Colloidal aggregation with sedimentation: concentration effects

The results of computer models for colloidal aggregation, that consider both Brownian motion and gravitational drift experienced by the colloidal particles and clusters, are extended to include concentrations spanning three orders of magnitude. In previous publications and for a high colloidal concentration, it was obtained that the aggregation crosses over from diffusion-limited colloidal aggregation (DLCA) to another regime with a higher cluster fractal dimension and a speeding up followed by a slowing down of the aggregation rate. In the present work we show, as the concentration is decreased, that we can still cross over to a similar regime during the course of the aggregation, as long as the height of the sample is increased accordingly. Among the differences between the mentioned new regimes for a high and a low colloidal concentration, the cluster fractal dimension is higher for the high concentration case and lowers its value as the concentration is decreased, presumably reaching for low enough concentrations a fixed value above the DLCA value. It is also obtained the fractal dimension of the sediments, arising from the settling clusters that reach the bottom and continue a 2D-like diffusive motion and aggregation, on the floor of the container. For these clusters we now see two and sometimes three regimes, depending on concentration and sedimentation strength, with their corresponding fractal dimensions. The first two coming from the crossover already mentioned, that took place in the bulk of the sample before the cluster deposition, while the third arises from the two-dimensional aggregation on the floor of the container. For these bottom clusters we also obtain their dynamical behavior and aggregation rate.Received: 7 January 2004, Published online: 25 March 2004PACS: 61.43.Hv Fractals; macroscopic aggregates (including diffusion-limited aggregates) - 82.70.Dd Colloids - 05.10.Ln Monte Carlo methods     

Explaining the length threshold of polyglutamine aggregation

The existence of a length threshold, of about 35 residues, above which polyglutamine repeats can give rise to aggregation and to pathologies, is one of the hallmarks of polyglutamine neurodegenerative diseases such as Huntington's disease. The reason why such a minimal length exists at all has remained one of the main open issues in research on the molecular origins of such classes of diseases. Following the seminal proposals of Perutz, most research has focused on the hunt for a special structure, attainable only above the minimal length, able to trigger aggregation. Such a structure has remained elusive and there is growing evidence that it might not exist at all. Here we review some basic polymer and statistical physics facts and show that the existence of a threshold is compatible with the modulation that the repeat length imposes on the association and dissociation rates of polyglutamine polypeptides to and from oligomers. In particular, their dramatically different functional dependence on the length rationalizes the very presence of a threshold and hints at the cellular processes that might be at play, in vivo, to prevent aggregation and the consequent onset of the disease.     

Temperature and concentration gradient effects on heat and mass transfer in micropolar fluid

We report here the development of collinear laser spectroscopy (CLS) system at VECC for the study of hyperfine spectrum and isotopic shift of stable and unstable isotopes. The facility is first of its kind in the country allowing measurement of hyperfine splitting of atomic levels using atomic beams. The CLS system is installed downstream of the focal plane of the existing isotope separator online (ISOL) facility at VECC and is recently commissioned by successfully resolving the fluorescence spectrum of the hyperfine levels in \(^{85,87}\)Rb. The atomic beams of Rb were produced by charge exchange of 8 keV Rb ion beam which were produced, extracted and transported to the charge exchange cell using the ion sources, extractor and the beam-line magnets of the ISOL facility. The laser propagating opposite to the ion / atom beam direction was allowed to interact with the atom beam and fluorescence spectrum was recorded. The experimental set-up and the experiment conducted are reported in detail. The measures needed to be carried out for improving the sensitivity to a level necessary for studying short-lived exotic nuclei have also been discussed.     

Effect of concentration on laser threshold of organic dye laser

The effect of concentration on laser threshold is examined theoretically and experimentally for Rhodamin B and Rhodamin 6G. Laser action is obtained in the region from 1 · 10^?6 mol to 2.5 · 10^?1 mol. Lasing in organic dye solutions which are excited by a pulslaser is possible in a broad concentration range. For high concentrations laser action is limited by fluorescence quenching and increasing losses (diffraction, thermal effects, and bleaching). For low concentrations the limit is given by the losses of the used experimental set-up. In the present work the influence of the concentration on the exciting intensity at laserthreshold is examined for Rhodamin B and Rhodamin 6G solutions.     

Rheology and DWS microrheology of concentrated suspensions of the semiflexible filamentous fd virus

Microrheology measurements were performed on suspensions of bacteriophage fd with diffusive wave spectroscopy in the concentrated regime, at different values of ionic strength. Viscosity vs. shear rate was also measured, and the effect of bacteriophage concentration and salt addition on shear thinning was determined, as well as on the peaks in the viscosity vs. shear curves corresponding to a transition from tumbling to wagging flow. The influence of concentration and salt addition on the mean square displacement of microspheres embedded in the suspensions was determined, as well as on their viscoelastic moduli up to high angular frequencies. Our results were compared with another microrheology technique previously reported where the power spectral density of thermal fluctuations of embedded micron-sized particles was evaluated. Although both results in general agree, the diffusive wave spectroscopy results are much less noisy and can reach larger frequencies. A comparison was made between measured and calculated shear modulus. Calculations were made employing the theory for highly entangled isotropic solutions of semiflexible polymers using a tube model, where various ways of calculating the needed parameters were used. Although some features are captured by the model, it is far from the experimental results mainly at high frequencies.     

Temperature dependent spall threshold of four metal alloys

Many aerospace components operate at elevated temperature. Such components are more vulnerable to impact and radiation environments than if cold. To evaluate the effects of these environments, an experimental program was undertaken in which four candidate missile structural materials were tested at various temperatures. The materials studied were 2024 aluminum alloy, 304 stainless steel, Haynes 25 cobalt alloy, and AZ-31B magnesium. A carbon foil radiative heater was used to provide initial temperatures up to 1300°K. The heated test specimens were then exposed to a high energy (~3 MeV) electron beam with a deposition time of 50nsec in the Boeing FX-75 facility. The results of these tests provided spall thresholds as a function of internal energy (temperature) for the various materials. Impact tests in the Boeing gas gun were also conducted to provide data on spall behavior at room temperature.     

Temperature effect on the threshold frequency of absorption in a quantum pseudodot

The threshold frequency of absorption in a quantum pseudodot under the influence of temperature and applied magnetic field is calculated. The threshold frequency of absorption is computed as a function of temperature and applied magnetic field. The linear and nonlinear dependence of the absorption threshold frequency on magnetic field and temperature has been showed. According to the results obtained from the present work, we find that the linear and nonlinear dependence of the absorption threshold frequency depends on used range of the temperatures and magnetic fields.     

A simplified model to predict the effects of aggregation on the absorption properties of soot particles

An exact electrostatics formulation for sphere clusters is used to predict the Rayleigh-limit radiative absorption properties of soot aggregates. In the near to mid IR wavelengths, it is shown that aggregation can result in absorption cross sections that are significantly larger than that predicted by an independent-sphere (Rayleigh–Gans) model. The relative increase in absorption increases with the number of spheres in the aggregate, and reaches an asymptote for aggregates containing 100–200 spheres. A simplified correlation is developed to predict the aggregate absorption cross section as a function of number of spheres and refractive index. Implications of the effects of aggregation on absorption and emission of thermal radiation by soot in flame and atmospheric environments are discussed.     

Spectral and threshold effects on recognition of speech at higher-than-normal levels

To examine spectral and threshold effects for speech and noise at high levels, recognition of nonsense syllables was assessed for low-pass-filtered speech and speech-shaped maskers and high-pass-filtered speech and speech-shaped maskers at three speech levels, with signal-to-noise ratio held constant. Subjects were younger adults with normal hearing and older adults with normal hearing but significantly higher average quiet thresholds. A broadband masker was always present to minimize audibility differences between subject groups and across presentation levels. For subjects with lower thresholds, the declines in recognition of low-frequency syllables in low-frequency maskers were attributed to nonlinear growth of masking which reduced "effective" signal-to-noise ratio at high levels, whereas the decline for subjects with higher thresholds was not fully explained by nonlinear masking growth. For all subjects, masking growth did not entirely account for declines in recognition of high-frequency syllables in high-frequency maskers at high levels. Relative to younger subjects with normal hearing and lower quiet thresholds, older subjects with normal hearing and higher quiet thresholds had poorer consonant recognition in noise, especially for high-frequency speech in high-frequency maskers. Age-related effects on thresholds and task proficiency may be determining factors in the recognition of speech in noise at high levels.     

General aspects of beam threshold effects in LEED

We present in this paper some general aspects of beam threshold effects in LEED concerning their observation, their extension and their detection. The observation is possible on LEED intensities but also on crystal to ground current and total reflected current. We discuss the various types of profiles due to threshold effects for LEED intensities. From universal relations coming from emergence conditions of a new beam we draw universal charts for the “speed of emergence” versus E, θ or φ. These charts are used for a discussion of the range in I(E) or I(φ) profiles, where threshold effects may affect the LEED intensities. The same relations are used for a discussion of the detection of threshold effects. Assuming a Gaussian distribution (in energy and angles) for the incident beam, we derive a “total equivalent experimental resolution” and we discuss the consequence on intensity profiles and on the optimal conditions of detection.     

Temperature effects on the transport properties of molecules

Recent experiments found an unusual temperature-induced large shift in the resonant-tunneling voltage of certain molecules. We report first-principles calculations showing that such behavior can be caused by the excitation of rotational modes of ligands. These modes have classical characteristics, i.e., the maximum excursion is dominant, while at the same time they have a significant effect on the energy levels responsible for resonant tunneling. The proposed mechanism of ligand rotations is unique to molecules and accounts for the fact that the effect is not seen in semiconductor nanostructures.     

Temperature and concentration effects in electron-stimulated desorption of sodium atoms from sodium layers adsorbed on tungsten coated with a gold film

The yield and energy distributions of sodium atoms upon electron-stimulated desorption from sodium layers adsorbed on tungsten coated with a gold film are investigated for the first time as functions of the thickness of the gold film, the concentration of deposited sodium, and the surface temperature. It is found that the energy distributions exhibit two peaks, namely, a narrow peak with a maximum at about 0.15 eV, whose intensity continuously increases with increasing temperature, and a broad peak with a maximum at about 0.35 eV, whose intensity either decreases or remains constant with increasing temperature. It is shown that both peaks arise as a result of the same excitation, which gives rise to different channels of electron-stimulated desorption of sodium atoms. Possible mechanisms of electron-stimulated desorption and the kinetics of destruction of the surface coating are discussed.     

Temperature effects on the two-magnon Raman-scattering of antiferromagnets

In this first of two papers we investigate the temperature effect on the spin wave energy and the two-magnon Raman scattering cross section for an antiferromagnet with rutile structure in the spin wave scheme. In the Heisenberg hamiltonian we consider the exchange interaction between magnetic neighbors up to the third order, an effective anisotropy field, and an external magnetic field. In the effective Raman hamiltonian an anisotropy factor, allowed by the crystal symmetry, is taken into account.The theory employs a Green's function method, where the Green's functions are obtained from equations of motion. The results are similar to published ones obtained with diagrammatic techniques.Extract from thesis, Munich, 1974     

Oxygen concentration effects on laser-induced grating spectroscopy of toluene

The effects of variation in oxygen concentration on laser-induced grating spectroscopy (LIGS) signals from toluene vapour in nitrogen/oxygen mixtures is investigated. The modulation of LIGS signals arising from the interference of counter-propagating acoustic waves with a stationary density perturbation induced by pulsed excitation of toluene by frequency quadrupled radiation from a Nd:YAG laser has been measured as a function of oxygen partial pressure at total gas pressures up to 8?bar. The modulation depth or signal contrast is found to vary in an unexpected way with oxygen partial pressure and the behaviour is ascribed to energy transfer to excited singlet states of the oxygen molecule and subsequent collisional quenching. A simple model of the energy transfer dynamics is presented that reproduces the observed behaviour and the potential for using the signal contrast of LIGS signals as a measure of oxygen concentration is discussed.     

Nuclear effects in positive pion electroproduction on the deuteron near threshold

Positive pion electroproduction from the deuteron near threshold has been considered within an approach based on the unitary transformation method. The gauge independence of the treatment is provided by using an explicitly gauge-independent expression for the reaction amplitude. The results of calculations for the kinematics of the experiments on forward-angle meson electroproduction accomplished at Saclay and Jefferson Laboratory are discussed and compared with those given by the impulse approximation. It is shown that the observed behaviour of the cross-sections is in accordance with the calculations based on the pion-nucleon dynamics. In particular, the pion production rate suppression in the )nn reaction compared to that for the )n one can be due to such nuclear medium effects as nucleon motion and binding along with Pauli blocking in the final nn state.Received: 28 October 2003, Published online: 29 June 2004PACS: 13.60.Le Meson production - 25.10. + s Nuclear reactions involving few-nucleon systems - 25.20.Lj Photoproduction reactions - 25.30.Rw Electroproduction reactionsL. Levchuk: leonid.levchuk@pd.infn.it     

The effects of neural synchronization and peripheral compression on the acoustic-reflex threshold

This study investigates the acoustic reflex threshold (ART) dependency on stimulus phase utilizing low-level reflex audiometry [Neumann et al., Audiol. Neuro-Otol. 1, 359-369 (1996)]. The goal is to obtain optimal broadband stimuli for elicitation of the acoustic reflex and to obtain objective determinations of cochlear hearing loss. Three types of tone complexes with different phase characteristics were investigated: A stimulus that compensates for basilar-membrane dispersion, thus causing a large overall neural synchrony (basilar-membrane tone complex-BMTC), the temporally inversed stimulus (iBMTC), and random-phase tone complexes (rTC). The ARTs were measured in eight normal-hearing and six hearing-impaired subjects. Five different conditions of peak amplitude and stimulus repetition rate were used for each stimulus type. The results of the present study suggest that the ART is influenced by at least two different factors: (a) the degree of synchrony of neural activity across frequency, and (b) the fast-acting compression mechanism in the cochlea that is reduced in the case of a sensorineural hearing loss. The results allow a clear distinction of the two subjects groups based on the different ART for the utilized types and conditions of the stimuli. These differences might be useful for objective recruitment detection in clinical diagnostics.