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14 February 2017
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Teaching is concentrated in the faculties and schools, and the departments deal with research, which is also conducted by institutes and chairs, at the same time responsible for knowledge promotion.
The term rheology is defined as the study of the deformation and flow of materials.Rotational rheometry is an analytical technique that allows the measurement of, among others, viscosity and viscoelastic properties (eg, G', G”, and δ) for a wide range of materials, including fluids, semisolids, and solids.
During rheometer analysis, the sample is placed in a given geometry and subjected to a controlled stress, measuring the resulting deformation.
In order to request the analysis of samples by rotational rheometry, you should contact the staff responsible for the equipment.
The toxicity and storage precautions of the samples must be reported.If unclaimed, surplus samples will be destroyed within one week of delivery of results.
In a rotational rheometry test the material is subjected to a shear stress.These stresses are produced in a rotational rheometer using different geometries, such as concentric cylinders (liquid samples) or parallel plates (semisolid samples).In the case of concentric cylinder geometry, the sample is located in the annular space between the fixed external cylinder (called the cup) and the internal one (mobile rotor).In the parallel plate geometry, the sample is placed between the fixed lower plate and the movable upper plate.
The dynamic viscosity is the main parameter that allows establishing the constitutive equations that relate stress and deformation to cutting.With rotational rheometry, flow curves can be obtained that relate the viscosity of the sample or the shear stress to the shear rate of deformation.
The other test mode with the rheometer is the oscillatory one, in which the viscoelastic response is monitored.In this way, the cutting storage (G') and loss (G'') moduli can be obtained as a function of temperature, deformation frequency or as a function of time in the case of reactions that modify the viscoelastic properties of the sample
The rotational rheometer is a very versatile piece of equipment, some of its main applications are:
In Newtonian fluids the relation between the shear stress and the speed of deformation is linear.This means that the viscosity can be considered constant (provided the temperature and the pressure are also constant).Moreover, in a Newtonian fluid, no minimum effort is needed for it to start to flow.
The rest of fluids, where either the relation between shear stress and speed of deformation is not linear or a minimum effort is needed for them to start to flow (yield stress), are considered non-Newtonian fluids.
Next the flow curves characteristic of Newtonian fluids and the main types of non-Newtonian fluids are shown:
In this case study the flow curves have been obtained from three types of commercial mayonnaises.These mayonnaises differ mainly in the fat content and type of oil used:
In accordance with the flow curves for figure 1.2, the mayonnaises have a non-Newtonian behaviour, given that they show a minimum shear stress for them to start to flow (flow effort or yield stress).
In a published study (Liu, H. et.al., 2007), it was concluded that the fat content in mayonnaises has a significant influence on viscosity curves, with yield stress higher as the fat content increases.This can be checked if the mayonnaises A and B are compared, both made with soya oil.Mayonnaise A, with a higher fat content, show a yield stress higher than in mayonnaise B.
It is also worth noting that the oil used in the mayonnaise affects the flow curve.So mayonnaise C, made with sunflower oil, shows a higher yield stress than mayonnaise A, even though both have the same fat content (70% in weight).
Liu, H.,Xu, X. M.,Guo, S. D. (2007).Rheological, texture and sensory properties of low-fat mayonnaise with different fat mimetics.LWT - Food Science and Technology, 40(6), 946–954
The oscillating measures in rheology are used to determine the gelation time of thermostable resins.These experiments consist of monitoring the changes in the storage module (G’) as well as in the loss module (G’’) ccording t time, using an oscillatory movementand cutting broadly and at a constant frequency (Fig.. 2.1).The cutting point for G’ and G'' is identified as the point of gelation.
The acquisition of this equipment has been co-financed by the European Regional Development Fund (ERDF) and the Recovery, Transformation and Resilience Plan (MRR).Ministerio de Ciencia, Innovación y Universidades
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