Viscoelastic properties of pig kidney in shear, experimental results and modelling

 In this paper, the results from a series of rheological tests of fresh pig kidney have been reported. Using a standard strain-controlled rheometer, the oscillation strain sweep experiment showed a linear viscoelastic strain limit of the order of 0.2% strain. To determine the components of dynamic moduli in terms of frequency, shear oscillation tests were done at strain 0.2% using a stress-controlled rheometer. Shear stress relaxation tests were carried out with a fixed strain of 0.2% and 0.02 s rise time. The model we have developed uses a multi-mode upper convected Maxwell (UCM) model with variable viscosities and time constants, to which we have added a Mooney hyper-elastic response, both multiplied by a damping function. Different forms of damping functions that control the non-linearity of strain-stress profile have been tested. The model was fitted to our experimental data, and matched the entire test data reasonably well with a single set of parameters. Received: 5 May 2000 Accepted: 16 March 2001     

Headgroup effect on drag reduction and rheological properties of micellar solutions of quaternary ammonium surfactants

Two sets of cationic surfactants each with essentially the same alkyl chains but different headgroup structures were studied to investigate the effects of surfactant headgroup structure on micelle microstructures, drag reduction (DR) and rheological properties at certain counterion and surfactant concentrations. Cetyldimethylethylammonium bromide (CDMEAB) was compared with alkyltrimethyl ammonium bromide (CnTAB) and benzyldimethyl(hydrogenated tallow)ammonium chloride (DMHTB) was compared with alkyltrimethylammonium chloride (CmTAC), respectively. Surfactants with larger headgroups showed lower high temperature limits for DR. CDMEAB systems have better DR abilities than CnTAB below room temperature but the opposite is true at higher temperatures. DMHTB has stronger counterion binding ability than CmTAC, giving better DR properties than CmTAC at low counterion concentration, but has a lower upper temperature limit for DR. These results provide further understanding of the self-assembly nature of threadlike micelles of cationic surfactants and guidance for design of effective surfactant structures to meet particular DR requirements.     

A new method to determine viscosity of liquids using vibration principles

A new method for determining viscosity of liquids is examined. The method employs the principles of vibration and measures the viscous damping due to the motion of a liquid placed in a cylindrical tube. The apparatus and the test liquid are treated as a dynamic system and the measured mechanical impedances are used to calculate energy dissipation due to the viscous damping. The newly designed apparatus is able to generate shear deformations in the liquid without using moving solid surfaces. A harmonic varying force with a frequency close to the resonance frequency of the system is applied through a piston and the resulting velocities of the oscillations generated in the system are measured. Liquids with higher viscosities result in lower velocities due to the higher damping. Analytical equations are provided to relate the viscous damping of the dynamic system to the viscosity of the liquids. The viscosities obtained from the proposed method are in good agreement with the ones obtained from standard rotational viscometry using a cone and plate geometry.     

Rheological behavior of concentrated suspensions as affected by the dynamics of the mixing process

The rheological characterizations of concentrated suspensions are generally carried out assuming “well-mixed” suspensions. However, the variation of the concentration distributions of the ingredients of the formulation, i.e., the “goodness of mixing”, the size and shape distributions of the particle clusters and the rheological behavior of the suspension all depend on the thermo-mechanical history that the suspension is exposed to during the mixing process. Here, various experimental tools are used for the characterization of the degree of mixedness (concentration distributions) of various ingredients along with the characterization of rheological material functions, wall slip behavior and the maximum packing fraction of a graphite/elastomer suspension. The degree of mixedness values of the ingredients of the suspensions processed using batch and continuous processes and under differing operating conditions were characterized quantitatively using wide-angle X-ray diffraction and thermo gravimetric analysis and were elucidated under the light of the electrical properties of the suspension as affected by the mixing process. Upon achieving better homogeneity of the graphite particles and the binder and decreases in the size and breadth of the size distributions of particle clusters (as inferred from electrical measurements and maximum packing fraction values), the elasticity (storage modulus) and the shear viscosity (magnitude of the complex viscosity from small-amplitude oscillatory shear and shear viscosity from steady torsional and capillary rheometry) of the suspension decreased significantly and the wall slip velocity values increased. These findings demonstrate the intimate relationships that exist between the rheological behavior of concentrated suspensions and the thermo-mechanical history that they are exposed to during the processing stage and suggest that the preparation conditions for suspensions should be carefully selected and well documented to achieve reproducible characterization of rheological material functions.     

Block copolymers at interfaces: Interactions with physiological media

Triblock copolymers (also known as Pluronics or poloxamers) are biocompatible molecules composed of hydrophobic and hydrophilic blocks with different lengths. They have received much attention recently owing to their applicability for targeted delivery of hydrophobic compounds. Their unique molecular structure facilitates the formation of dynamic aggregates which are able to transport lipid soluble compounds. However, these structures can be unstable and tend to solubilize within the blood stream. The use of nanoemulsions as carriers for the lipid soluble compounds appears as a new alternative with improved protection against physiological media. The interfacial behavior of block copolymers is directly related to their peculiar molecular structure and further knowledge could provide a rational use in the design of poloxamer-stabilized nanoemulsions. This review aims to combine the new insights gained recently into the interfacial properties of block copolymers and their performance in nanoemulsions. Direct studies dealing with the interactions with physiological media are also reviewed in order to address issues relating metabolism degradation profiles. A better understanding of the physico-chemical and interfacial properties of block copolymers will allow their manipulation to modulate lipolysis, hence allowing the rational design of nanocarriers with efficient controlled release.     

Photodegradation of isotactic poly(1-butene): Multiscale characterization

The effect of photodegradation in isotactic poly(1-butene) (PB-1) have been investigated using rheology, differential scanning calorimetry and infrared spectroscopy. Two commercially available grades of PB-1 with different average molecular weight were chosen. Specimens prepared by compression moulding were UV irradiated in the interval from 0 to 70 h. UV-induced changes in molecular structure have been followed by evolution of rheological properties, thermal properties and degradation by-products. Thermal analysis showed significant changes in crystallization behaviour influencing morphology and resulting thermal properties. Moreover it has been confirmed that the degradation significantly retards the phase transformation. Rheological measurement has been found as an effective method for determination of early stages of photodegradation of PB-1.     

Simulations of thixotropic liquids

In thixotropic liquids the rheological properties depend on the liquid’s deformation history. Clay suspensions (as encountered in oil sands mining) are a prominent example. Activated clay particles form a network. As a consequence of (ionic) transport limitations, the network is not an instantaneous feature; it takes time to build up, and also to break down, the latter as a result of deformation in the liquid. In this paper a procedure for detailed simulations of flow of viscous thixotropic liquids is outlined. The local viscosity of the liquid relates to the level of integrity of the network. The time-dependence of the liquid’s rheology is due to the finite rate with which the network in the liquid builds up or breaks down. This concept has been incorporated in a lattice-Boltzmann discretization of the flow equations. With this methodology the way thixotropic liquids are mobilized in mixing tanks and the drag force experienced by dense assemblies of coarse particles (e.g. sand) in thixotropic liquids have been studied.     

A new belt-type apparatus for neutron-based rheological measurements at gigapascal pressures

We have developed a new solid-media apparatus for performing rheological investigations at multi-gigapascal pressures. The pressure cell consists of a simple belt design and fits in a modified 250 tonne Paris–Edinburgh press. Elastic strains are measured by neutron diffraction, on the ENGIN-X experimental station at ISIS. Stresses are estimated from the measured strains in combination with published values of the elastic moduli. As an exemplair of the method, we present data from initial deformation experiments on pyrope garnet at 1.5 GPa and 873 K. Data collection times are as short as 60 min and the elastic strain resolution is better than 10^?4. We anticipate, however, that by interrupted testing, strain rates as low as 10^?9/s, or lower, will be measurable.