Frequency Sweep Measurements Research Articles

Study on the rheological properties of heat-induced whey protein isolate–dextran conjugate gel

Effect of glycosylation on the rheological properties of whey protein isolate (WPI) during the heat-induced gelation process was evaluated. Significant changes in browning intensity, free amino groups content and SDS-PAGE profile showed that the conjugate of WPI and dextran (150 kDa) was successfully prepared using the traditional dry-heating treatment. For the conjugate, during the heating and cooling cycle, the curves of G′ and G″ were considerably shifted to lower values and their shapes varied comparing to the corresponding spectra of initial WPI and WPI + dextran mixture. After holding at 25 °C, G' reached a value of about 2200 Pa, only a tenth of the value that obtained in the initial WPI gel. Moreover, frequency sweep measurements revealed that the stiffness of gel was greatly reduced in the conjugate, although a typical elastic gel was still formed. All data showed that the rheological properties of thermal gelation could be modified upon the covalent attachment of dextran.

Development of a 3.5-mm Coaxial Microcalorimeter for RF and Microwave Power Standards at KRISS

This paper introduces a newly constructed type-N coaxial microcalorimeter that will serve as a primary power standard in Korea Research Institute of Standards and Science. The detailed design scheme of the microcalorimeter is presented, and its characteristics are evaluated. The performance of an adiabatic line was simulated with a finite-element-method-based tool and predicted. The measured insertion loss of the assembled adiabatic line was less than 0.15 dB. Frequency and power sweep measurements were performed on a reference standard to find a substituted power level that minimized the dual-element error. We also propose evaluation standards that can be used for the measurement of radio frequency loss. The correction factor for the effective efficiency of the reference standard was experimentally evaluated. We also present an uncertainty evaluation process and the corrected effective efficiency of a type-N coaxial reference standard.

56.2: Analysis and Interpretation of Degradation Mechanism of OLED with p‐doping Layer

AbstractWe demonstrate that the new structure show an outstanding EL Lifetime that is 2 times higher than in the reference device. We explain the degradation mechanism of p‐doping layer on the charge injection in organic light‐emitting diode. Generally, p‐doping layer has the ability to reduce the interface barrier between Anode and HTL, the role of simplifying the multi‐layer. But, except of power consumption, there is not the detailed report of analyzing the degradation depending on the p‐doping efficiency which means the relative yield of charge transfer complex between p‐dopant and HTL. In order to understand the underlying cause and mechanism of degradation in OLED consisted of p‐doped system, Impedance Spectroscopy (IS) was used. Using AC with frequency sweep and DC measurement, it's possible to differentiate between bulk and interface properties and find their effect on the degradation mechanism. Using IS we will show how different factors contribute to the overall degradation mechanism, and how by changing them we can achieve the lifetime improvement.

Compositional analysis and rheological characterization of gum tragacanth exudates from six species of Iranian Astragalus

The sugar composition and viscoelastic behaviour of Iranian gum tragacanth exuded by six species of Astragalus was investigated at a concentration of 1.3% and varying ionic strength using a controlled shear-rate rheometer. Compositional analysis of the six species of gum tragacanth by high-performance anion-exchange chromatography with pulsed amperometric detection suggested the occurrence of arabinose, xylose, glucose, galactose, fucose, rhamnose and galacturonic acid residues in the gum structure; however, the proportions of each sugar varied significantly among the gums from the different species of Astragalus, and this variation led to interesting differences in functional properties. Rheological measurements performed on dispersions of the six species of gum tragacanth demonstrated viscoelastic properties. The mechanical spectra derived from strain sweep and frequency sweep measurements indicated that the different gum tragacanth dispersions had distinctive viscoelastic behaviours. Investigation of the viscoelastic properties of the different gum dispersions in the presence of NaCl revealed that the addition of NaCl could lead to slight to drastic decreases in the G′, G″ or η ∗ values of the various gums. In general, the results indicated that the six varieties of gum tragacanth studied exhibited significantly different rheological properties; therefore, these different gums may find use in a variety of applications as stabilisers, thickeners, emulsifiers and suspending agents depending on their rheological behaviour.

Rheological and textural properties of sorghum-based formulations modified with variable amounts of native or pregelatinised cassava starch

Sorghum bread was made from native or pregelatinised cassava starch and sorghum flour in the ratio 10:90, 20:80, 30:70, 40:60 and 50:50. The other ingredients, measured on flour-weight-basis, were water (100%), sugar (6.7%), egg white (6%), fat (2%), salt (1.7%) and yeast (1.5%). The dynamic oscillatory behaviours of the batters were affected by the amount and type of starch. In the amplitude sweep measurements, increasing concentration of native starch decreased storage modulus, whereas increasing concentration of pregelatinised starch increased the linear viscoelastic range of the batters. In the frequency sweep measurements, the loss factor of batters treated with native starch declined with increasing frequency. Batters treated with 10 or 20% pregelatinised starch showed declining loss factors, whereas batters treated with 40 or 50% pregelatinised starch showed increasing loss factors with increasing frequency. Sorghum-based batters containing native starch gave bread with better crumb properties than batters containing pregelatinised starch. Crumb firmness and chewiness declined with increasing native or pregelatinised starch concentration. Crumb adhesiveness of breads containing pregelatinised starch increased with increasing starch content but was not affected by native starch. Cohesiveness, springiness and resilience increased with increasing native starch content, but were minimally affected by increasing pregelatinised starch content.

A Hybrid Measurement Approach for Wideband Characterization and Modeling of Power Transformers

This paper describes a hybrid procedure for wideband characterization and modeling of power transformer behavior from frequency sweep measurements. The approach is based on measuring corresponding sets of voltage and current vectors that are associated with short circuit and open circuit conditions. These vectors are stacked side-by-side in two matrices which are used for computing the short circuit admittance matrix. This hybrid approach is shown to give more information about the transformer behavior than the traditional approach of measuring the admittance matrix directly under short-circuit conditions, which tends to corrupt the small eigenvalues associated with open-circuit conditions. From the obtained admittance matrix, a pole-residue model is extracted via vector fitting and passivity enforcement, thereby giving a model ready for EMTP simulation. The accuracy of the small eigenvalues is retained by using a modal formulation in the fitting and passivity enforcement step. Application to a distribution transformer shows that the hybrid approach can greatly improve the accuracy of time-domain simulations involving high-impedance terminations.

The impact of concentration, temperature and pH on dynamic rheology of psyllium gels

Structural aspects of the psyllium gum prepared from the seed husk of the plant of Plantago ovata Forsk was characterized by dynamic rheology and microscopy. Dynamic rheological properties of psyllium gel in the linear viscoelastic region, as a function of concentration (2, 2.5 and 3% w/w), temperature (5–95 °C) and pH (2.5–10) were investigated. Mechanical spectra of the psyllium gels were obtained by frequency sweep measurement classified into that of weak gels because G′ was larger than G″ throughout the tested frequency range and the separation of the two moduli (tan δ) was greater than 0.1. The phase angle increased with temperature and a peak associated with gel melting appeared at about 40 °C. All gels at different pH presented a typical weak gel spectrum. Scanning electron microscopy showed porous structures with different pore-size distribution for psyllium gels under different conditions in terms of concentration, pH and temperature.

Study of Transformer Resonant Overvoltages Caused by Cable-Transformer High-Frequency Interaction

Power transformers can fail from dielectric stresses caused by electromagnetic transients. In this paper, we focus on a special phenomenon where excessive overvoltages arise due to resonance. This situation can take place when a transformer on the high-voltage side is connected to a cable and the low-voltage side is unloaded. Very high overvoltages can then result on the low-voltage side from transient events that cause a weakly attenuated overvoltage on the cable with a dominant frequency matching a resonance peak in the transformer voltage ratio. Laboratory tests on a 11-kV/230 V distribution transformer show that a step voltage excitation on a 27-m cable produces 24-p.u. overvoltage on the open low-voltage side. The voltage waveforms are accurately reproduced by a black-box model obtained from frequency sweep measurements. Simulations show that overvoltages as high as 43 p.u. could occur with the most unfavorable cable length. It is further shown that the following situations can lead to high overvoltages on an unloaded transformer low-voltage side: 1) ground fault initiation at the far cable end, 2) cable energization from a busbar with several other cables connected, 3) cable energization from another cable with the same length, and 4) capacitor bank energization at the far cable end.

Strain-rate frequency superposition in large-amplitude oscillatory shear

In a recent work, Wyss [Phys. Rev. Lett. 98, 238303 (2007)] have noted a property of "soft solids" under oscillatory shear, the so-called strain-rate frequency superposition. We extend this study to the case of soft solids under large-amplitude oscillatory shear (LAOS). We show results from LAOS studies in a monodisperse hydrogel suspension, an aqueous gel, and a biopolymer suspension and show that constant strain-rate frequency sweep measurements with soft solids can be superimposed onto master curves for higher harmonic moduli with the same shift factors as for the linear viscoelastic moduli. We show that the behavior of higher harmonic moduli at low frequencies in constant strain-rate frequency sweep measurements is similar to that at large strain amplitude in strain-amplitude sweep tests. We show surface plots of the harmonic moduli and the energy dissipation rate per unit volume in LAOS for soft solids and show experimentally that the energy dissipated per unit volume depends on the first harmonic loss modulus alone, in both the linear and the nonlinear viscoelastic regime.

Composition-Insensitive Highly Viscous Wormlike Micellar Solutions Formed in Anionic and Cationic Surfactant Systems

We investigated phase behavior and rheological properties of aqueous micellar phase formed in water/cocoyl glutamate neutralized with triethanol amine (CGT-n)/hexadecyl trimethylammonium salt (CTAB or CTAC) systems, where n is a degree of neutralization. Micellar phase appears in wide composition range with respect to the surfactant mixing fraction in ternary phase diagrams at 25 degrees C. At high mixing fraction of cationic surfactant in the water/CGT-n/CTAB systems, one can observe a highly viscous micellar phase in which worm-like micelles are expected to form. Contrary to conventional systems in which worm-like micelles are formed, the zero-shear viscosity of the micellar solution in the water/CGT-n/CTAB system with n=1.2 increases with the addition of cationic cosurfactant and once decreases after a maximum, then increases again and decreases after the second maximum. At n=1.5 and 2, highly viscous solution is observed in the relatively wide range of surfactant mixing fraction instead of two maxima of the viscosity curve observed at n=1.2. In the case of CTAC instead of CTAB we can observe narrow composition range for the maximum viscosity. Frequency sweep measurements were performed on the highly viscous samples in the water/CGT-1.5/CTAB system. Typical viscoelastic behavior of worm-like micellar solutions is observed; i.e. the curves of storage (G') and loss (G") moduli make a crossover and the data points of G' and G" can be fitted to the Maxwell model. Relaxation time against the mixing fraction of two surfactants behaves similarly to the zero-shear viscosity change, whereas the plateau modulus continuously increases in the plateau region for the zero-shear viscosity curve.

Morphology, rheology and crystallization behavior of polylactide composites prepared through addition of five-armed star polylactide grafted multiwalled carbon nanotubes

Functionalized multiwalled carbon nanotubes (F-MWNTs) were prepared by covalent grafting of five-armed star polylactide (fa-PLA), and were characterized by thermogravimetric analysis (TGA), nuclear magnetic resonance (NMR) spectroscopy and Raman spectroscopy. A series of polylactide (PLA)/F-MWNTs composites was prepared via coagulation method. Several techniques were applied to investigate the effects of F-MWNTs on the morphology, melt rheology, and crystallization and melting behaviors of the PLA composites. The optical microscope (OM), field-emission scanning electron microscope (FE-SEM) and transmission electron microscopy (TEM) observations demonstrated that, in comparison with the case of PLA filled with pristine MWNTs, F-MWNTs case showed improved dispersion and interfacial adhesion. Oscillatory frequency sweep measurements showed that addition of about 2.0 wt% F-MWNTs led to a solidlike response where a percolated network structure formed, and the composites exhibited remarkable improvement of rheological properties in the melt state as compared with that of neat PLA. DSC measurements showed that F-MWNTs acted as a nucleating agent to enhance crystallization when below the percolation concentration, while also acted as a hindrance to retard crystallization above the percolation concentration. The double melting peaks on the DSC curves were attributed to melting of the crystals formed in the cold crystallization stage and the melting-recrystallization–remelting (mrr) event during heating, respectively.

Nature of steady flow in entangled fluids revealed by superimposed small amplitude oscillatory shear

We carry out a systematic investigation into steady-state shear behavior of six entangled solutions based on a superposition of continuous shear and small amplitude oscillatory shear (SAOS). During steady shear in the shear thinning regime, the superimposed SAOS frequency sweep measurements reveal characteristics of viscous liquids, e.g., terminal dynamics, on the experimental time scale of the reciprocal shear rate. The residual entanglement network retains the same level of elastic stiffness as the equilibrium system does. Consistent with the convective constraint release idea, chains in the network are forced to pass around each other as they must do so to undergo steady flow. When such a sample is examined at significantly short time scales, chains are unable to pass around and the signature of this residual entanglement is that the storage modulus is greater than the loss modulus at higher frequencies than the applied shear rate. The particle-tracking velocimetric observations confirm that whether shear banding is present or not does not affect the basic “terminal flow” character revealed by the superimposed SAOS.

In situ gelling properties of chitosan-thioglycolic acid conjugate in the presence of oxidizing agents

The rheological behaviour of chitosan-thioglycolic acid conjugate (chitosan-TGA) in the presence of four oxidizing agents was investigated. Chitosan-TGA was synthesized via amide bond formation between the primary amino group of chitosan and the carboxylic acid group of thioglycolic acid. The sol–gel phase transition of the polymer was determined by rheological measurements. Moreover, cytotoxicity of the gel in combination with each oxidizing agent was evaluated utilizing LDH and MTT assay. The modified chitosan displayed 1053 ± 44 μmol/g thiol groups. Results of rheological studies showed that 1% (m/v) chitosan-TGA without any oxidizing agents became gel within 40 min. In contrast, when the oxidizing agents hydrogen peroxide, sodium periodate, ammonium persulfate and sodium hypochlorite were added, respectively, gelation took place within a few minutes. Within 20 min, hydrogen peroxide having been added in a final concentration of 25.2 nmol/L increased dynamic viscosity of 1% (m/v) chitosan-TGA up to 16,500-fold. This can be explained by the formation of inter- and/or intramolecular disulfide bonds which were indirectly verified via the decrease in thiol groups. Additionally, evidence of an increase in cross-linking of thiolated chitosan as a function of time was provided by frequency sweep measurements. Furthermore, viability of Caco-2 cells having been incubated with chitosan-TGA/oxidizing agent systems assessed by MTT assay was 70–85% and the percentage of LDH release was only in case of the chitosan-TGA/ammonium persulfate system significantly ( p < 0.05) raising compared to the negative control. According to these results, chitosan-TGA/oxidizing agent combinations might be a promising novel in situ gelling system for various pharmaceutical applications such as a controlled drug release carrier or for tissue engineering.

Generation of Pseudo Time-Domain Holography from Frequency-Swept Measurements

This paper presents the methodology for generating pseudo-time-domain holography from frequency-swept measurements. This is an approximation to the time-domain holography (TDH) patented by the authors (J. Marti-Canales et al., 2000, J. Marti-Canales et al., 2002). It opens new possibilities for antenna diagnostics using conventional instrumentation, in the absence of time-domain measurements. Practical examples using two spaceborne antennas are provided and discussed.

Rheological study of the effect of shearing process and κ-carrageenan concentration on the formation of whey protein microgels at pH 7

Abstract Gelation under shear during phase separation is evaluated as potential process to prepare microstructured functional ingredients with tunable rheological properties. The physical and chemical reactions leading to gelation have been extensively studied under quiescent conditions. However, a clear understanding of the correlation between microstructure and bulk rheological properties under shear is still lacking. Our work reports a number of interesting new findings, regarding WPI–κ-car gel behaviour. Steady shear flow behaviour and dynamic viscoelastic properties of WPI–κ-car (pH 7.0) were evaluated at 80 °C. The steady viscosity was determined at shear rates from 0.02 to 200 s −1 . Frequency sweep measurements of the dynamic viscosity were performed in the angular frequency range of 0.015–15 Hz at a strain of 0.5%. Confocal microscopy was used to look at the gels microstructure after gelation and demixing process. The effect of shear treatment applied during heating was studied on 5% WPI mixes with varying κ-car concentrations (0%, 0.2%, 0.3%, 0.5% and 0.6%). A steep increase in viscosity was observed at κ-car concentration more than 0.3% and was related to segregating conditions. It was concluded that the low shear Newtonian viscosity for κ-car in isolation changed linearly with κ-car concentration. However, in mixtures, the low shear Newtonian viscosity changed non-linearly with κ-car concentration for non-sheared as well as for sheared systems. The effect of varying shear rates (3.6, 14.4, 43.2 and 86.4 s −1 ) was also studied on a 5% WPI–0.5% κ-car mixture. A competition arises between the gelation and the phase separation depending on whether shear is applied during or after gelation. Applying shear during gelation modifies the microstructure and the viscoelastic properties of the microgels. This work brings further understanding on the segregating interactions taking place between whey proteins and κ-car when used as functional ingredients in dairy products.

Recent Developments on Impedance Measurements With DSP-Based Ellipse-Fitting Algorithms

In this paper, recent advances of a new digital-signal-processor (DSP)-based impedance measurement instrument under development are presented. The digital signal processing algorithms are based on ellipse fitting for the extraction of the acquired sine signal parameters. so that the impedance magnitude and phase can be determined. Special attention is devoted to the improvement of the algorithm's efficiency, i.e., by enabling the acquisition of a large number of samples by processing nonconsecutive data segments with no extra memory requirements. This capability is shown to reduce the experimental uncertainties of the estimated parameters. The systematic errors caused by the two different acquisition channels are measured and taken into account. The combined experimental measurement uncertainty is evaluated for the frequency sweep measurement of a particular impedance.

Effect of trypan blue on the elasticity of the human anterior lens capsule

To evaluate the effect of trypan blue (VisionBlue) staining on the elastic properties of the human anterior lens capsule. Center for Vision Science, Ruhr University Eye Hospital, Bochum, Germany. Nineteen anterior capsules were obtained from 19 human eyes at the time of cataract operation. Two same-size strips were prepared from each capsule; 1 was exposed to trypan blue staining for 10 seconds, and the other was a control. The elasticity measurements were performed on both specimens using a modified rheometer. The mean age of the patients was 75.3 years +/- 13.3 (SD) (range 35 to 89 years). Sixty-three measurements of 19 suitable specimen pairs were performed. In 13 pairs, the elasticity measurements were eligible for statistical analysis. The stress value at the tearing point was statistically significantly lower in the treated group (mean 169 +/- 55 Pa) than in the control group without staining (mean 252 +/- 67 Pa) (P = .023). The difference in stress between the stained and control samples at 3 seconds and 5 seconds was not statistically significant (P = .093 versus P = .316). Dynamic frequency sweep measurements showed a statistically significant elevation of the viscosity modulus in the samples stained with trypan blue. Trypan blue staining affected the biomechanical properties of the human lens capsule and led to a significant reduction in elasticity and an increase in stiffness. These results confirm clinical reports of changes in the capsule by dye and should be taken into consideration in dye-enhanced cataract surgery.

Rheological properties of carboxymethyl cellulose (CMC) solutions

In this study, we investigated the way of predicting two critical concentrations of sodium carboxymethyl cellulose (CMC) solutions using simple experimental procedures with a rotational rheometer. It was found that, above a critical shear rate, all CMC solutions (0.2 to 7 wt.%) exhibit shear-thinning behavior and the flow curves could be described by the Cross model. A first critical CMC concentration c*, transition to semidilute network solution, was determined using the following methods (1) study of the flow curve shapes, (2) Cross model parameters, (3) plot of the specific viscosity vs the overlap parameter, and (4) empirical structure–properties relationships. Furthermore, both creep and frequency-sweep measurements showed that the solutions behaved as viscoelastic materials above a second critical CMC concentration c** (transition to concentrated solution). The characterization of CMC solutions was completed with a time-dependent viscosity study that showed that the CMC solutions exhibited strong thixotropic behavior, especially at the highest CMC concentrations.

Constant Strain Frequency Sweep Measurements on Granite Rock

Like many materials, granite exhibits both nonlinear acoustic distortion and slow nonequilibrium dynamics. Measurements to date have shown a response from both phenomena simultaneously, thus cross-contaminating the results. In this Letter, constant strain frequency sweep measurements eliminate the slow dynamics and, for the first time, permit evaluation of nonlinearity by itself characterized by lower resonance frequencies and a steeper slope. Measurements such as these are necessary for the fundamental understanding of material dynamics, and for the creation and validation of descriptive models.

Nonlinear Viscoelasticity of Sorbitan Tristearate Monolayers at Liquid/Gas Interface

The interfacial rheology of sorbitan tristearate monolayers formed at the liquid/air interface reveal a distinct nonlinear viscoelastic behavior under oscillatory shear usually observed in many 3D metastable complex fluids with large structural relaxation times. At large strain amplitudes (gamma), the storage modulus (G') decreases monotonically whereas the loss modulus (G'') exhibits a peak above a critical strain amplitude before it decreases at higher strain amplitudes. The power law decay exponents of G' and G'' are in the ratio 2:1. The peak in G'' is absent at high temperatures and low concentration of sorbitan tristearate. Strain-rate frequency sweep measurements on the monolayers do indicate a strain-rate dependence on the structural relaxation time. The present study on sorbitan tristearate monolayers clearly indicates that the nonlinear viscoelastic behavior in 2D Langmuir monolayers is more general and exhibits many of the features observed in 3D complex fluids.