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• Non-linear viscoelasticity  (1)
• Rheological equation  (1)
• culture  (1)
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• 1
Electronic Resource
Springer
Rheologica acta 38 (1999), S. 14-25
ISSN: 1435-1528
Keywords: Key words Fumed silica ; Colloidal suspensions ; Non-linear viscoelasticity ; Rheological models
Source: Springer Online Journal Archives 1860-2000
Topics: Chemistry and Pharmacology , Physics
Notes: Abstract Suspensions of fumed silica exhibit a wide range of rheological properties depending on the nature and magnitude of the interparticle forces. In a non-polar fluid, the particles interact through hydrogen bonding and can form a three-dimensional network. The microstructure formation is responsible for the non-linear viscoelastic behavior of fumed silica suspensions, even at very small strain. These non-linear rheological properties have been studied in small amplitude oscillatory experiments as a function of particle size, surface treatment of particles, suspending medium polarity and solids concentration. The non-linear viscoelastic behavior is characterized by a non-sinusoidal waveform of the signal response. For suspensions in a non-polar fluid, both the elastic and the loss moduli are shown to be sensitive to the strain amplitude: the elastic modulus is decreasing with increasing strain whereas the loss moduli is initially increasing with strain. We have chosen to examine the dissipated energy which is clearly related to the breakup of the suspension structure. A comparison of model predictions and the experimental data shows the limitations of these models, recently proposed in the literature to describe the behavior of colloidal suspensions.
Type of Medium: Electronic Resource
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• 2
Electronic Resource
Springer
Rheologica acta 27 (1988), S. 241-254
ISSN: 1435-1528
Keywords: Rheological equation ; dilute polymersolution ; polyelectrolyte ; shear thickening ; polyacrylamide solution
Source: Springer Online Journal Archives 1860-2000
Topics: Chemistry and Pharmacology , Physics
Notes: Abstract A mathematical model based on the diffusion-convection equations is used to describe the rheological properties of dilute polymer solutions. The model uses a second-order conformation tensor as a measure of the internal strain; this avoids the mathematical complexity resulting from the use of a more detailed description of the macromolecules and also avoids the necessity of introducing additional ad-hoc assumptions (closure approximations) commonly used in other molecular theories. The rheological equation is obtained in terms of the rate-of-deformation tensor $$\dot \gamma$$ and a scalar functionf(σ) relating the extra stress tensorσ to the internal strain tensorc. The functionf(σ) depends on the physical insight introduced in the Helmholtz free energyA(c) of the solvent-polymer system. This approach is illustrated for an intra-molecular potential of a “FENE-charged” type. The concept of an isotropic, but conformation-dependent, friction coefficient, is also introduced to account for the “coil-stretch” transformation of macromolecules in solution. Viscosity and first normal-stress data, of partially hydrolyzed polyacrylamide solutions, (polyelectrolytes) are analyzed and compared to the model predictions in steady shear and elongational flows.
Type of Medium: Electronic Resource
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• 3
Electronic Resource
New York, NY [u.a.] : Wiley-Blackwell
Biotechnology and Bioengineering 39 (1992), S. 511-521
ISSN: 0006-3592
Keywords: plant cell ; Catharanthus roseus ; suspension ; culture ; mixing ; helical ribbon impeller ; bioreactor ; Chemistry ; Biochemistry and Biotechnology
Source: Wiley InterScience Backfile Collection 1832-2000
Topics: Biology , Process Engineering, Biotechnology, Nutrition Technology
Notes: A double helical-ribbon impeller (HRI) bioreactor with a 11-L working volume was developed to grow high-density Catharanthus roseus cell suspensions. The rheological behavior of this suspension was found to be shear-thinning for concentrations higher than 12 to 15 g DW · L-1. A granulated agar suspension of similar rheological properties was used as a model fluid for these suspensions. Mixing studies revealed that surface baffling and bottom profiling of the bioreactor and impeller speeds of 60 to 150 rpm ensured uniform mixing of suspensions. The HRI power requirement was found to increase singnificantly for agar suspensions higher than 13 g DW · L-1, in conjunction with the effective viscosity increase. Oxygen transfer studies showed high apparent surface oxygen transfer coefficients (kLa ∼4 to 45 h-1) from agar suspensions of 30 g DW · L-1 to water and for mixing speeds ranging from 120 to 150 rpm. These high surface kIa values were ascribed to the flow pattern of this bioreactor configuration combined with surface bubble generation and entrainment in the liquid phase caused by the presence of the surface baffles. High-density C. roseus cell suspension cultures were successfully grown in this bioreactor without gas sparging. Up to 70% oxygen enrichment of the head space was required to ensure sufficient oxygen supply to the cultures so that dissolved oxygen concentration would remain above the critical level (≥10% air saturation). The best mixing speed was 120 rpm. These cultures grew at the same rate (∼0.4 d-1) and attained the same high biomass concentrations (∼25 to 27 g DW · L-1, 450 to 500 g filtered wet biomass · L-1, and 92% to 100% settled wet biomass volume) as shake flask cultures. The scale-up potential of this bioreactor configuration is discussed.