João M. Maia

Rheological monitoring of structure evolution and development in stirred yoghurt

The objective of the present work was to study the rheological changes of yoghurt at different stages in the manufacturing process and to try to gain some measure of understanding of the morphological changes that occur. Rheological measurements were made on yoghurt samples for different post-sampling periods, allowing the study of the influence of time in the structure and flow properties of the material to be performed. This was accomplished using both steady and oscillatory rheometry. Up and down shear rate sweeps, constant and oscillatory shear measurements were carried out. The study of the effect of temperature allowed the observation of two distinct regions of temperature dependency, in all samples, the transition point being approximately 25°C. The structural and rheological changes that occurred during post-incubation were observed during the analysis of several yoghurt samples, indicating that there is a partial structure recovery upon cessation of flow.

Heat transfer and rheology of stirred yoghurt during cooling in plate heat exchangers

In the present work an experimental investigation was conducted to obtain a correlation for the determination of convective heat transfer coefficients of stirred yoghurt in a plate heat exchanger. A rheological study was carried out in order to characterise the stirred yoghurt flow behaviour, evaluating its dependency both on shear rate and temperature. A shift in the temperature dependency was evidenced at 25 °C. It is also shown that the material shows a complex flow behaviour, changing from a Bingham fluid to a power-law fluid at shear stresses in excess of approximately 6.7 Pa. As regards the heat transfer behaviour of the non-Newtonian stirred yoghurt a correlation for the convective heat transfer coefficient was obtained that reveals the large effects of the thermal entry length due to the high Prandtl numbers and to the short length of the plate heat exchanger.

Measuring uniaxial extensional viscosity using a modified rotational rheometer

Knowledge of the extensional behaviour of polymer melts is extremely important due to the industrial relevance of extensional flows in common processing techniques and sequences such as blow moulding, film blowing, fibre spinning, melt flow through extrusion dies and injection mould filling. One of the main problems both researchers and industrialists come across is the fact that, unlike shear flows, steady-state extensional flows are not easy to generate and maintain experimentally. This fact limits the extent to which one can characterise the materials and, therefore, the degree of optimisation of the productive process. In this paper, a modification to a commercially available controlled rate rotational rheometer is proposed in order to produce a cheap, easy to set-up, flexible extensional rheometer. This is based on the well-known Meissner-type extensional rheometer and makes use of the accurate velocity control and torque measurement possibilities of the rotational apparatus. In this case, the adaptation was performed on a TA Instruments Weissenberg Rheogoniometer, but the idea is applicable to most other similar devices. The feasibility of the modification will be discussed and confirmed, results being presented for two materials at different temperatures. These include the calculation of transient uniaxial extensional viscosity and a study of rupture conditions.

Online monitoring techniques for studying evolution of physical, rheological and chemical effects along the extruder

Abstract During extrusion and/or compounding, polymeric systems may be subjected to a complex thermo-mechanical-chemical environment, therefore monitoring the evolution of physical, rheological and chemical effects along the extruder is an important tool assisting process understanding and optimisation. Online monitoring concepts that allow sample collection, rheology measurements and RTD characterisation at specific locations along the screw axis of an extruder are presented. Each practical set-up is presented, its operation is described and the results obtained are validated experimentally. Finally, examples of the use of the tools developed for the study of specific polymer systems are presented and discussed.

An experimental study on the criteria for failure of polymer melts in uniaxial extension: The test case of a polyisobutylene melt in different deformation regimes

Failure of polymer liquids under flow has been the subject of a relatively strong computational, theoretical, and experimental effort over the years, despite which a clear picture of the phenomenon is still nonexistent. For example, there are still arguments in the literature whether the maximum in engineering stress, also known as the Considere limit for failure, is a point of true yielding of the polymer network or simply a purely elastic mechanical instability and if this onset of yielding also corresponds to the true onset of non-homogeneous deformation, as expressed for example by necking. The main objective of the present work is to contribute to the ongoing discussion on this matter by studying a linear polyisobutylene melt in terms of its failure and rupture behavior in both the viscoelastic and purely elastic deformation regimes of deformation showing that homogeneous flow still occurs after the maximum in engineering stress and quantifying this deviation. This is ultimately connected with the ob...

Rheological measurements along an extruder with an on-line capillary rheometer

An on-line capillary rheometer concept using an in-house built sampling and testing device, as well as a commercially available portable rotational rheometer, is proposed. Off-line validation was carried out by checking the behaviour of a standard silicone oil. On-line validation was provided through the comparison of the viscosity of two commercial polymer melts with that obtained via conventional off-line experiments. The apparatus allows the quick measurement of the evolution of the rheology of polymer systems along the length of an extruder. The ability to characterise rheologically reactive systems is illustrated by monitoring the peroxide induced degradation of polypropylene along a twin screw extruder.

Linear and non-linear dynamics of entangled linear polymer melts by modified tunable coarse-grained level Dissipative Particle Dynamics

Dissipative particle dynamics (DPD) is a well-known simulation method for soft materials and has been applied to a variety of systems. However, doubts have been cast recently on its adequacy because of upper coarse-graining limitations, which could prevent the method from being applicable to the whole mesoscopic range. This paper proposes a modified coarse-grained level tunable DPD method and demonstrates its performance for linear polymeric systems. The method can reproduce both static and dynamic properties of entangled linear polymer systems well. Linear and non-linear viscoelastic properties were predicted and despite being a mesoscale technique, the code is able to capture the transition from the plateau regime to the terminal zone with decreasing angular frequency, the transition from the Rouse to the entangled regime with increasing molecular weight and the overshoots in both shear stress and normal-stress differences upon start-up of steady shear.

The effect of interfacial properties on the deformation and relaxation behavior of PMMA/PS blends

This work aims at studying the role of interface properties on the rheological behavior of non-compatibilized and compatibilized polymer blends. Blends of polymethylmetacrylate (PMMA) with polystyrene (PS) and PS functionalized with oxazoline (PSOX) with concentrations of up to 20 w/w of the dispersed phase were used. It was observed that until a critical concentration is reached the increase in PSOX content leads to a significant increase in (a) the elasticity at low frequencies and (b) the relaxation time after cessation of flow, both in shear and extension. This points to a likely significant role played by interface elasticity. Since no chemical reactions occur between PMMA and the oxazoline groups of PSOX, the latter is probably caused by the partial miscibility between PMMA and PSOX. Beyond this critical concentration, the amount of PSOX does not have a significant influence on the rheological behavior of the blends. In order to gain an insight into the relaxation dynamics of the droplets and interf...

Microstructure and rheology of soft to rigid shear-thickening colloidal suspensions

The shear rate-dependent rheological properties of soft to rigid colloidal suspensions are studied using computational models. We show that a contact force defined based on an elasto-hydrodynamic deformation theory captures an important rheological behavior of colloidal suspensions: While near hard-sphere particles exhibit a strong and continuous shear thickening the evolves to a constant viscosity state, soft suspensions undergo a second shear-thinning regime at high Peclet numbers when the hydrodynamic stresses become larger than the modulus of the colloidal particles. We measure N1 and N2 to be large and negative in the shear-thickening regime; however, for soft spheres at the onset of second shear-thinning N2 reduces in magnitude and eventually becomes positive. We show that for near hard-sphere suspensions, colloidal pressure, shear stress, and normal stress difference coefficients tend to diverge near the maximum packing fraction while P>σ>N1>N2.

Bridging the gap between microstructure and macroscopic behavior of monodisperse and bimodal colloidal suspensions

Colloidal suspensions exhibit a transition from shear-thinning to shear-thickening behavior as the shear rate increases. Despite all the experimental and computational studies, an understanding of the structure of suspensions in different flow regimes remains controversial. In this work, a dissipative particle dynamics model was employed to perform a comprehensive study of the rheological and morphological behaviors of monodisperse and bimodal suspensions over a wide range of shear rates. The interplay between rheology and structure indicates that hydroclusters are formed in the shear-thickening regime, whereas interparticle interaction is responsible for the shear-thinning response at low stresses. The effect of particle size, ratio, and combination in bimodal systems have also been investigated and quantitative agreement with existing experimental data was found. Thus, it was possible for the first time to perform a comprehensive study on different aspects of the bimodal dispersions and correlate the macroscopic behavior with the microstructure in different flow regimes.