(8) cfu/mL) were detected in the effluent. The effect of (i) addition of a dispersant, (ii) linear velocity of injection, (iii) cell concentration, (iv) salinity (v) temperature, and (vi) the presence of a residual oleic phase were determined experimentally. A lower reduction in permeability and a higher effluent bacterial concentration were obtained in the presence of dispersant, high injection velocities, low salinities, and at a higher temperature. Macroscopic measurements at different linear velocities and in the presence or absence of dispersants suggest that the formation of reversible microaggregates and multiparticle hydrodynamic exclusion may be the primary mechanisms for bacterial retention and permeability reduction. (c) 1994 John Wiley & Sons, Inc.

Use of polyamino acid analogs of biomineral proteins in dispersion of inorganic particulates important to water treatment

Synthetic polyamino acids (peptides) based on the structure and activity of matrix proteins isolated from oyster shell and other biomineral structures have been identified for use in the prevention of mineral scaling. Matrix proteins are polyanionic and are thought to act as regulators of crystallization during the development of skeletal and other mineral structures. These proteins and their synthetic analogs also contain hydrophobic regions that may enhance their surface-active properties. The dispersion activity of a variety of polyamino acids that are matrix protein analogs has been evaluated in bench-top tests using inorganic mineral particles. Dispersion activities were measured using particles of iron oxide, kaolin, calcium carbonate, and calcium phosphate (hydroxyapatite). The activity was measured by increases in the spectrophotometric absorbance of test particle suspensions in the presence of dispersants. The increases in absorbance were due to turbidity resulting from the production of smaller particle sizes or slower rates of settling. The results suggest that biopolymers composed of polyanionic polyamino acids may be effective as dispersants. Polyamino acids containing a hydrophobic or phosphorylated domain attached to a polyaspartate backbone demonstrate enhanced activity over polyaspartate. These polyamino acids display comparable activity to commercially available hydrocarbon-based polymeric dispersants. An economical process for the manufacture of polyamino acids by thermal polycondensation is under development. Clearly, non-toxic and biodegradable polyamino acids present a desirable alternative to toxic non-biodegradable polymers in a number of applications such as detergents and cooling tower additives.

Properties of concentration waves in presence of nonlinear sorption, precipitation/dissolution, and homogeneous reactions: 2. Illustrative examples

Theoretical properties of breakthrough curves (BTCs) of solutes subject to nonlinear sorption, precipitation/dissolution and homogeneous reactions are compared with numerical simulations. The example treated here involves heterovalent ion exchange on clays, ionization of carbonic acid and water, and dissolution of gaseous carbon dioxide and of solid calcite. Despite the complexity of the solid‐fluid interaction mechanism (13 reacting species and six reactions), the number of waves, the possible existence of an unretarded wave, and the expressions for the concentrations of the stoichiometric tracers are easily derived from the stoichiometric matrix. The location of peaks or waves and their broadening characteristics due to the nonlinear equilibria are estimated exclusively from the compositions of the injected and initial solutions. All of these theoretical properties are confirmed by numerical simulations, even in the presence of dispersion. The number of waves and the stoichiometric tracers are shown to depend on whether or not the pore solution equilibrates with a gas phase. This property illustrates the strong sensitivity of the breakthrough curves to the interaction mechanism. Finally, calculating the BTCs of the stoichiometric tracers from the experimental BTCs of the liquid phase species allows one to discriminate among rival interaction mechanisms without requiring a numerical model and knowledge of the equilibrium constants.

The Rayleigh-Brillouin Light Scattering Spectrum of Dense Argon Gas Between 70 and 260 MPa at T = 298 K: the Sound Velocity and the Specific Heats Ratio γ

Abstract Measurements of the Rayleigh–Brillouin spectrum in dense argon gas at T = 298 K and pressures between 70 and 260 MPa are reported. No dispersion in the sound velocity was found up to frequencies of the order of 4 GHz, and low and high frequency measurements agree within 0.5%. This permits the use of hypersonic velocity for deriving thermodynamic quantities for this system. From our measurements we have derived the density behavior of the specific heats ratio γ. Moreover, in absence of dispersion, we have shown that Brillouin scattering gives the sole possibility of measuring both the very low and high sound velocities in fluids, at the same time, therefore giving the direct possibility of checking either the absence or presence of dispersion.

Photofixation of carbon dioxide by a hydroxo-oxobis(8-quinolyloxo)vanadium(V)-MV2+-ethylenediaminetetraacetic acid dispersion system

A new type of organometallic complex semiconductor: hydroxo-oxobis(8-quinolyloxo)vanadium(V) (VOQ2OH) was explored as a photocatalyst for the reduction of CO2 to formic acid and formaldehyde in aqueous medium. The band gap Eg of this organometallic complex was determined and found to be equal to 1.5 eV. The complex semiconductor was used for the photoreduction process either (i) as colloidal dispersion in presence of methyl viologen (MV2+ as electron acceptor with ethylenediaminetetraacetic acid (EDTA) as sacrificial electron donor or (ii) as colloidal dispersion without any additive. The best performance for the photoreduction of CO2 of the present system was found when the quantity of VOQ2OH suspended is 0.66 g l−1. [MV2+] = 5 × 10−3 mol l−1, [EDTA] = 1 × 10−3 mol l−1 and pH 5 (citrate buffer). The highest concentrations of formic acid and formaldehyde produced under optimum conditions were 8.2 × 10−6 mol l−1 and 3.95 × 10−5 mol l−1 respectively. The photosensitizing action was proved by the photoresponse spectrum of VOQ2OH. The effects of pH, temperature, incident light intensity and reaction time were also studied and, according to the experimental results, a general reaction mechanism for the photofixation of CO2 was suggested.

Co-propagation of pulses with steepening and phase modulation effects

We consider the copragation of a high intensity pump and a low intensity signal in the presence of dispersion, phase modulation and steepening effects. The possibility of obtaining large compression for the signal pulse is investigated.

Effects of dispersion in additive-pulse mode-locked lasers

The transient process of pulse shortening in a laser with a nonlinear external cavity is studied by numerical calculation. The effect of normal fiber dispersion on the steady-state output parameters is investigated. The main features of the pulse-shortening process can be understood by simple arguments. The numerical simulations indicate that pulse shortening is accompanied by an increase in the chirp of the master resonator pulse. It is shown that a chirped master resonator pulse in the presence of dispersion diminishes the pulse shortening and results in longer stationary pulse. If the chirp is strong enough, pulse shortening is even reversed into pulse lengthening. Our conclusions are supported by experimental observations.

A multi-torque model for the effects of dispersants and slurry viscosity on ball milling

Diverse ball milling conditions were employed by changing ball size, media charge, mill speed and slurry hold-up in order to study the effect of polymeric dispersant aids on the grinding of dense slurries. In the absence of a dispersant, as the pulp thickens, there occurs an increasingly disproportionate split of the grinding charge between cascading and cataracting-centrifuging masses, resulting in a complex relationship between the instantaneous viscosity and mill torque. In the presence of dispersants, the partitioning of charge in a relatively fluid suspension occurs to a much lesser extent. A multi-torque mill model has been derived which tracks the overall mill torque as a function of changing viscosity of the pulp, permits estimation of the charge split between cascading and cataracting-centrifuging regimes and explains the occurrence of a peak torque value at some intermediate stage. The size distributions of the ground product are a function of the cumulative energy input only, even when the power drawn by the mill varies appreciably with milling conditions and/or grinding time. Implication is that the grinding rates are proportional to the mill torque and the latter can be used directly to monitor and analyze the mill performance and role of grinding aids, notwithstanding the fact that the slurry may undergo major changes in its rheological character.

Doppler-free resonances of the second-order Raman scattering

An analysis is made of the possibility of observation of Raman scattering resonances free of the influence of the Doppler effect. Such Doppler-free resonances result from the excitation of a standing wave of Raman vibrations in a gas by two standing optical waves differing in frequency by half the Raman frequency. Complete compensation of the Doppler shifts occurs as a result of simultaneous interaction of atomic particles with two pairs of counterpropagating waves. Scattering of traveling pump waves by a standing wave of Raman vibrations in the fifth order with respect to the field gives rise to resonances in the radiation at the Stokes and anti-Stokes frequencies. Coherent scattering is observed in the presence of dispersion. A resonance in the number of particles excited to the upper level of a Raman transition appears in the eighth order in the field. The expressions obtained are applied to the specific example of neon.

Effect of background activity of a net of excitatory neuron-like elements on conduction of excitation.

1. Investigations on a mathematical model have shown that the presence of an optimal system of parameters of neuron-like excitatory elements (EE) and of connections between them is essential for the undistorted conduction of excitation along a network composed of them. If these parameters deviate from their optimal values excitation will be transmitted along the net either with attenuation or with amplification, due to the presence of divergence and convergence of influences of EE of the preceding layer of the net on the succeeding layer. 2. Optimal values of the parameters of EE and of the connections between them for undistorted conduction of excitation are not strictly determined. Deviations of some parameters can be compensated by changes in others. 3. The presence of dispersion of thresholds of EE induces chaos in the working of the net, weakens the degree of synchronization in the activation of EE to an external agency, and impairs the detection of activity of input EE at the output of the net. 4. Background activity of the net, both tonic and periodic, above the threshold for impulse generation prevents the appearance of activation of input EE at the output of the net, whereas near-threshold tonic activity of the net facilitates the conduction of excitation along it. 5. Periodic below-threshold activity of the net facilitates the conduction of excitation along it only in the presence of optimal proportions of frequencies and phase shift of coherent rhythmic oscillations of excitability of EE, and also of the time required for excitation to be transmitted from the input of the net to its output. If those conditions are not present, the presence of below-threshold rhythmic oscillations of excitability (including high-frequency oscillations) in the net will limit the conduction of excitation along it by comparison with tonic below-threshold activation.

Power-dependent polarization of optical pulses

The theory of the power dependence of the polarization of optical pulses in birefringent optical fibers with Kerr-like nonlinearity is presented in detail. Fundamental discussions are given of the nature of the polarization, and the coupled Schrödinger equations satisfied by the components of the polarization are derived in detail from first principles. It is shown how the Cartesian and circularly polarized field representations relate to each other and to the existing literature. Extensions that include self-steepening and applications to twisted fibers are also given. It is argued that, since high-order solitons evolve on a much shorter length scale, relative to the soliton length, than lower-order solitons, a more useful indication of the significance of dispersion is given by some pulse-compression distance based on the initial compression rate of the pulse, owing to self-phase modulation. This evidence is presented numerically for an initial condition using a 100-W pulse for a fiber having αp = 1.8 × 10−9 as its nonlinear coefficient and assuming an initial pulse width of 3.3 psec. The effect of dispersion and birefringence on a nonlinear pulse with the power equally distributed between the fast (y) and slow (x) axes of a birefringent optical fiber is given for a fiber with an intrinsic birefringence that gives a beat length of 20 m. In the evolution of the fields for zero group-velocity dispersion, it is shown that a portion of the power transfers from the fast axis the slow axis and back and continues to oscillate back and forth between the two. The evolution in the presence of dispersion (〈β″〉 = 5 × 10−26) without any intrinsic birefringence shows that this case is close to the standard N = 3 soliton case. In both of the above cases, the structure of the pulse evolves as it travels down the z axis, but there is no overall breakup of the pulse. If both dispersion and the assumed birefringence are present, splitting into two separate pulses is seen, with, in addition, some permanent transfer of energy from the fast to slow mode.

Stability of alumina dispersions in presence of surface active agents

In order to understand the suspension properties of concentrated alumina slips in presence of surface active agents, with particular reference on their flocculation behaviour, electrophoresis, rheology and sedimentation, experiments were carried out on a commercial alumina powder in three cationic surface active agents in the pH range 3–11. The results suggest that the alumina powder is associated with some acidic surface impurity and all three cationic additives are adsorbed on to the surface of alumina particles at all pH values investigated. While the rheological and sedimentation behavior of all slips prepared in water and the additive with the smallest polymer chain length depend upon the electrophoretic mobility, the surface charge appears to have no effect on the stability of alumina slips prepared in additive solutions containing longer polymer chain lengths.