Strain-controlled Rheometer Research Articles

Time dependent flow in equimolar micellar solutions: transient behaviour of the shear stress and first normal stress difference in shear induced structures coupled with flow instabilities

Recently we studied time dependent structural changes that are coupled with flow instabilities (Fischer 1998; Wheeler 1998; Fischer 2000). Within a stability analysis, a classification scheme for the feedback circuit of coupled shear-induced structure and flow instabilities was derived by Schmitt et al. (1995) and applied to our samples. Here, inhomogeneous flow layers of different concentration and viscosity are generated by shear-induced diffusion (spinodal demixing) and, as consequence, one no longer observes a homogeneous solution but a type of shear banding that is seen here for the first time. In this paper we present the behaviour of the first normal stress difference observed in the critical shear-rate regime where transient shear-induced structure is coupled with flow instability. Similar to the oscillations of the shear stresses (strain-controlled rheometer) one observes oscillations in the first normal stress difference. This behaviour indicates that elastic structures are built up and destroyed while the shear-induced structures occur and that the induced phase is more elastic than the initial one. Oscillations of shear stress and first normal stress difference are in phase and indicate that both phenomena are caused by the same mechanism.

Viscoelastic properties of MR fluids

Mechanical properties of magnetorheological (MR) fluids are classified into pre-yield and post-yield regions according to whether the shear stress is below or above the yield stress. MR fluids within the pre-yield region exhibit viscoelastic properties; and these properties are important for understanding MR suspensions, especially for vibration damping applications. MR suspensions composed of reduced iron powders dispersed in silicone oil are utilized to study the viscoelastic properties with the help of a strain-controlled rheometer with plate-plate configuration. Two types of experiments, i.e. strain-amplitude sweep mode and frequency-sweep mode, were carried out to investigate the viscoelastic properties of MR fluids: (a) strain-amplitude sweep mode, the strain amplitude was swept from 0.0001 to 0.001 at a fixed driving frequency of 10 Hz. (b) Frequency-sweep mode, the driving frequency was swept from 1 Hz to 100 Hz at a constant strain amplitude of 0.001. The results show that both storage modulus G´ and loss modulus G´´ decrease with the increment of the strain amplitude, however, the loss factor increases with the increment of the strain amplitude. On the other hand, both the storage modulus and loss modulus increase with the increment of frequency, which is different from the loss factor. Moreover, the effects of magnetic field and volume fraction on the viscoelastic properties are also investigated. The higher the magnetic field, the higher the storage modulus and loss modulus, and the lower the loss factor. The higher the volume fraction, the higher the storage modulus and the loss factor.

Determination of the viscosity of an amorphous drug using thermomechanical analysis (TMA).

To evaluate thermomechanical analysis (TMA) as a technique for determining the viscosity of amorphous pharmaceutical materials. This property of amorphous drugs and excipients is related to their average rate of molecular mobility and thus to their physical and chemical stability. Indomethacin was selected as a model amorphous drug whose viscosity has previously been reported in the literature. A Seiko TMA 120C thermomechanical analyzer was utilized in isothermal penetration mode to determine the viscosity of the amorphous drug over the maximum possible range of temperatures. Using a cylindrical penetration geometry it was possible to accurately determine the viscosity of amorphous indomethacin samples by TMA over the temperature range from 35 to 75 degrees C. The results were consistent with those reported in the literature using a controlled strain rheometer over the range 44-75 degrees C. The limiting lower experimental temperature for the TMA technique was extended to significantly below the calorimetric glass transition temperature (Tg approximately 42 degrees C), thus allowing a direct experimental determination of the viscosity at Tg to be made. Thermomechanical analysis can be used to accurately determine the viscosity of amorphous pharmaceutical materials at temperatures near and above their calorimetric glass transition temperatures.

Molecular mobility of supercooled amorphous indomethacin, determined by dynamic mechanical analysis.

To determine the viscosity and the frequency-dependent shear modulus of supercooled indomethacin as a function of temperature near and above its glass transition temperature and from these data to obtain a quantitative measure of its molecular mobility in the amorphous state. Viscoelastic measurements were carried with a controlled strain rheometer in the frequency domain, at 9 temperatures from 44 degrees to 90 degrees C. The viscosity of supercooled indomethacin shows a strong non-Arrhenius temperature dependence over the temperature range studied, indicative of a fragile amorphous material. Application of the viscosity data to the VTF equation indicates a viscosity of 4.5 x 10(10) Pa.s at the calorimetric Tg of 41 degrees C. and a T0 of -17 degrees C. From the complex shear modulus and the Cole-Davidson equation the shear relaxation behaviour is found to be non-exponential, and the shear relaxation time at Tg is found to be approximately 100 sec. Supercooled indomethacin near and above its Tg exhibits significant molecular mobility, with relaxation times similar to the timescales covered in the handling and storage of pharmaceutical products.

Nonlinear viscoelasticity and the thrombelastograph: 1. Studies on bovine plasma clots

To study the possible role of nonlinear viscoelastic effects in the thrombelastograph (TEG), clotting of bovine plasma was studied by both thrombelastography and with a controlled strain rheometer. Clot rheology is dominated by elastic effects at frequencies of interest. There is a well-defined regime of linear elasticity for strains less than around 2%, while at larger strains the clots show significant strain hardening. Oscillatory shear applied during clotting has little effect on the resulting clot provided that the strain is less than 2%, but leads to substantial weakening of clots formed at larger strains. The TEG operates within the regime of nonlinear elasticity, significantly obscuring the interpretation of TEG amplitude in terms of an elastic modulus. Comparing the results of standard TEG experiments with those conducted with a modified TEG, having no oscillation during clotting, shows that deformation during standard thrombelastography leads to weaker clots than are produced under quiescent conditions.