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Rheology 1 Classification of liquids 1. Newtonian materials Follow Newton’s law Viscosity is constant e.g., water, true solutions and diluted suspension 2. Non- Newtonian Do not follow Newton’ law Viscosity changes with changing the shear force E,g. colloidal dispersion, emulsions and gels A. time independent materials Shear thinning Shear thickening B. time dependent materials Thixotropic Newtonian systems These systems have constant viscosity where η = F / G. When we plot a rheogram of G against F then we become a straight line passing through the origin, the slope of which is equal to the reciprocal of viscosity, a value referred to as the fluidity Φ, Φ = 1 / η 3 Newtonian systems Newtonian systems like water, simple organic liquids, true solutions and dilute suspensions. 4 Newtonian Viscosities of Common Liquids Material Temperature Viscosities (C) (poise) Water 20 0.0100 50 0.0055 99 0.0028 Ethanol 20 0.0120 50 0.0070 Benzene 20 0.0065 50 0.0044 Glycerin 20 15 Castor oil 20 10.3 Newtonian flow G G F F = Shear stress; G = Shear rate; = Viscosity Coefficient 6 Characteristics of Newtonian flow 1. The passage through the origin indicates that even a mild force can induce flow in these systems. 7 Characteristics of Newtonian flow 2. The linear nature of the curve shows that the viscosity (η) of a newtonian liquid is a constant unaffected by the value of the rate of shear. Thus a single determination of viscosity from the shear stress at any given shear rate is sufficient to characterize the flow properties of a Newtonian liquid. 8 FLOW CHARACTERISTICS OF NON-NEWTONIAN SYSTEMS Do not follow the simple Newtonian relationship i.e. when F is plotted against G the rheogram is not a straight line passing through the origin i.e. viscosity is not a constant value. Such as colloidal dispersions, emulsions, suspensions and ointments, etc. 9 FLOW CHARACTERISTICS OF NON-NEWTONIAN SYSTEMS There rheograms represents three types of flow: - Plastic - Pseudoplastic - Dilatant. 10 1. Non-Newtonian flow: plastic G G Yield value F (f) 11 1. Plastic Flow Such materials are called Bingham bodies The curve is linear over most of its length corresponding to that of a Newtonian fluid. 12 1. Plastic Flow However, the curve does not pass through the origin but rather intersects the shearing stress axis at a particular point referred to as the Yield value or Bingham Yield value 13 1. Plastic Flow Contrary to a Newtonian liquid that flows under the slightest force, a Bingham body does not flow until a definite shearing stress equal to the yield value is applied. Below the yield value the system acts as an elastic material. plastic systems resembles Newtonian systems at shear stresses above the yield value. 14 1. Plastic Flow The slope of the rheogram is termed mobility, analogous to fluidity in Newtonian systems and its reciprocal is known as the Plastic viscosity, U. U = (F - f) G Plastic systems are shear-thinning systems 15 1. Plastic Flow 16 Explanation of Plasticity: Flocculated particles in a concentrated suspensions usually show plastic flow The yield value is because the van der Waals forces between adjacent particles, which must be broken first before flow can occur The more flocculated the suspension the higher will be the yield value 17 Explanation of Plasticity: 18 2-Pseudoplastic Flow A large number of pharmaceutical products, including natural and synthetic gums, e.g. liquid dispersions of tragacanth, sodium alginate, methyl cellulose and Na-carboxymethylcellulose show pseudoplastic flow. 19 2-Pseudoplastic Flow As a general rule pseudoplastic flow is exhibited by polymers in solution, in contrast to plastic systems which are composed of flocculated particles in suspension. 20 Non-Newtonian flow: pseudoplastic Shear-thinning G G F 21 2-Pseudoplastic Flow Curve for a pseudoplastic material begins at the origin consequently, in contrast to Bingham bodies, there is no yield value and no part of the curve is linear. 22 2-Pseudoplastic Flow The viscosity of a pseudoplastic substance decreases with increasing rate of shear. (shear-thinning systems) 23 2-Pseudoplastic Flow ►Newtonian system is completely described by η, the viscosity. ►Plastic system is described by the yield value and U, the plastic viscosity. ►Pseudoplastic systems which can not be described by a single value are expressed by: FN = η’ G 24 2-Pseudoplastic Flow *When N = 1, equation the flow is Newtonian *As N rises the flow becomes increasingly non Newtonian. *The term η’ is a viscosity coefficient. The logarithmic form is a straight line equation log G = N log F – log η’ A straight line is obtained when log G is plotted against log F 25 Explanation of pseudo-plasticity Most of the pseudoplastic materials consist of long chain molecules which are disarranged at rest, but As the shearing stress is increased, the normally-disarranged molecules begin to align their long axes in the direction of flow. 3-Dilatant Flow 27 3-Dilatant Flow Dilatant systems exhibit an increase in resistance to flow (viscosity) with increasing rates of shear. “ shear thickening systems”. Such systems actually increase in volume when sheared and are hence termed dilatant. When the stress is removed, a dilatant system returns to its original state of fluidity 28 Non-Newtonian flow: dilatant Shear-thickening G G F 29 3-Dilatant Flow Dilatant flow is the reverse of that possessed by pseudoplastic systems. The equation: FN = η’ G can be used to describe dilatancy in quantitative terms. In this case, N is always less than 1 and decreases as the degree of dilatancy increases. as N approaches 1, the system becomes increasingly Newtonian in behaviour 30 3-Dilatant Flow Substances possessing dilatant flow properties are invariably suspensions containing a high concentration (about 50 percent or greater) of small, deflocculated particles. Flocculated would be expected to possess plastic, rather than dilatant flow characteristics. 31 Explanation of dilatancy At rest, the particles are closely packed with the interparticle volume, or voids. The amount of vehicle in the suspension is sufficient, however, to fill this volume and permits the particles to move relative to one another at low rates of shear. 32 Explanation of dilatancy As the shear stress is increased, the bulk of the system expands or dilates. Such expansion leads to a significant increase in the interparticle void volume and the amount of vehicle remains constant and become insufficient to fill the increased voids between the particles. Accordingly, the resistance to flow increases because the particles are no longer completely wetted or lubricated by the vehicle. Thus, the suspension will set up as a firm paste Explanation of dilatancy Examples of non-Newtonian fluids Shear thinning: Ketchup, toothpaste, blood, paint, nail polish, whipped cream, and face-cream work the opposite way. They start off relatively thick and viscous but become more runny if you subject them to forces. Shear thickening: Cornstarch, custard, Some slurries and pastes thicken up when you subject them to forces. 35

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deformation and flow, Society of Rheology, polymer rheology, Food rheology, Journal of Rheology, rheology testing, rheological properties, time scales, The Society, shear rate

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posted: | 4/4/2011 |

language: | English |

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