Volume 21, Issue 3

Rheological behaviour of culture broth stands as a fundamental parameter in bioprocess performances because it affects simultaneously the heat and mass transfer as well as the flow pattern. On-line measurements of rheological behaviour are hardly compatible with the operating condition with respect to accurate and stringent conditions imposed by cell culture strategy. Our scientific and technical objectives are (i) to develop and identify an experimental device enabling on-line rheometry and (ii) to discuss and compare on-line and off-line measurements. In this aim, a bioreactor was equipped with a derivation loop including a specific on-line rheometric device as well as additional physical and biological measurements (specific density, mass flow rate, electrical conductivity, pH, pO 2 and temperature) during microbial cell cultures. In a first time, friction curves of calibrated ducts were established with Newtonian and non-Newtonian shear-thinning fluids. In a second time, axenic cultures with two microorganisms (bacteria and yeast exhibiting different sizes) were investigated in pure oxidative culture in order to produce biomass under high cell concentrations: ~ 40 to 110 g CDW /l for E. coli (bacteria) and ~ 75 to 105 g CDW /l for Y. lipolytica (yeast). Cell broths exhibited Newtonian behaviour for E. coliand shear-thinning behaviour for Y. lipolytica, which were both dependant on biomass concentration. On-line and off-line rheological measurements are consistent for E. coli and Y. lipolytica, but significantly differed. On-line estimated viscosity appears higher than off-line apparent viscosity. Several assumptions in relation with microorganism physiology and metabolism (size, morphology, surface properties, concentration, biological activity) could be formulated in agreement with scientific literature. On-line rheology brings new insight to investigate complex interaction between physical and biological phenomena.

The interaction between magnetic particles in a bed fluidized by a gas is determined by the magnetizing action of an externally applied magnetic field. As the strength of the field is increased there comes a point at which the bed transits from a fluidlike to a solidlike stable state. Interparticle attractive forces induced by the applied field causes chainlike agglomeration of the particles, which confers the stabilized structure with a mechanical strength. In this paper we report experimental results on the yield stress of fluidized beds of fine magnetic particles stabilized by an externally applied magnetic field. Our results show that, in analogy with magnetorheological fluids (MRFs), particle structuring determines essentially the yield stress of magnetofluidized beds (MFBs). Moreover, our work shows that the dependence of the yield stress on particle size, which stands as a controversial issue in the study of MRFs, can be understood from the analysis of the jamming transition as affected by the size of the particles and the strength of the field.

A set of 31 cosmetic emulsions, as the most frequent cosmetic dispersions, comprising lotions and creams (o/w, w/o), was analyzed by rheological procedures (RheoStress 300, Thermo Fischer Scientific) and by sensory profiling. The power law model was used for pseudoplastic body lotions and the Herschel-Bulkley model for viscoplastic creams to get basic rheological parameters (apparent viscosity, consistency parameter, yield stress value, plastic viscosity and flow behaviour index). The content of TiO2 in sun lotions probably caused better agreement with viscoplastic creams. Rheological analysis proved to be more suitable for the storage stability testing of the emulsion than sensory evaluation. Psychorheology was applied as a suitable complex method. Rheological parameters were compared to sensory texture attributes (removing from a package, ease of spreading, skin feel and thickness). Almost 60% of relationships among rheological and sensory parameters were statistically significant (P = 0.05). Considering relationships only between rheological and sensory characteristics (with each other), 46 % were statistically significant (P = 0.05). In the case of apparent viscosity and removing the lotion from a bottle the relationship was reliable enough (correlation coefficient 0.91) to estimate the sensory attribute by fast rheological measurement. The other statistically significant relationships (correlation coefficients 0.53-0.80) proved that the sensory texture acceptability of a cosmetic emulsion could be partly predicted by rheological analysis.

In this study the steady flow viscosity, complex viscosity and relative viscosity of PP/FeSi composite melts with filler contents up to x =0.7 (70 vol.%) of spherical Iron Silicon (FeSi) microparticles (in 10 vol.% steps) with diameter of d < 106 mm have been investigated. Plate-plate and capillary rheometry at different shear rate and angular frequency in the range from 0.12 to 16000 s -1 (rad/s) were used. The results show an inflection point at high filler contents x ≥ 0.4 (40 vol.%) and low shear rates (< 1 s -1 ) caused by particle/particle interactions. With increasing shear rate and angular frequency the typical shear thinning flow behaviour of polymer melts was found for all investigated filler contents. The viscosity increases with increasing filler content at constant shear rate and angular frequency. Time-temperature-superposition (TTS) and Cox-Merz relation were fulfilled at filler contents up to x =0.3 (30 vol.%). The results of complex viscosity were normalized to a superimposed master curve. The three parameter of the new modified Carreau model were calculated for PP melt and shifted by two factors depending on the filler content. Based on these modifications, the complex viscosity was calculated for each filler content up to x = 0.3. The complex viscosity graphs were reduced to a general master curve of the investigated PP/FeSi composites up to x = 0.3. Finally, the relative viscosity of the examined composites shows the best fit to the Quemada model.