Structural Kinetic Model Research Articles

Effect of Concentration and Temperature on Flow Properties of Alyssum homolocarpum Seed Gum Solutions: Assessment of Time Dependency and Thixotropy

Effects of different shear rates (14, 25, and 50 s −1 ), gum concentrations (3%, 3.5%, and 4%), and temperatures (5-65 °C) on flow properties of Alyssum homolocarpum seed gum solutions were investigated using a rotational viscometer. The experimental data were fitted with three time-dependent rheological models, namely second-order structural kinetic model, Weltman model, and first-order stress decay model with a non-zero stress value. The rate constant and extent of viscosity strongly depended on the shear rate, gum concentration, and temperature. It was found that A. homolocarpum seed gum samples exhibited shear thinning and thixotropic behavior for all concentrations and temperatures. The amount of structural breakdown decreased with shear rate, but it did not have a general trend with concentration and temperature. The extent of thixotropy increased with increasing gum concentration and decreased with increasing temperature and shear rate. In this work, the decay rate constant generally increased with increasing shear rate; however, it did not have any trend with concentration and temperature.

Modeling the Structural Breakdown of Solder Paste Using the Structural Kinetic Model

Solder paste is the most important strategic bonding material used in the assembly of surface mount devices in electronic industries. It is known to exhibit a thixotropic behavior, which is recognized by the decrease in apparent viscosity of paste material with time when subjected to a constant shear rate. The proper characterization of this time-dependent rheological behavior of solder pastes is crucial for establishing the relationships between the pastes’ structure and flow behavior; and for correlating the physical parameters with paste printing performance. In this article, we present a novel method which has been developed for characterizing the time-dependent and non-Newtonian rheological behavior of solder pastes and flux mediums as a function of shear rates. We also present results of the study of the rheology of the solder pastes and flux mediums using the structural kinetic modeling approach, which postulates that the network structure of solder pastes breaks down irreversibly under shear, leading to time and shear-dependent changes in the flow properties. Our results show that for the solder pastes used in the study, the rate and extent of thixotropy was generally found to increase with increasing shear rate. The technique demonstrated in this study has wide utility for RD and for qualifying new flip-chip assembly lines.

Kinetic and Structural Modeling Mechanisms of Melatonin Transport from an Electrolytically Regulated Salted-out PLGA Scaffold

This study focused on optimizing the mechanism of zero-order active pre-programmed release of melatonin from a salted-out PLGA scaffold. A Box—Behnken design, modeled the formulations, required for optimizing the melatonin entrapment efficiency (EE), mean dissolution time at 30 days (MDT30) and the release rate constant ( k). Response Surface Methodology depicted the influence of NaCl, CaCl2, and AlCl3 on the release kinetics. Qualitative structural kinetic modeling and quantitative mathematical modeling of release data supported the kinetic events, interaction parameters, and melatonin transport phenomena that resolved the constraints governing the rate and extent of melatonin release. A salted-out PLGA chain was evaluated by rheological studies and braided rope-coiling and nonbraided nonrope coiling with dynamic simulations capturing the coherent structural transitions in the turbulent release medium with the influence of salts on the swelling or erosion, energy dissipation, and subsequent melatonin release. The release was mainly governed by erosion and not affected by time-dependent diffusion resistance (Hopfenberg model; n = 0.95; R2 = 0.96; ke = 0.11—4.69×10-3mm/min; D = 0.110—0.893 × 10 -8 cm2/s; Debrelease = 0.016—1.312). Swelling parameters confirmed that polymer swelling did not significantly influence melatonin release (δ = 0.232.00 mm, v = 0.027—0.181 cm/s, S w = 0.010—0.542). EE values ranged between 46% and 90% and were dependant on the salt type and concentration. AlCl3 and NaCl blends increased the k values (0.0050) indicating their significance in melatonin release. The optimal scaffold (EE = 95%; MDT30 = 1; k = 0.0050) was predicted to comprise 1.1451 and 0.8264 w/v of NaCl and AlCl3, respectively, with the exclusion of CaCl2 in order to achieve zero-order kinetics over 30 days. The kinetic modeling approach enabled a qualitative and quantitative description of melatonin release patterns from the salted-out PLGA scaffolds thus facilitating the manipulation and prediction of drug release from PLGA modification by salting-out.

Flow properties and thixotropy of selected hydrocolloids: Experimental and modeling studies

The flow curves and time-dependent rheological behavior of two traditional Iranian hydrocolloids obtained from Salep tubers (wild terrestrial orchids) and Balangu seeds (Lallemantia royleana) have been investigated. All samples exhibited thixotropic behavior at the concentrations used and both in forward and backward measurements were characterized by the power law model. At the constant shear rate, the apparent viscosity decreased rapidly with time of shearing within the first 100 s for Salep and 50 s for Balangu samples, and approached a constant value corresponding to a steady state after approximately 450 and 250 s, respectively. The breakdown rate of all samples accelerated at higher shear rates. Four time-dependent models namely the second order structural kinetic model, Weltman model, first-order stress decay models with zero and non-zero equilibrium stress values were used to describe the thixotropy behavior and different parameters of these models were analyzed. The results showed that the rate of the thixotropic breakdown increased with increasing shear rates for Salep, whereas, the extent of thixotropy increased with shear rate for Salep and decreased for Balangu. In this study, the selected hydrocolloids were adequately modeled by the first-order stress decay model with non-zero equilibrium stress value.

Compressive and shear properties of flocculated kaolinite–polyacrylamide suspensions

Abstract The influence of cationic and anionic polyacrylamides (PAM) on the compressive and shear properties of kaolinite suspensions has been investigated at pH 7. The compressive properties show cationic PAM based suspensions to be less compressible than the anionic PAM based suspensions. On the other hand, rheological measurements show that the kaolinite–PAM system is thixotropic with a measurable yield stress. A structural kinetic model (SKM) was used to characterize the thixotropic behaviour of the suspension, while the viscoelastic properties were measured using oscillatory and creep measurements. The shear properties show similar trends to compressive properties. It is found that the magnitudes of the yield stress, critical stress, elastic and viscous modulus are strongly dependent upon floc structure; with greater values being observed for the cationic PAM than for the anionic PAM. The difference in the compression or shear sensitivity of the flocculated slurries may be attributed to floc structure-related adsorption. The strong electrostatic interactions between the kaolinite surface and cationic polymer chains produce stronger and less compressible flocs. This work has been very successful in establishing the link between compressive and shear properties for the flocculated kaolinite–PAM suspensions.

Rheological properties and thixotropy model of concentrated aqueous silicon slurry for gel casting

Abstract The effect of poly(acrylic acid) ammonium (PAA-NH4) dispersant on high solids content silicon suspensions was rheologically characterized. The viscosity and fluidity exhibited a strong dependence on the solids concentration, but when subjected to certain amount of dispersant PAA-NH4 adsorption and high speed shear sweep investigations, the suspensions demonstrated shear-thinning and thixotropic behavior as a result of changes to the suspension microstructure. Zeta potential and sedimentation character showed that the dispersant was adsorbed on silicon for efficient stabilization at high solids loadings. A general structural kinetics model was presented to describe the flow behavior of the thixotropic systems. The effect of shear on structure breakdown and build-up, as well as the effect of Brownian motion on build-up were taken into account to the kinetic equation for the structure parameter. Due to the PAA-NH4 absorption modality, thixotropic model characterized the PAA-NH4 concentration-dependent behavior of the suspensions over a wide range of PAA-NH4 concentration.

The stability and robustness of metabolic states: identifying stabilizing sites in metabolic networks

The dynamic behavior of metabolic networks is governed by numerous regulatory mechanisms, such as reversible phosphorylation, binding of allosteric effectors or temporal gene expression, by which the activity of the participating enzymes can be adjusted to the functional requirements of the cell. For most of the cellular enzymes, such regulatory mechanisms are at best qualitatively known, whereas detailed enzyme-kinetic models are lacking. To explore the possible dynamic behavior of metabolic networks in cases of lacking or incomplete enzyme-kinetic information, we present a computational approach based on structural kinetic modeling. We derive statistical measures for the relative impact of enzyme-kinetic parameters on dynamic properties (such as local stability) and apply our approach to the metabolism of human erythrocytes. Our findings show that allosteric enzyme regulation significantly enhances the stability of the network and extends its potential dynamic behavior. Moreover, our approach allows to differentiate quantitatively between metabolic states related to senescence and metabolic collapse of the human erythrocyte. We think that the proposed method represents an important intermediate step on the long way from topological network analysis to detailed kinetic modeling of complex metabolic networks.

Physicochemical characterization of grifolan: Thixotropic properties and complex formation with Congo Red

Grifolan is a branched (1,3)-β-glucan extracted from the fungus Grifola frondosa. Its physicochemical properties were investigated by studying the visible spectrum of Congo Red–glucan complexes and the rheological behavior. The analysis of the visible spectrum of Congo Red–glucan complexes indicated that the alkali-extracted grifolan was in a triple-helix conformation. The triple-helix structure remained unaltered in the temperature range studied (5–25 °C) or when varying the sucrose content (0–20%), but disappeared at a urea concentration of 5.0 M. The alkali-extracted grifolan formed a gel at subambient temperatures. The thixotropic viscosity profile of grifolan gels was fitted by a structural kinetics model, and the thixotropy was attributed to the breakdown of aggregates of triple helices under shear. The rate of structure breakdown was found to increase both with increasing shear rate and temperature. The amount of gel structure available for shear-induced structure breakdown was shear-rate independent, but decreased with increasing temperature. The presence of sucrose or urea in grifolan gels resulted in significant changes in thixotropy, which were attributed to the enhancing effect of sucrose on the strength of aggregates of triple helices and the loss of triple-helix structure due to the breaking of intermolecular hydrogen bonds by urea.

Structural kinetic modeling of metabolic networks.

To develop and investigate detailed mathematical models of metabolic processes is one of the primary challenges in systems biology. However, despite considerable advance in the topological analysis of metabolic networks, kinetic modeling is still often severely hampered by inadequate knowledge of the enzyme-kinetic rate laws and their associated parameter values. Here we propose a method that aims to give a quantitative account of the dynamical capabilities of a metabolic system, without requiring any explicit information about the functional form of the rate equations. Our approach is based on constructing a local linear model at each point in parameter space, such that each element of the model is either directly experimentally accessible or amenable to a straightforward biochemical interpretation. This ensemble of local linear models, encompassing all possible explicit kinetic models, then allows for a statistical exploration of the comprehensive parameter space. The method is exemplified on two paradigmatic metabolic systems: the glycolytic pathway of yeast and a realistic-scale representation of the photosynthetic Calvin cycle.

A structural kinetics model for thixotropy

A general structural kinetics model is presented to describe the flow behaviour of thixotropic systems based on inelastic suspending media. The total stress is divided in structure-dependent elastic and viscous contributions. The kinetic equation for the structure parameter takes into account the effect of shear on structure breakdown and build-up, as well as the effect of Brownian motion on build-up. The relaxation and deformation of the flocs is also included. Both the kinetic and the relaxation equations contain a distribution of time constants. The predictions of this model, as well as those of two representative models from the literature, are compared with experimental data using an objective method for parameter estimation. Model validation is based on stress transients resulting from sudden changes in shear rate, including both structure build-up and breakdown. The predictions of the viscous and elastic components are evaluated separately using stress jump experiments with steady and non-steady state starting conditions.

A cellular automata model for simulating fed-batch penicillin fermentation process

A cellular automata model to simulate penicillin fed-batch fermentation process (CAPFM) was established in this study, based on a morphologically structured dynamic penicillin production model, that is in turn based on the growth mechanism of penicillin producing microorganisms and the characteristics of penicillin fed-batch fermentation. CAPFM uses the three-dimensional cellular automata as a growth space, and a Moore-type neighborhood as the cellular neighborhood. The transition rules of CAPFM are designed based on mechanical and structural kinetic models of penicillin batch-fed fermentation processes. Every cell of CAPFM represents a single or specific number of penicillin producing microorganisms, and has various state. The simulation experimental results show that CAPFM replicates the evolutionary behavior of penicillin batch-fed fermentation processes described by the structured penicillin production kinetic model accordingly.

Rheological Behaviour and Time Dependent Characterisation of Gilaboru Juice (Viburnum opulus L.)

The flow curves and time-dependent rheological behaviour of gilaboru ( Viburnum opulus L.), a traditional drink in the middle Anatolia region of Turkey, with different solid concentrations (59.7, 56.3, 53.1, 43 and 35°Brix) were studied at different temperatures (5-60°C) using a controlled stress rheometer. Gilaboru samples exhibited thixotropic behaviour for all concentrations, both in the forward and backward measurements were characterised by the power law. A single equation was proposed for the apparent viscosity 1. Temperature played a major role in determining the magnitudes of the apparent viscosity 1. The completely destructed gilaboru samples flow curves were also measured after subjecting the samples to a high shear rate for 2h. After eliminating thixotropy by shearing, samples showed shearthinning properties that fitted well to the power law model. Three models were used to describe the time-dependent behaviour, namely, the second-order structural kinetic, Weltman and first-order stress decay models. Among the models, the first-order stress decay model fitted well compared to the second- order structural kinetic and Weltman models.

Chemical structured kinetic model for xanthan production

A chemical structured kinetic model for xanthan gum production is proposed involving both the carbon source metabolism and the nitrogen metabolism into the cell. This model considers eight lumped reactions (synthesis of amino acids, both non forming and forming bases, nucleic acids synthesis, both RNA and DNA, xanthan production, total sugar metabolism, oxidative phosphorylation and maintenance energy) and eight key compounds (biomass, ammonium, RNA, DNA, intracellular proteins, xanthan, sucrose and dissolved oxygen). Six runs under different temperatures and different initial nitrogen concentrations have been carried out. Parameter values of the model have been calculated by fitting experimental data of the six runs, using a multiple-response non-linear regression technique coupled to a fourth order Runge-Kutta algorithm. The kinetic model with the parameter values calculated is able to describe in close agreement with experimental data the concentration evolution of the eight key compounds previously mentioned in all the runs performed. Moreover, the structured kinetic model is able to predict the behavior of the system when some operational conditions are changed, such as temperature and initial nitrogen concentration, and also different oxygen transport rates, predicting different xanthan production rates depending on the operational conditions and medium composition (nitrogen source concentration).

Flow properties of sweetened sesame paste (halawa tehineh)

Sweetened sesame paste, known as halawa tehineh, is a food product that consists of heat-treated sugar, citric acid and tehineh. Some natural flavor may be added, for instance vanilla and nuts. The steady and time-dependent flow properties of halawa tehineh were investigated in the temperature range between 25–45 °C. The flow behavior of halawa tehineh was observed to be pseudoplastic and thixotropic at all temperatures measured, and the apparent viscosity–shear rate data can be described by a power law model. Increasing the temperature increases the flow behavior index and significantly reduces the consistency coefficient. The temperature dependency of the consistency coefficient is given by an Arrhenius-type relationship. A second order structural kinetic model describes the thixotropic behavior of halawa tehineh well. An empirical equation was developed for predicting the rate of thixotropic structural breakdown as a function of shear rate and temperature.

Flow behaviour of semi-solid dairy desserts. Effect of temperature

The flow behaviour of seven commercial samples of natillas, a popular semisolid dairy dessert in Spain, was analysed at the two common consumption temperatures, 5°C and 25°C, with special attention to their time-dependent properties. All samples showed flow time dependence at both temperatures tested. For the same temperature, differences in the form and in the magnitude of the hysteresis loop were observed among samples, these differences being more noticeable at 5°C. An analysis of variance of the thixotropic area values, considering sample and temperature as factors, showed a significant interaction between these factors, the corresponding F value (F int ) being 53.21 (p = 0.000), which indicates that the effect of temperature on this area was different depending on samples. Similar results were obtained by analysing Weltmann parameters A and B, from shear stress decay experiments. Shear stress decay data fitted well to a second-order structural kinetics model [(η 0 - η e )/(η - η e )] = kt + 1, and the sample-temperature interaction was also significant on parameter k from this model (F int = 5.46, p = 0.004). Flow of samples, after eliminating thixotropy, fitted well to the Herschel-Bulkley model (σ = σ 0 + Kγ n ). Here again, sample-temperature interactions were significant for σ 0 (F int = 5.40, p = 0.005) and for η 1 (F int = 6.63, p = 0.002). Although the extent of the differences varied among samples, in general, time dependence, plasticity and consistency were lower and pseudoplasticity was slightly higher at the higher temperature.

Modelling the time-dependent rheological behavior of semisolid foodstuffs

Many of food products exhibited the thixotropic behavior, in which, the apparent viscosity of material decreases with time of shearing at constant shear rate. The structural kinetic model (SKM) was used to characterize the thixotropic behavior of three different kinds of food products. Foods selected for analysis represent the fluid and semisolid food materials. They include milled sesame, concentrated yogurt and mayonnaise. The SKM postulates that the change in the rheological behavior is associated with shear-induced breakdown of the internal structure of the food product. This model for the structure decay with time at constant shear rate assumes nth order kinetics for the decay of the material structure with a rate constant, k. The dependence of the degree and the extent of thixotropy of the materials on the temperature, composition and shear history of the food product was determined.

Bioreactor operating strategy inThalictrum rugosum plant cell culture for the production of berberine

Optimal substrate feeding strategy in bioreactor operation was investigated to increase the production of secondary metabolite in a high density culture of plant cell. It was accomplished by the previously proposed structured kinetic model that describes the cell growth and synthesis of the secondary metabolite, berberine, in a batch suspension culture ofThalictrum rugosum. Four types of operation strategies for sugar feeding intoT. rugosum culture were proposed based on the model, which were the periodic fedbatch operations to maintain the cell activity, the cell viability, and the specific production rate, and the perfusion operation to maintain the specific production rate. From the simulation results of these strategies, it could be found that the periodic fed-batch operation and the perfusion operation could achieve the higher volumetric production of berberine (mg berberine/L) and specific production yield (mg berberine/g dry cell weight) than those of batch cultures. Although the highest productivity (mg berberine/day) of berberine could be achieved by the periodic fed-batch operation to maintain the cell activity compared with the other strategies in the periodic fed-batch operations, the specific production yield was low due to the higher maximum dry cell weight than other cases. The periodic fed-batch operation to maintain cell viability resulted in the highest volumetric production of berberine and specific production yield compared with the other strategies. In the cases of maintaining the specific production rate, the per-formance of the periodic fed-batch operation was better than that of the perfusion operation in the respect of the volumetric production and productivity of berberine. In order to increase the volumetric production of berberine and to get the highest specific production yield, the periodic fed-batch operation to maintain cell viability could be chosen as the optimal operating strategy in high density, culture ofT. rugosum plant cell.

Metabolic structured kinetic model for xanthan production

A metabolic structured kinetic model for xanthan gum production is described and fitted to experimental data obtained in a batch stirred tank reactor. The model is able to describe the evolution of biomass, consumption of carbon, nitrogen, dissolved oxygen, and production of xanthan at different temperatures. The description of growth is carried out by means of an unstructured model taking the nitrogen source as the limiting nutrient, employing the logistic equation which fits adequately the experimental data of the increase in biomass concentration as a function of initial concentrations of biomass and nitrogen source. The model proposed is metabolically structured because it takes into account the carbon source metabolism into the cell, describing xanthan production, carbon source, and dissolved oxygen consumption in a structured way according to a reaction network as a simplified scheme of the intracellular metabolism. The limiting nutrient of xanthan production and carbon source consumption is supposed to be dissolved oxygen. The parameter estimation has been performed by fitting to experimental data, employing an integral method coupling a fourth-order Runge-Kutta algorithm to a multiple-response nonlinear regression technique. Runs performed at four different temperatures (25, 28, 31, and 34°C) are fitted and described by the model.

Simulation and Profit Estimation for Baker's Yeast Continuous Production

In baker's yeast continuous fermentation process, profit optimization is essentially the final target of all process control and scheduling measures. In this paper, an earlier proposed structured kinetic model for baker's yeast Saccharomyces cerevisiae was tested and modified with industrial fermentation data. With the support of the process model system, the profit function of the whole process is estimated This makes a further profit optimization possible.

Application of a new structured model to tobacco cell cultures

A new structured kinetic model has been formulated and applied to batch suspensions of Nicotiana tabacum. This model has been developed by representing culture interactions with pathways designated for structural component production, secondary metabolite synthesis, and cellular respiration. Additional provisions were made to distinguish growth-competitive secondary metabolite production from non-growth-competitive secondary metabolite production. Parameters for kinetic rate expressions within the model were estimated based upon experimental observations utilized in conjunction with numerical optimization techniques. Using these parameters, culture growth, substrate uptake, cell respiration, and total phenolics production were all successfully correlated to experimental data from shake flask cultures of N. tabacum.