Structural Kinetic Model Research Articles

Rheological properties of soy protein isolate solution for fibers and films

For the first time, rheological behavior of soy protein isolate (SPI) solution with high concentration for fibers and films was studied. The results showed that SPI could be readily dissolved in urea/sodium sulfite aqueous system, and the SPI solution is suitable for wet spinning and film casting. The effects of concentration, temperature, and shear rates on the rheological properties of SPI solution were studied. Viscosity of SPI solution decreases with increasing temperature, which is good for the preparation of solution with high concentration and regenerated SPI fibers/films with good properties. All solution samples exhibited thixotropic behavior, which were different from traditional polymer solution for wet spinning and film casting process. Both in forward and backward measurements were carried out by the Power Law model. Under constant shear rates (80-160 s−1), 18 wt% SPI solution was chosen to study the dependence of viscosity on time. Second order structural kinetic model, Weltman model, first-order stress decay models with zero and none-zero equilibrium stress values were used to fit the rheological data. Among the four models, Weltman model is more appropriate to fit the thixotropic data.

Curve-fitting equation for prediction of the start-up stress overshoot of an oil-based drilling fluid

Drilling fluids usually gel at rest in order to avoid cuttings to precipitate over the drill bit when circulation is interrupted. At flow start-ups, pumping pressures higher than the steady-state circulation pressure are usually required to surpass the gel strength. The gel-liquid transition may have significant importance, especially in ultra-deep waters where high pressures and low temperatures take place. In the current work, controlled shear rate rheometric tests were conducted to investigate the yielding of an oil based drilling fluid. An algebraic equation, that accounts for both shear rate and shear history, is proposed to predict the gel breaking. This equation requires less fitting parameters than the current structural kinetic models available in literature, and is quite useful to represent the pressure peaks that take place during drilling fluid flow start-ups. The equation fit to rheometric data and is able to predict satisfactorily the start-up shear stress of a gelled drilling fluid at different shear rates.

Shear time dependent viscosity of polystyrene-ethylacrylate based shear thickening fluid

In this study, the influence of the shear rate and shear time on the transient viscosity of polystyrene-ethylacrylate based shear thickening fluid (STF) is investigated. If the shear rate is stepwise changed, it is found that both the viscosity and critical shear rate are affected by the shear time. Above the critical shear rate, the viscosity of the STF with larger power law exponent (n) increases faster. However, the viscosity tends to decrease when the shear time is long enough. This phenomenon can be responsible for the reversible structure buildup and the break-down process. An effective volume fraction (EVF) mechanism is proposed to analyze the shear time dependent viscosity and it is found that viscosity changes in proportion to EVF. To further clarify the structure evolution, a structural kinetic model is studied because the structural kinetic parameter (λ) could describe the variation in the effective volume fraction. The theoretical results of the structural kinetic model agree well with the experimental results. With this model, the change in viscosity and EVF can be speculated from the variation of λ and then the structure evolution can be better illustrated.

Influence mechanism of torrefaction on softwood pyrolysis based on structural analysis and kinetic modeling

The influence mechanism of torrefaction on biomass pyrolysis was studied in detail. 13C CP/MAS NMR was employed to characterize the evolution of functional groups during torrefaction, indicating that hemicellulose was extensively deacetylated and depolymerized; the crystalline structure of cellulose was destroyed; and the aryl ether linkages and propyl side branches in lignin were cleaved. A distributed activation energy model with three combined Gaussian functions was introduced to analyze the pyrolysis kinetics of biomass. The mean activation energies for three parallel reactions remained unchanged after torrefaction, while the weighing factors proved that their contributions to devolatilization largely changed. Acetic acid yield significantly decreased after torrefaction, with reduction of the yields of 5-hydroxymethylfurfural and levoglucosan because of the depolymerization of carbohydrates during torrefaction. Due to the dissociation of propyl branches and the demethylation of methoxyls in lignin, pyrolysis of torrefied biomass yielded more guaiacol and catechol and less phenols with C4-propyl groups.

A quantitative study of the forbidden and intercombination transitions arising from the Li-like Al autoionizing levels

This work presents results from detailed quantitative description of the level population kinetics responsible for the relatively high line-intensity of the forbidden and intercombination transitions arising from autoionization states. The goal is to investigate the influence of different atomic processes on the emission spectrum of the laser-produced aluminum plasma obtained at the nhelix-laser test bed facility at GSI (PRA 63, 032716). In the experiment a Nd-glass YAG laser beam having 50 J pulse energy, 15 ns full width half maximum duration and 1.064 μm wavelength is focused on a massive Al target. To simulate the experimental reported spectrum, the population kinetics is investigated based on a collisional radiative model. The theoretical spectroscopic investigation reported here indicates the existence of anomalous high intensity X-ray intercombination and two-electron transitions arising from autoionizing states in Li-like Al ion. Detailed atomic structure calculation and kinetics modeling is performed to find out the individual states involved, and the population density distribution over them. Presented results are useful for femtosecond-laser produced plasma experiments and diagnostics.

The influence of basil seed gum, guar gum and their blend on the rheological, physical and sensory properties of low fat ice cream

In the present paper, the functionality of basil seed gum (BSG), guar gum (GG) & their blend (MGB, 50:50), at the concentration of 0.35, 0.45, 0.50 and 0.55%, related to the rheological, physical and sensory characteristics of low fat ice cream (2.5% fat) was compared with full fat sample (10%) as control. Shear thinning and thixotropic behaviors of all mixes were well described by Herschel–Bulkley and second-order structural kinetic models, respectively. The results showed that BSG and MGB, by producing high consistency coefficient (k), yield stress (τ0), thixotropy rate constant (k), extent of thixotropy (η0/η∞) and low flow behavior index (n) values, provided satisfactory rheological properties in low fat ice cream. Fat reduction resulted in some defects which to some extent compensated by increasing levels of fat replacers. BSG & MGB exhibited a more shear-sensitive thixotropic nature than GG mixes and strongly favored perception of creaminess which was more than full fat mixes in some concentrations. In addition, they depressed the coldness and coarseness perception of low fat mixes. Addition of BSG reduced the meltdown rate and extent of melting at all concentration compared to the GG and MGB mixes, which two latter gum systems showed almost equal value of these properties. Furthermore, significant correlation between the values of melting rate data and rate of breakdown parameters of the mixes was observed. The results presented herein showed that BSG and its blend with GG are very suitable fat replacers/stabilizers for low fat ice cream.

Prelude to rational scale-up of penicillin production: a scale-down study

Penicillin is one of the best known pharmaceuticals and is also an important member of the β-lactam antibiotics. Over the years, ambitious yields, titers, productivities, and low costs in the production of the β-lactam antibiotics have been stepwise realized through successive rounds of strain improvement and process optimization. Penicillium chrysogenum was proven to be an ideal cell factory for the production of penicillin, and successful approaches were exploited to elevate the production titer. However, the industrial production of penicillin faces the serious challenge that environmental gradients, which are caused by insufficient mixing and mass transfer limitations, exert a considerably negative impact on the ultimate productivity and yield. Scale-down studies regarding diverse environmental gradients have been carried out on bacteria, yeasts, and filamentous fungi as well as animal cells. In accordance, a variety of scale-down devices combined with fast sampling and quenching protocols have been established to acquire the true snapshots of the perturbed cellular conditions. The perturbed metabolome information stemming from scale-down studies contributed to the comprehension of the production process and the identification of improvement approaches. However, little is known about the influence of the flow field and the mechanisms of intracellular metabolism. Consequently, it is still rather difficult to realize a fully rational scale-up. In the future, developing a computer framework to simulate the flow field of the large-scale fermenters is highly recommended. Furthermore, a metabolically structured kinetic model directly related to the production of penicillin will be further coupled to the fluid flow dynamics. A mathematical model including the information from both computational fluid dynamics and chemical reaction dynamics will then be established for the prediction of detailed information over the entire period of the fermentation process and thereby for the optimization of penicillin production, and subsequently also benefiting other fermentation products.

In silico optimization and low structured kinetic model of poly[(R)-3-hydroxybutyrate] synthesis by Cupriavidus necator DSM 545 by fed-batch cultivation on glycerol

Glycerol was utilized by Cupriavidus necator DSM 545 for production of poly-3-hydroxybutyrate (PHB) in fed-batch fermentation. Maximal specific growth rates (0.12 and 0.3h−1) and maximal specific non-growth PHB production rate (0.16gg−1h−1) were determined from two experiments (inocula from exponential and stationary phase). Saturation constants for nitrogen (0.107 and 0.016gL−1), glycerol (0.05gL−1), non-growth related PHB synthesis (0.011gL−1) and nitrogen/PHB related inhibition constant (0.405gL−1), were estimated. Five relations for specific growth rate were tested using mathematical models. In silico performed optimization procedures (varied glycerol/nitrogen ratio and feeding) has resulted in a PHB content of 70.9%, shorter cultivation time (23h) and better PHB yield (0.347gg−1). Initial concentration of biomass 16.8gL−1 and glycerol concentration in broth between 3 and 5gL−1 were decisive factors for increasing of productivity.

Time-independent and time-dependent rheological characterization of vegetable-based infant purees

Flow behavior of vegetable-based infant purees was analyzed at different temperatures (5–65°C) giving particular attention to their time-dependent properties in a shear rate range (5–200s−1). Power law model parameters describing flow behavior of samples depended on kind of infant puree, its water content and measurement temperature. Arrhenius model was used to explain temperature effect on apparent viscosity at 50s−1. Infant purees exhibited thixotropic behavior for all temperatures tested. For the same temperature, differences in hysteresis loop magnitudes were observed among purees, being more noticeable at lower temperatures. Two models were used to describe the time-dependent behavior, namely Weltman model, and second-order structural kinetic model. For all infant purees, the initial shear stress and the extent of thixotropy increased and decreased significantly with increases in shear rate and temperature. The breakdown rate of puree associations also accelerated at higher shear rates, but no trend was observed with temperature.

A MATLAB toolbox for structural kinetic modeling

Structural kinetic modeling (SKM) enables the analysis of dynamical properties of metabolic networks solely based on topological information and experimental data. Current SKM-based experiments are hampered by the time-intensive process of assigning model parameters and choosing appropriate sampling intervals for Monte-Carlo experiments. We introduce a toolbox for the automatic and efficient construction and evaluation of structural kinetic models (SK models). Quantitative and qualitative analyses of network stability properties are performed in an automated manner. We illustrate the model building and analysis process in detailed example scripts that provide toolbox implementations of previously published literature models. The source code is freely available for download at http://bioinformatics.uni-potsdam.de/projects/skm. girbig@mpimp-golm.mpg.de.

Systematic analysis of stability patterns in plant primary metabolism.

Metabolic networks are characterized by complex interactions and regulatory mechanisms between many individual components. These interactions determine whether a steady state is stable to perturbations. Structural kinetic modeling (SKM) is a framework to analyze the stability of metabolic steady states that allows the study of the system Jacobian without requiring detailed knowledge about individual rate equations. Stability criteria can be derived by generating a large number of structural kinetic models (SK-models) with randomly sampled parameter sets and evaluating the resulting Jacobian matrices. Until now, SKM experiments applied univariate tests to detect the network components with the largest influence on stability. In this work, we present an extended SKM approach relying on supervised machine learning to detect patterns of enzyme-metabolite interactions that act together in an orchestrated manner to ensure stability. We demonstrate its application on a detailed SK-model of the Calvin-Benson cycle and connected pathways. The identified stability patterns are highly complex reflecting that changes in dynamic properties depend on concerted interactions between several network components. In total, we find more patterns that reliably ensure stability than patterns ensuring instability. This shows that the design of this system is strongly targeted towards maintaining stability. We also investigate the effect of allosteric regulators revealing that the tendency to stability is significantly increased by including experimentally determined regulatory mechanisms that have not yet been integrated into existing kinetic models.

A novel and efficient epoxy/chitosan cement slurry for use in severe acidic environments of oil wells—Structural characterization and kinetic modeling

In this study, the biopolymer chitosan was used to synthesize a new epoxy/chitosan cement slurry. The features of the new slurry were evaluated in relation to a standard cement slurry (w/c=0.5). A kinetic study of the interaction epoxy/chitosan slurry/HCl was performed to simulate the use of the new slurry in environmental-friendly acidizing procedures of oil wells. The experimental data were well fitted to a three-parameter kinetic model. The analysis of the kinetic modeling suggests that surface reactions constitute the main interactions at the interface epoxy/chitosan-modified cement slurry/HCl. The characterization of the slurries was performed by FTIR, XRD, TG/DTG and solid-state reflectance spectroscopy. The results have pointed out that the main features of the new cement slurry were preserved, even after long-term contact with HCl in aqueous solution. The results of this study underline the excellent features of the new epoxy/chitosan-modified cement slurry for using in environmental-friendly acidizing procedures of oil wells.

Rheological and microstructural characterization of WPI-stabilized O/W emulsions exhibiting time-dependent flow behavior

The rheological behavior of oil-in-water (O/W) emulsions stabilized by whey protein isolate (WPI) and its relationship with the microstructural changes caused by shearing was studied. O/W emulsions (50, 55, and 60 g oil/100 g) were made using ultrasound and their rheological properties were determined by: flow curve test, constant shear rate test, and hysteresis loop test. Microstructural changes were evaluated in terms of droplet size and droplet size distribution. Emulsions containing 50 and 55 g oil/100 g showed a Newtonian behavior, whereas those with 60 g oil/100 g exhibited shear-thinning behavior. Under constant deformation, the apparent viscosity of the emulsions decreased with time. The hysteresis loop test revealed that increasing oil content increased the degree of thixotropy of the emulsions. Moreover, before and after the constant deformation test droplet size distributions did not show differences, indicating that the decrease in the apparent viscosity may be promoted by breakdown and further deformation and/or reorganization of oil droplets flocs. In turn, experimental data obtained from the constant shear rate test was fitted to a structural kinetic model. The rate constant values showed no particular trend with oil content and shear rate, implying that probably wall slip occurred at high shear rates and high oil contents.

Flow behaviour of carrot puree: modelling the influence of time, temperature and potato flakes addition

SummaryRheological properties of carrot puree were investigated in the 1–1000 s−1 shear rate range with the objective of modelling the influence of time, temperature and addition of potato flakes on the flow behaviour. Carrot puree exhibited a shear‐thinning behaviour that was well described by the Ostwald‐de Waele (Power Law) model with a flow behaviour index of 0.34 (±0.02) at 20 °C. The time‐dependent behaviour was characterised by a second‐order Structural Kinetic Model. The decay of the structural parameter with time was found to be independent of shear rate. The Arrhenius model was used to explain the effect of temperature in the range from 4 to 60 °C. The dry matter was increased by adding potato flakes (0–5%). A power law model (for the concentration) and the Arrhenius relationship (for the temperature) were combined to simultaneously describe the effects of temperature and concentration. This study provides essential data for equipment and process design.

Morphology influence on nanoscale magnetism of Co nanoparticles: Experimental and theoretical aspects of exchange bias

Co-based nanostructures ranging from core-shell to hollow nanoparticles were produced by varying the reaction time and the chemical environment during the thermal decomposition of Co2(CO)8. Both structural characterization and kinetic model simulation illustrate that the diffusivities of Co and oxygen determine the growth ratio and the final morphology of the nanoparticles. Exchange coupling between Co and Co-oxide in core/shell nanoparticles induced a shift of field-cooled hysteresis loops that is proportional to the shell thickness, as verified by numerical studies. The increased nanocomplexity when going from core/shell to hollow particles, also leads to the appearance of hysteresis above 300 K due to an enhancement of the surface anisotropy resulting from the additional spin-disordered surfaces.

Transformation of porous structure of anodic alumina films formed during galvanostatic anodising of aluminium

A method was developed revealing even slight variations of structural features around the barrier layer of porous anodic alumina films formed in H 2SO 4 electrolyte at conditions different from those of best ordering. This is based on the combination of a suitable structural–kinetic model with the average size of imprints of barrier layer units on the Al surface determined by FESEM. The galvanostatic growth of films in steady state is accompanied by rise of anodic potential with time depending on temperature. Simultaneously structural features around the barrier layer like cell width, pore base diameter and barrier layer thickness increase with time and anodic potential. The ratios of their values remain constant in time but depend on temperature. A proportionality rule thus applies explained by demand for constant Al 3+ and O 2− transport numbers in steady state. The enlargement of the above structural features occurs simultaneously with termination of some pores due to their narrowing resulting from drop of ionic conductance in pores and/or barrier layer. The surface density of cells/pores across the film increases to film surface, where it becomes highest, established around the start of second transient stage. The porosity near barrier layer remains constant with time but increases with temperature. The anodising ratio and average field strength across the barrier layer generally slightly increases and decreases respectively with time while the relevant trends are affected by temperature. FESEM examination of film surfaces showed structure affected by metallurgical texture, altered however to deeper layers, and surface structure changes due to chemical attack of oxide by the electrolyte.

Thixotropic Characteristics of Asphalt Binder

Asphalt binder has thixotropic characteristics that significantly affect its overall performance, but specifically fatigue performance, healing performance, and high temperature performance. Therefore, a clear understanding of the thixotropy of asphalt binder, together with proper modeling, is useful in researching the performance of the material. However, research in this area is still in the early stages of investigation. For the study reported in this paper, the thixotropy of four types of asphalt binder is examined by using various methods to establish corresponding thixotropy models. The results show that the exponential thixotropy model, the extended exponential function, and the structural kinetic model can be established by using different methods, and they all reflect the thixotropy of asphalt binder to some extent.

Rheology and Processing of Ni Based Nano-Composite Pastes for Electronic Assembly

Lead-free solder paste printing process accounts for majority of the assembly defects in the electronic manufacturing industry. In the stencil printing process, the solder paste must be able to withstand low and high shear rates, which results in continuous structural breakdown and build-up. The study investigates the effect of nickel additional to the thixotropic behaviour of lead-free Sn/Ag/Cu solder pastes using the structural kinetic model. A hysteresis loop test and constant shear test is utilized to investigate the thixotropic behaviour of the pastes using parallel plate rheometry in temperature 30 oC. In this study, the shear rates were increased from 0.01 s-1 to 10s-1 and the second curve was a result of decreasing shear rate from 10s-1 to 0.01s-1. For the constant shear test, the samples were subjected to two constant shear rates of 0.1s-1 and 1s-1. The constant shear rate test was designed to study the structural breakdown and build-up of the paste materials. From this investigation, hysteresis loop test was shown to be an effective test method to differentiate the extent of structural recovery in the solder pastes. The results of the structural parameter values obtained from the kinetic model showed a good correlation to the breakdown and build-up of the paste structure.

Rheological Characterization of Clay-PolyesterComposites

Polymer-clay composites, which consist of clay particles dispersed in a polymeric matrix, have been used in different applications. Clay fillers are widely used in polymers as ways to produce cost-effective, strong, and energy efficient materials. The compounding and inclusion of particulate fillers in the polymers to get a homogenous material is a rather complex process. The processing of these materials, like mixing and moulding is strongly dependent on the particle-particle and particle-polymer interactions. Therefore, the fundamental understanding of the rheological properties of the polymer-clay composites is very important in the design of their processing. In this study, the rheological behaviour of bentonite clay dispersed in unsaturated polyester was investigated. Herschel-Bulkley model and Structural Kinetic model were used to describe the dependence of the apparent viscosity of the composite on shear rate and shearing time, respectively. The effects of the filler/polyester ratio and filler size on the rheological properties of the composite were studied.

On the stability of metabolic cycles

We investigate the stability properties of two different classes of metabolic cycles using a combination of analytical and computational methods. Using principles from structural kinetic modeling (SKM), we show that the stability of metabolic networks with certain structural regularities can be studied using a combination of analytical and computational techniques. We then apply these techniques to a class of single input, single output metabolic cycles, and find that the cycles are stable under all conditions tested. Next, we extend our analysis to a small autocatalytic cycle, and determine parameter regimes within which the cycle is very likely to be stable. We demonstrate that analytical methods can be used to understand the relationship between kinetic parameters and stability, and that results from these analytical methods can be confirmed with computational experiments. In addition, our results suggest that elevated metabolite concentrations and certain crucial saturation parameters can strongly affect the stability of the entire metabolic cycle. We discuss our results in light of the possibility that evolutionary forces may select for metabolic network topologies with a high intrinsic probability of being stable. Furthermore, our conclusions support the hypothesis that certain types of metabolic cycles may have played a role in the development of primitive metabolism despite the absence of regulatory mechanisms.