Search

Ethylene vinyl acetate (EVA) is a random copolymer of ethylene and varying amounts of vinyl acetate that interfere with poly-ethylene chain packing reducing crystallinity, thus improving transparency and lowering the melting temperature to 40°C–60°C. The material viscoelastic properties in its working conditions may thus depend on the crystallinity degree. The crystallization process is here rheologically studied in non-isothermal conditions and the frequency spectra are measured at different temperatures to investigate the viscoelasticity of EVA. Coupling the crystallization kinetics and the viscoelastic spectra at different temperatures, that is, at different degree of crystallinity, we here determine two independent shift factors, one for the time-crystallinity shift, the other for the time-temperature shift, so to propose a new time-temperature-crystallinity-superposition to reconcile all the data on a single master curve. In this way, the experimentally observable frequency range has been widened significantly so to detect all the relaxation times of the material from the shortest to the largest ones.

The use of biogas in internal combustion engines presents many advantages. Nevertheless, the CO2 content in the fuel negatively affects the combustion process, reducing combustion speed and stability. The presence of hydrogen in the biogas would improve its combustion characteristics. This paper investigates the combustion of biogas in a Controlled Auto Ignition (CAI) engine by means of numerical simulations. A model of the combustion system was developed for analyzing the use of biogas in a Chemkin environment. The biogas considered in this paper naturally contains hydrogen as a result of a new anaerobic digestion process. The CAI internal combustion engine model allowed the evaluation of engine performance and emissions. Different hydrogen contents were investigated to find the optimal fuel composition for reducing nitrogen oxides emission. Exhaust Gas Recirculation (EGR) was adopted for controlling in-cylinder temperature and therefore fuel auto-ignition. In fact, the presence of very reactive chemical species in the EGR, such as nitric oxide (NO), resulted to have an important impact on the onset of combustion. The results demonstrated that biogas containing hydrogen allow a reduction of NOx emissions with respect to a conventional biogas. An optimal biogas composition was identified, allowing a 32% reduction in NOx emission compared to the conventional biogas. The reaction mechanism for NO formation did not change with biogas composition, with an important contribution coming from prompt NOx formation mechanism. The main differences were in the reaction rates, higher for the conventional biogas.

We study the asphaltene microstructure in a heavy crude oil in response to the addition of solvents of different qualities by measuring the intrinsic viscosity and the Huggins coefficient. Dilution with good solvents (xylene and cyclohexanone) causes the native asphaltene clusters swelling, while that with poor solvents (a diesel oil and a C11-C14 alkanes mixture) may induce different microstructural rearrangements. The mixture of C11-C14 alkanes induces the aggregation of clusters, while the diesel oil their shrinkage. Moreover, if the diesel is added in small quantities instead of cluster shrinkage it may induce cluster aggregation thus forming larger “macro-clusters”. These structures survive after further dilution in both poor and good solvents and their formation is always accompanied by a reduction of the Huggins coefficient that implies the better solubility of these self-aggregated structures in the used solvent. Finally, an unusual negative Huggins coefficient is measured, when the macro-clusters are formed and the solution is diluted in a good solvent. This might be due to a prevalence of the intra-macro-cluster interactions on the inter-macro-cluster ones.

Anaerobic digestion is an established technology to simultaneously produce biogas and treat different kinds of wastes. This work deals with water buffalo manure digested under mesophilic (37°C) and thermophilic (55°C) conditions with initial pH varying from 6.0 to 8.7. The digestion is investigated following the evolution of the biogas composition in time, in terms of the volume fraction of CH4, CO2 and their ratio. The growths of the biogas products are interpolated with a Gompertz equation and in all the investigated cases the methane productivity is above 63% with a CH4/CO2 ratio rather high and equal to about 2. In mesophilic conditions, an initial pH of 7.0 must be preferred to optimize the methane yield and to minimize the time required to attain it. The thermophilic conditions resulted very promising since they almost halved the time required to reach the final yield. The speed up of the process at 55°C, in our case, resulted to be due to the increase of the digestion kinetic more than to the selection of a new metabolic pathway.

The paper describes a numerical study of the combustion of hydrogen enriched methane and biogases containing hydrogen in a Controlled Auto Ignition engine (CAI). A single cylinder CAI engine is modelled with Chemkin to predict engine performance, comparing the fuels in terms of indicated mean effective pressure, engine efficiency, and pollutant emissions. The effects of hydrogen and carbon dioxide on the combustion process are evaluated using the GRI-Mech 3.0 detailed radical chain reactions mechanism. A parametric study, performed by varying the temperature at the start of compression and the equivalence ratio, allows evaluating the temperature requirements for all fuels, moreover, the effect of hydrogen enrichment on the auto-ignition process is investigated. The results show that, at constant initial temperature, hydrogen promotes the ignition, which then occurs earlier, as a consequence of higher chemical reactivity. At a fixed indicated mean effective pressure, hydrogen presence shifts the operating range towards lower initial gas temperature and lower equivalence ratio and reduces NOx emissions. Such reduction, somewhat counter-intuitive if compared with similar studies on spark-ignition engines, is the result of operating the engine at lower initial gas temperatures.

Heavy crude oil samples extracted from the same well showed some rheological differences mainly in terms of viscosity and response to the addition of a small amount of dodecyl benzen sulfonate. Also slight differences in the activation energy and in the viscosity dependence on a solvent concentration were observed. The crude oil samples, also separated in maltenic and asphaltenic fractions, were fully characterized by elemental and proximate analysis, solvent extraction, thermogravimetry, mass spectrometric and spectroscopic analyses. No significant differences were found showing that the samples have the same composition and molecular structure and demonstrating that the different rheological behavior has to be related only to a different microstructural organization that is not sensed by the chemical–physical tests. This result helps to complete the picture of the complex correlation between rheological and chemical–physical properties of heavy crude oils. While it is accepted that chemical-physically different samples may, or may not, have a different rheology, we show for the first time that unperturbed samples with the same chemical–physical properties may exhibit differences in the rheological behavior.

The effect of the soil organic content on the stability of a soil involved in a rapid mud flow is here experimentally investigated. The soil is collected form a site where a catastrophic landslide took place (Cervinara – AV) and it is chemically treated to selectively remove different aliquots of its original organic carbon. In particular, The Dissolved Organic Carbon ( i.e. the carbon soluble in water)results the 6% of the soil organic carbon and it is removed with a mild chemical treatment to obtain the DOC 6 sample, the 77% and 89% of the Total Organic Carbon ( i.e. the pool of oxidizable soil organic carbon)is removed with a strong chemical treatment to obtain the TOC 77 and TOC 89 sample, respectively. The stabilizing effect of the organic carbon is investigated by following the evolution of the particle size distribution of soils induced by a mild mixing of diluted slurries and we showed that the particle size distribution of the original soil sample is unaffected by the slurry mixing, while those of DOC 6,TOC 77 and TOC 89 evolve during time, revealing the breakup of soil aggregates. Our findings highlight the stabilizing effect of SOC in the investigated liquid slurries and, furthermore, they suggest that the organic carbon quality, more than its quantity, plays a crucial role in the soil stability. Indeed, it is enough to remove the Dissolved Organic Carbon to register a soil disaggregation process comparable to that observed for TOC 77 and TOC 89 samples. This suggests that Dissolved Organic Carbon is the fundamental organic fraction stabilizing the slurry microstructure.

In the present work rheological and mechanical properties of PEEK-PEI blends were investigated. Besides the pure components, blends with PEI concentration ranging from 10% to 90% in mass were considered. Oscillatory experiments in controlled atmosphere were conducted at different frequencies and temperatures. The frequency responses at different temperatures allowed using the TTS principle to reconstruct the master curves. All systems showed a shear thinning behavior and a flux index increasing with the percentage of PEI. The zero-shear viscosity was computed with the implementation of the Cross model and showed a decreasing behavior with the percentage of PEI. The relaxation time estimated from the crossover value of storage and loss moduli didn't change significantly with blend composition, suggesting the non-sensibility of the elasticity of the system. Lastly, tensile tests were executed to investigate the dependence of Young modulus in the different blends.

Dilute polymer blends and immiscible liquid emulsions are characterized by a globular morphology. The dynamics of a single drop subjected to an imposed flow field has been considered to be a valuable model system to get information on dilute blends. This problem has been studied either theoretically by developing exact theories for small drop deformations or by developing simplified models often based on phenomenological assumptions. In this paper, a critical overview of the available models for the dynamics of a single drop is presented, discussing four different systems, namely the Newtonian system, where a single Newtonian drop is immersed in an infinite Newtonian matrix; the non-Newtonian system, where at least one of the components, the drop fluid or the matrix one, is non-Newtonian; the confined Newtonian system, where the matrix is confined and wall effects alter the drop dynamics; and the confined non-Newtonian system.

In this work, the microconfined shear deformation of a droplet in an equiviscous non-Newtonian immiscible fluid is investigated by modeling and experiments. A phenomenological model based on the assumption of ellipsoidal shape and taking into account wall effects is proposed for systems made of non-Newtonian second-order fluids. The model, without any adjustable parameters, is tested by comparison with experiments under simple shear flow performed in a sliding plate apparatus, where the ratio between the distance between the confining walls and the droplet radius can be varied. The agreement between model predictions and experimental data is good both in steady state shear and in transient drop retraction upon cessation of flow. The results obtained in this work are relevant for microfluidics applications where non-Newtonian fluids are used.

Stress-induced demixing of a polymer solution is studied in the mainframe of the two-fluid theory. A true phase separation is predicted to take place and the theory has been reformulated so to be a...

The simple phenomenological model developed by Maffettone and Minale (J Non-Newt Fluid Mech 78:227-241, 1998) for the deformation of a single ellipsoidal drop in a viscous flow is extended here to predict the drop deformation in confined viscous flows. The model is capable of describing the transient evolution of an ellipsoidal drop subjected to a generic flow field. The steady-state predictions are analytical and recover the theoretical limits of Shapira and Haber (Int J Multiph Flow 16:305-321, 1990) for steady small deformation of a drop in a confined simple shear flow. Model predictions are compared with data available in the literature that cover a wide range of parameter values, and the agreement is good.

This work deals with the study of the effect of the soil organic carbon (SOC) content on the rheology of a concentrated natural slurry made with soil collected from a site involved in a catastrophic rapid mudflow. We considered the soil with all the original SOC and with various fractions of it, selectively removed with specific chemical treatments. In this way, we obtained samples with different quantities of SOC having different solubilities and degrees of complexation with the soil mineral matrix. Flow curves and yield stresses of slurries, with a soil volume fraction of 40%, are experimentally determined. Flow curves show a marked shear-thinning behaviour and, at each imposed shear rate, the viscosity decreases by reducing the content of SOC. Similarly, the yield stress decreases while reducing SOC. Our findings highlight the stabilizing effect of SOC in the investigated liquid slurries and, furthermore, they suggest that the organic carbon quality, more than its quantity, plays a crucial role in slurry rheology. In particular, the dissolved organic carbon is the fundamental organic fraction stabilizing the slurry microstructure. This work deals with the study of the effect of the soil organic carbon (SOC) content on the rheology of a concentrated natural slurry made with soil collected from a site involved in a catastrophic rapid mudflow. We considered the soil with all the original SOC and with various fractions of it, selectively removed with specific chemical treatments. In this way, we obtained samples with different quantities of SOC having different solubilities and degrees of complexation with the soil mineral matrix.Flow curves and yield stresses of slurries, with a soil volume fraction of 40%, are experimentally determined. Flow curves show a marked shear-thinning behaviour and, at each imposed shear rate, the viscosity decreases by reducing the content of SOC. Similarly, the yield stress decreases while reducing SOC. Our findings highlight the stabilizing effect of SOC in the investigated liquid slurries and, furthermore, they suggest that the organic carbon quality, more than its quantity, plays a crucial role in slurry rheology. In particular, the dissolved organic carbon is the fundamental organic fraction stabilizing the slurry microstructure.

The rheological behavior of a non-Brownian Newtonian concentrated suspension is investigated under oscillatory shear at different strain amplitudes and at two frequencies. The data are in agreement with the literature showing that the complex viscosity varies both with the number of oscillations imposed to the material and with the strain amplitude. We here also show that the response to the oscillatory shear depends on the applied frequency. This result is unexpected for a Newtonian suspension and implies the existence of some mechanism able to introduce a characteristic time into the system. Can it be cage elasticity?

The shape evolution of a liquid drop immersed in an immiscible liquid is studied under transient flow conditions. The drop is Newtonian and buoyancy free; the external liquid is Newtonian and subjected to shear flow reversal. Three model systems, polydimethylsiloxane/polyisobutylene, polybutene/silicone oil, and silicone oil/polybutene, all Newtonian under the experimental conditions investigated, have been selected to have a range of viscosity ratios. The three drop axes and the drop orientation within the shear plane are independently measured. The results are compared with the predictions of a phenomenological model. The agreement between experimental results and theory is good. Peculiar behavior of the orientation angle has been observed and correctly predicted. The results are also used to explain some rheological features typical of immiscible polymer blends.

The flow-induced concentration changes in polymers are analysed using the two-fluid theory of Doi and Milner, which predicts migration in flows with either curvilinear trajectories or non-uniform shear rates. Here we show that, even in simple shear flows, migration can be relevant since it can make the stationary solution with uniform concentration linearly unstable in fast flows.

Production of articles with suitable mechanical properties is a major issue in the plastic industry. Blending low price immiscible polymers is a good alternative to synthesising new monomers to obtain high performance polymers. In such a case, the mechanical features of the final product depend not only on the properties of the constituent but also on the morphology of the blend (Vinckier et al., 1996). In this work, we consider dilute blends consisting in drops dispersed in a continuous matrix. In these conditions, when the blend is subjected to a viscous flow, both the deformation of the drops and the stress response are only slightly affected by the hydrodynamic interactions between droplets. It so appears that a good description of the deformation of a single drop immersed in a matrix subjected to flow is of value in determining the stress field of the flowing blend.

The dynamic transition of nematics in (slow) shear from flow-aligning to tumbling behaviour is here predicted by using the rigid rod-like molecular model together with an extension of the Maier-Saupe mean-field potential. Such an extension, already available in the literature with reference to equilibrium data, includes interactions of rank higher than the basic quadrupolar one considered by Maier and Saupe. The theoretical predictions are then compared with the few existing sets of dynamic data. Although an acceptable qualitative agreement is found, the theory generally overpredicts the value of the relevant ratio α3/α2 of Leslie coefficients.

The effect of processing conditions (essentially the flow conditions) on the mophology of incompatible, immiscible polymer blends was investigated. In simple shear flow the steady-state mophology, that is, droplet size, of such blends is a single-valued finction of the shear rate whenever a critical shear rate is exceeded. Below this critical value it becomes uely dificult to reach a dynamic equilibrium between droplet breakup and coalescence, which gives rise to a hysteresis region for the size of the inclusions. Such regions were identified and studied for model-blend systems. The critical shear rate decreases with increasing concentration of the droplet phase. A large ratio of droplet over matrix viscosity seems to have the opposite effect. The hysteresis region is bounded by the critical curues for droplet breakup and coalescence. The accessibility of this re gion can therefore be used to evaluate models for the said phenomena in nondilute systems. Droplet breakup is nearly independent of droplet concentration, while elasticity of the dispersed phase seems to facilitate breakup. As expected, coalescence is enhanced by higher concentrations. Droplet elasticity induces a certain degree of “rigidity” to the interface, and therefore the mobile interface model for coalescence is not suitable for such systems.