Etiket Arşivleri: Rheology
Rheology is the science of flow and deformation of matter and describes the interrelation between force, deformation and time. The term comes from Greek rheos meaning to flow.Rheology is applicable to all materials, from gases to solids. The science of rheology is only about 70 years of age. It was founded by two scientists meeting in the late 1920s and finding out they have the same need for describing fluid flow properties. The scientists were Professor Marcus Reiner and Professor Eugene Bingham. The Greek philosopher Heraclitus described rheology as panta rei – everything flows. Translated into rheological terms by Marcus Reiner; this means everything will flow if you just wait long enough. Fluid rheology is used to describe the consistency of different products, normally by the two components: viscosity and elasticity. By viscosity is usually meant resistance to flow or thickness and by elasticity usually stickiness or structure. Rheological relationships help us to understand the fluids we are working with so that we can either know how they are behaving or force them to behave according to our needs. Once a correlation has been developed between rheological data and product behaviour, the procedure can then be reversed and rheological data may be used to predict performance and behaviour…..
Efficient Rheology Control Additives Rheology is defined as “the study of the change in form and the flow of matter embracing elasticity, viscosity and plasticity.” We concern ourselves with viscosity, further defined as “the internal friction of a fluid caused by molecular attraction, which makes it resist a tendency to flow.” Water is an invaluable solvent and vehicle but it is not without certain deficiencies. It is, above all, watery. The exercise of rheological control over water is a significant challenge for formulators. It is often too thin, it is generally runny and it is invariably unsupportive for insoluble particulates. Formulators are, therefore. forced to adjust water to suit their needs. Fortunately, this can be readily accomplished through the use of rheology modifiers. Specific control of water is enabled by the careful application of one or more of the rheology modifiers available for use in aqueous compositions. Familiarity with the rheological nuances of a particular modifier can at times make the difference between an exceptional formulation and a routine one. What follows is an overview of the hydrocolloids based additives most commonly used to control water. The intent is to make the formulator sufficiently familiar with the fundamental nature of each of these materials, so as to facilitate proper selection.
The terms used to characterize rheology are defined as follows:
Newtonian – The viscosity of such fluids will not change as the shear rate is varied. Water and thin motor oils show typical Newtonian behavior.
Non-Newtonian – The viscosity of such fluids will change as the shear rate is varied. There are several types of non-Newtonian flow behavior, characterized by the way a fluid’s viscosity changes in response to variations in shear rate. The most common types of nonNewtonian fluids you may encounter include:
• Pseudoplastic – This type of fluid will display a decreasing viscosity with an increasing shear rate. Probably the most common of the non-Newtonian fluids, pseudo-plastics include paints, emulsions and dispersions of many types. This type of flow behavior is
sometimes called “shear-thinning.” Moreover, they immediately recover their nonsheared viscosity once shear is removed.
• Dilatant – Increasing viscosity with an increase in shear rate characterizes the dilatant fluid. Although rarer than pseudoplasticity, dilatancy is frequently observed in fluids containing high levels of deflocculated solids such as clay slurries, candy compounds,
corn starch in water and sand/water mixtures. Dilatancy is also referred to as “shearthickening” flow behavior.
• Plastic – This type of fluid will behave as a solid under static conditions. A certain amount of force must be applied to the fluid before any flow is induced; this force is called the “yield value.” Tomato ketchup is a good example of this type of fluid; its yield value will often make it refuse to pour from the bottle until the bottle is shaken or struck, allowing the ketchup to gush freely. Once the yield value is exceeded and flow begins, plastic fluids may display Newtonian, pseudoplastic or dilatant flow characteristics.
Yield Value – Yield value indicates the minimum force (the yield stress) that must be applied to a liquid to start disrupting the structure imparted by the rheology modifier, so that flow can occur. In practical terms, solids, oils and gases are trapped and segregated by this structure unless gravity or buoyancy can exert a force greater than the yield stress. Some fluids display a change in viscosity with time under conditions of constant shear rate. There are two categories to consider:
Thixotropy – Thixotropic fluids show a time-dependent response to shear. When subjected to a constant shear rate, they will decrease in viscosity over time. Often this is seen as a large initial viscosity loss, followed by gradual further loss. Once shear is removed, thixotropic fluids recover their viscosity, but over a period of time, not instantaneously. These fluids are also considered to be pseudoplastic, but only in that they show decreasing viscosity in response to increasing shear rate. Thixotropy is frequently observed in materials such as greases, heavy printing inks and paints.
Rheopexy – This is essentially the opposite of thixotropic behavior, in that the fluid’s viscosity increases with time as it is sheared at a constant rate. Rheopectic fluids are rarely encountered. Both thixotropy and rheopexy may occur in combination with any of the previously discussed flow behaviors or only at certain shear rates. The time element is extremely variable; under conditions of constant shear, some fluids will reach their final viscosity value in a few seconds, while others may take up to several days.
In addition, the term “synergism” is used to indicate that a combination of two rheology control additives provides a stronger rheological effect (e.g., viscosity or yield value) than would be anticipated by adding the individual contribution of each additive.
Food rheology is the material science of food. Rheology is the science of flow and deformation of matter and describes the interrelation between force, deformation and time. In this experiment, the rheological behavior of solid pekmez with different solid contents (75.4, 71.6 and 67.1 Brixes) was studied in the temperature range of 10, 20 and 30°C using a controlled stress rheometer. Solid pekmez was found to exibit non-Newtonian behavior. However, diluted samples were Newtonian.
Rheology is the study of the deformation and flow of matter. In practice, rheology is principally concerned with extending the "classical" disciplines of elasticity and (Newtonian) fluid mechanics to materials whose mechanical behaviour cannot be described with the classical theories. It is also concerned with establishing predictions for mechanical behaviour (on the continuum mechanical scale) based on the micro- or nanostructure of the material, e.g. the molecular size and architecture of polymers in solution or the particle size distribution in a solid suspension.
There are two common tests for foods. One is a creep/recovery test to determine the yield stress of the food suspension, which is an indication that the material has structure. It is this structure that stabilizes the suspension. In this test a sample is subjected to a constant shearing stress and the amount that it deforms or flows is measured. The second commonly used test is an oscillatory test where the sample is vibrated at a controlled amplitude and frequency, and we measure the storage modulus (G’) and loss modulus (G”). The ratio of the loss to storage modulus, called tan delta, is a useful and sensitive rheological parameter for predicting the stability of suspensions. Both the storage modulus and the yield stress serve to quantify the amount of structure in a material.
In the experiment of rheology the liquid sample’s textural characteristics have been measured. After calculations from datas the textural structre has been observed if it was newtonion or non newtonion type.Generally while measuring the rheological charecteristics, temperature was an important effective parameters. In non newtonion type of fluid there iare two type; time dependent and time independent. Also except temperature shear rate and time are the other effective parameters while measuring the flow of fluid.
Rheology is defined as the science of deformation and flow of matter. The term itself originates from Greek rheos meaning to flow. Rheology is applicable to all types of materials, from gases to solids. The science of rheology is young, only about 70 years of age, but its history is very old. In the book of Judges in the Old Testament the prophetess Deborah declared “The mountains flowed before the Lord…”. Translated into rheological terms by professor M. Reiner, this expression means everything flows if you just wait long enough, a statement that is certainly applicable to rheology. It was also described by the Greek philosopher Heraclitus as “panta rei” – everything flows. Professor Reiner, together with Professor E. Bingham, was the founder of the science of rheology in the mid-20s. Rheology is used in food science to define the consistency of different products. Rheologically the consistency is described by two components, the viscosity (“thickness”, lack of slipperiness) and the elasticity (“stickiness”, structure). In practice, therefore, rheology stands for viscosity measurements, characterisation of flow behaviour and determination of material structure. Basic knowledge of these subjects is essential in process design and product quality evaluation.