Small-amplitude Oscillatory Strain Research Articles

Orthogonal superposition rheometry of model colloidal glasses with short-ranged attractions

Attractive colloidal glasses are unique as their dynamical arrest is a combination of entropic crowding effects and energetic bonds formation. When such systems are subjected to flow, their dynamics are activated in a way which differs from hard-sphere glasses that melt through a “convective cage release mechanism.” Here, we investigate the microscopic dynamics by measuring the relaxation spectrum during flow using orthogonal superposition rheometry. A small amplitude oscillatory strain is imposed perpendicularly to a steady shear flow, and superposition moduli are measured. Brownian dynamic simulations are utilized complementary to extract the moduli from mean-squared displacements using the generalized Stokes–Einstein relation. At low Péclet number, a crossover frequency between elastic and viscous moduli is detected, representing the relaxation time associated with shear-induced particles escape from their frustration (localization) under flow. For the repulsive glass, this is related to shear-induced cage renewal of particles. For attractive glasses, however, when particles escape their localized length (bonds), they move with no further hindrance with the escape time, which is independent of attraction strength and interestingly faster than that in the repulsive glass. This is attributed to particle localization at shorter length scales due to bonding. At high Péclet, a second low frequency crossover is observed and a low frequency elastic dominated response emerges. This elastic response may originate from slow relaxation of hydroclusters or be a consequence of more intricate nature of superposition moduli. At high frequencies, both orthogonal moduli increase relative to quiescent state due to shear-induced cage deformation, which slows down in-cage dynamics.

Waterless single screw extrusion of pasta-filata cheese: Process design based on thermo-rheological material properties

Process design of a waterless single screw extrusion of pasta-filata cheese was established based on the material properties during the major steps plasticization, conveying and texturization. Thermo-rheological properties of model cheese curds (pH 5.8 adjusted by citric acid) were determined by small amplitude oscillatory strain experiments (20 – 80 °C) to narrow a promising range of the process temperature ϑprocess. It was shown that ϑprocess should be set according to the following relation: ϑgel-sol (full plasticization) ≤ ϑproduct ≤ ϑtanδmax (peak flowability). Critical shear rates (γ˙crit) of plasticized model cheese were analyzed by a capillary rheometer and translated to the screw speeds (SS) by considering the default configuration of the waterless extruder. Upscaling of process conditions was evaluated with regards to product homogeneity and chemical composition, resulting in the most promising parameters, ϑprocess = 63 °C and SS = 10 rpm. Experiments using fermented and ripened cheese curd (pH 5.0) corroborated the proposed process design strategy.

Harmonic strain-optical response revealed in the isotropic (liquid) phase of liquid crystals

A strong optical birefringence is observed when applying a small amplitude oscillatory strain to the liquid phase of a liquid crystal. This unpredicted birefringence is found to oscillate at the same frequency as the driving frequency, with frequencies down to 0.01 Hz. This birefringence is visible up to 15 °C above the liquid crystal transition. This opto-dynamic property is interpreted as a result of a coupling of the orientational pretransitional fluctuations existing in the isotropic phase and long range elastic interactions recently identified in liquids. The conversion of the mechanical wave in an optical response is shapeable. Two examples of synchronized periodic signals are shown: the sine and the square waves. The optimization of the signal is analyzed using a Heaviside-step shear test. This optical property is immediately exploitable to design low energy on/off switching materials.

Effect of alkalis on konjac glucomannan gels for use as potential gelling agents in restructured seafood products

Four dispersions of 3% glucomannan in water, deacetylated with 5% 0.6 N and 1 N KOH (lots L1 and L2) and 0.6 N and 1 N NaOH (lots L3 and L4) as gelling agents, were evaluated for use in raw restructured seafood products. Several properties (pH, moisture content, water binding capacity, cooking loss and lightness) together with puncture data (breaking force and breaking deformation) were determined after 1 and 10 days of chilled storage at 5 °C. All these data were analyzed together with different viscoelastic parameters obtained at small amplitude oscillatory strain (SAOS) after 1 day of chilled storage, showing that L1 and L4 samples were the most suitable gels for incorporation in raw restructured fish products. In both cases the highest stress ( σ max) and strain ( γ max) amplitude values were found in the linear viscoelastic (LVE) range; however, L1 showed both high strain amplitude and breaking deformation values. Moreover, creep and recovery (transient) data showed that L1 was the most time-stable gel with the highest elasticity and the lowest relaxation exponent ( n). L4 gel showed strong rigidity, i.e. the highest values of breaking force and storage moduli ( G′) and the highest n value, making it less gel-like. Both L1 and L4 gels became significantly less gel-like over 10 days of chilled storage due to the loss of gel strength ( S) and a noticeable increase of n. These chilled storage effects were more intense in L4 than in L1.

Rheological properties of yeasted and nonyeasted wheat doughs developed under different mixing conditions

AbstractBACKGROUND: In order to explore the differences in rheological behaviour of full formula and flour/water doughs due to mixing, small‐amplitude oscillatory strain (SAOS) rheological measurements were made on fully developed nonyeasted and inactivated yeasted dough formulations mixed at various speeds using the Brabender Farinograph and the Bohlin ReoMixer. These results were compared with large‐strain empirical rheological results (including Kieffer rig uniaxial extension) as well as baking test results to determine differences due to mixer speed and/or flow distribution.RESULTS: The uniaxial extension and baking tests detected mixing speeds with incomplete dough development. Above those speeds, energy input to peak development was relatively constant in the Farinograph. Extensibility trends showed increases with speed in the yeasted dough samples, which were attributed to variation in fermentation time during mixing to peak torque at different speeds. While SAOS results did not show differences due to mixing speed, they did detect differences between the yeasted and nonyeasted dough formulations, as well as significant differences (P < 0.01) between yeasted doughs mixed in the two different mixers.CONCLUSION: The results indicate that known differences in the distribution of elongational and shear flows in the two mixers impact the development of dough structure during mixing. Copyright © 2008 Society of Chemical Industry

Instrumental Aspects of Dynamic Two-Dimensional Infrared Spectroscopy

Dynamic two-dimensional infrared (2D IR) correlation maps are a convenient means of examining the information contained in time-resolved IR spectra. Dynamic 2D IR spectra can be collected with the use of either dispersive or Fourier transform (FT) IR spectrometers. Use of a step-scanning FT-IR spectrometer has advantages over conventional rapid-scan FT-IR spectrometry when one is acquiring time-resolved IR data on time scales faster than about 0.1 s, because the spectral multiplexing is removed from the time domain. Dynamic IR spectra of atactic polystyrene (undergoing a small-amplitude oscillatory strain) collected on both dispersive and FT instrumentation are compared. Although the dispersive approach produces higher signal-to-noise ratios over small spectral regions, the multiplex advantage makes the FT approach attractive when broader spectral coverages are required. The first vibrational circular dichroism (VCD) spectrum [of (–)- α;-pinene] collected on a step-scanning interferometer is also presented.

Dynamic infrared linear dichroism study of high-density and low-density polyethylene near the β-transition temperature

High-density polyethylene (HDPE) and low-density polyethylene (LDPE) films under small-amplitude oscillatory strain were examined using time-resolved infrared (IR) linear dichroism spectroscopy. A negative dynamic dichroism at 1473 cm −1 and a positive dynamic dichroism at 1463 cm −1 were observed at +32°C (above the β-transition temperature, T β. This observation is interpreted to mean that above T β the orthorhombic crystallographic b axis reorients parallel to the direction of applied strain, while the crystallographic a axis reorients perpendicular to the strain direction for both HDPE and LDPE. Interestingly, the dynamic dichroism of both bands changes sign at −50°C (below T β), indicating that the dynamic reorientation directions for the crystalline a and b axes are shifted below T β. This result clearly shows that different deformation mechanisms operate above and below T β. Even though the IR peaks in this region are made up of overlapping bands from both crystalline and amorphous components, they are easily resolved in this experiment. The high spectral resolution was achieved because the electric dipole transition moments associated with individual bands reorient in different directions, at different rates, and to different extents under dynamic strain.

Dynamic IR Studies of Microdomain Interphases of Isotope-Labeled Block Copolymers

ABSTRACTBy probing the localized segmental motion of isotope-labeled block copolymers, the physical nature of the interphase region between microphaseseparated domains of block polymers was examined. Dynamic infrared linear dichroism (DIRLD) spectroscopy, which measures the reorientations of submolecular structures induced by a small-amplitude oscillatory strain, was combined with specific isotope-labeling using deuterium-substituted monomers. The latter technique enabled us to differentiate the dynamic responses of well-defined parts of block segments, e.g., near the segment junction, chain end, or middle of the block. The degree of segmental interactions near the interphase region of styrene-isoprene diblock copolymers were studied as a function of the segment location and temperature. The reorientational motion of the polystyrene segment, especially near the block junction, was monitored around the glass transition temperature of the polyisoprene matrix. From this result, the degree of segmental mixing in the interphase region which leads to local plasticization of the polystyrene segment was determined.

A Spectrometer for Measuring Time-Resolved Infrared Linear Dichroism Induced by a Small-Amplitude Oscillatory Strain

A spectrometer for detecting dynamic infrared linear dichroism (DIRLD) induced by a small-amplitude oscillatory strain is described. The strain-induced dynamic variations of absorbance and linear dichroism, as well as the normal static absorbance and dichroism, are measured simultaneously as functions of IR wavenumber, temperature, and strain frequency. The phase (temporal) relationships between the dynamic optical signals and the applied strain are also obtained. The instrument is sensitive enough to detect dynamic optical signals on the order of 10−4 absorbance units with a time resolution of about 14 μs. The dynamic dichroism signals arise from the strain-induced temporary reorientation of dipole-transition moments associated with the molecular vibrations of chemical functional groups. Both the rate and extent of reorientations are strongly influenced by the local molecular environment of the functional groups. Because of the specificity of IR absorbance bands to the individual submolecular structures, DIRLD spectroscopy is especially suited for the study of the intra- and intermolecular interactions and the detection of subtle changes in the local molecular environment. Example DIRLD spectra of atactic polystyrene are presented to demonstrate the potential utility of this spectroscopic technique.