“παντα ρει” (everything flows … ) - Heraclito de Samos (500 A.C.) RHEOLOGY Contents RHEOLOGY: a). Definition and Fundamental concept. b). Properties contributing to rheological behaviors. c). Graphic presentation of rheological data. UNIT OBJECTIVES: After the end of this lecture, student will be able to: Define rheology adequately. Describe various deformations Define viscosity and coefficient of viscosity Define different term used in rheology adequately. Classify various types of fluid systems Distinguish different system of flow and deformation. UNIT OBJECTIVES Explain newtonian flow adequately. List various examples of Newtonian fluids Explain non-newtonian flow adequately List various examples of non-Newtonian fluids Distinguish various reograms of newtonion & non newtonion fluids. RHEOLOGY rheo – to flow logos – science ology – the study of is the study of the flow of materials that behave in an interesting or unusual manner. is the study of deformation and flow of matter. science concerned with the deformation of matter under the influence of stresses. EXAMPLES MAYONNAISE, PEANUT BUTTER, CHOCOLATE, BREAD DOUGH, PAINTS, INKS, ROAD BUILDING MATERIALS, COSMETICS, DAIRY PRODUCTS, ETC. SIGNIFICANCE OF RHEOLOGY SIGNIFICANCE OF RHEOLOGY The study of viscosity of true liquids, solutions, dilute and concentrated colloidal systems is of much importance in this study It is involved in the mixing and flow of materials, their packaging into containers, the pouring from the bottle, extrusion from a tube or a passage of the liquid to a syringe needle. SIGNIFICANCE OF RHEOLOGY Can affect the patient’s acceptability of the product, physical stability, biologic availability, absorption rate of drugs in the gastrointestinal tract. Influence the choice of processing equipments in the pharmaceutical system RHEOLOGY Definition: As the science concerned with the deformation of matter under the influence of stress, which may be applied perpendicularly to the surface of the body (tensile stress) tangentially to the surface of the body (a shearing stress) or at any other angle to the surface of the body. TYPES OF DEFORMATION: ELASTIC DEFORMATION PLASTIC DEFORMATION TYPES OF DEFORMATION: ELASTIC DEFORMATION: it is the spontaneous and reversible deformation. the work spent for the deformation is recoverable when the body returns to its original position after the removal of the applied stress. TYPES OF DEFORMATION: PLASTIC DEFORMATION: it is the permanent and irreversible deformation. the work spent for the deformation is dissipated as heat. It is not recoverable mechanically after force is removed. PLASTIC DEFORMATION is also referred to as flow and is exhibited by viscous bodies. TYPES OF FLUIDS: NEWTONIAN FLUIDS: NON NEWTONIAN FLUIDS TYPES OF FLUIDS: NEWTONIAN FLUIDS: fluids that obey Newton's law are called as Newtonian fluids NON NEWTONIAN FLUIDS: fluids that donot obey Newton's law are called as Newtonian fluids Time –independent Time –dependent NEWTONIAN SYSTEMS FLOW CHARACTERISTICS OF NEWTONIAN FLUIDS Newtonian (Newtonian Law of Flow) “the higher the viscosity of a liquid, the greater is the force per unit area (shearing stress) required to produce a certain rate of shear” WHAT IS SHEARING STRESS? Shear – is a stress which is applied parallel or tangential to a face of a material, as opposed to a normal stress which is applied perpendicularly. Shear stress Measured in (SI unit): pascal Commonly used symbols: τ Expressed in other quantities:τ = F / A Newtonian (Newtonian Law of Flow) Force Area Stress, F A L h Strain L h d Shear Rate dt Shear rate: Change in velocity/distance measured in direction of flow at right angles EXPLANATION OF TERMS USED: SHEAR: is the movement of material relative to parallel layer. SHEAR STRESS (F’): is the force per unite area required to bring about flow (F/A) SHEAR RATE (S) difference in velocity dv, between two planes of liquids separated by distance dr (i.e. dv/dr) F/A α dv/dr STRAIN Strain is the absolute amount of distortion which occurs, expressed as a fraction of an original dimension of the unstressed sample. Strain is dimensionless. VISCOSITY DEFINITION: As the measure of resistance of a fluid to deformation under shear stress. It describes the fluids internal resistance to flow . It may be thought as a measure of fluid friction. UNITS: Poise,centipoise Pascal,dynes/cm2 Stokes, centistokes VISCOSITY Application of a shearing force to a fluid usually causes it to flow because fluid cannot support a strain for very long periods of time. When the force is removed, irreversible deformation occurs. VISCOSITY The shear stress that causes a particular rate of shear is obtained by dividing the shearing force by the surface area to which the shearing force is tangentially applied. The ratio of the applied shear stress to the rate of shear is called coefficient of viscosity. FLUIDITY: it is the reciprocal of viscosity Ø = 1/ h KINEMATIC VISCOSITY: It is the absolute viscosity divided by the density of liquid at a specified temperature Kinematic viscosity = viscosity /density The unite is Stoke (s) or centistoke(cs) Thank you Any questions ?? For reference: PHYSICAL PHARMACY BY AGARWAL Pg:76-106 For further reference: Cooper and Gunn’s Tutorial Pharmacy Edited by S. J. Carter, 6th edition, 2005 (Pages: 115-127) Remington, The science and practice of Pharmacy, 21st edition, vol. 1, 2005 (Pages: 338-356) Newtonian Fluid Newtonian behaviour:Viscosity remains constant no matter what the shear rate. From IDF 1982 Consistency curve for a Newtonian fluid. EXAMPLES OF NEWTONIAN FLUIDS Water Milk Vegetable oils Fruit juices Sugar and salt solutions FLOW CHARACTERISTICS OF NEWTONIAN FLUIDS For a Newtonian fluid, the viscosity, by definition, depends only on temperature and pressure (and also the chemical composition of the fluid if the fluid is not a pure substance), not on the forces acting upon it. NON-NEWTONIAN SYSTEMS NON - NEWTONIAN FLUIDS Many pharmaceutical liquids like concentrated suspensions and emulsions do not follow Newton’s equation because the value of η varies with the rate of shear. Therefore, it is usual to consider the apparent viscosities of these systems at particular rate of shear, where the apparent viscosity, ηapp, is the ratio of shear stress to shear rate at a given point on the flow diagram. NON - NEWTONIAN FLUIDS In a non-Newtonian fluid, the relation between the shear stress and the strain rate is nonlinear, and can even be time-dependent. Therefore a constant coefficient of viscosity cannot be defined. A ratio between shear stress and rate of strain (or shear-dependent viscosity) can be defined, this concept being more useful for fluids without time-dependent behavior. NON - NEWTONIAN FLUIDS Non-Newtonian Examples An inexpensive, non-toxic example of a non-Newtonian fluid is a suspension of starch (e.g. cornflour) in water,(uncooked imitation custard, being a suspension of primarily cornflour, has the same properties). NON - NEWTONIAN FLUIDS The three types of Non Newtonian fluids are plastic, pseudoplastic and dilatant, in which the apparent viscosity varies with the rate of shear. NON NEWTONION FLOW NON NEWTONION FLOW (Time –independent) SIMPLE PLASTIC FLOW PSEUDOPLASTIC FLOW DILATANT FLOW SIMPLE PLASTIC FLOW When the graph is seen, the line does not pass through the origin of graph but arises at some point on the shear stress axis. This indicates that a certain shearing stress must be exerted before flow begins, which is called yield value. Contd.. The quantitative behavior of these systems is usually expressed in terms of the Bingham equation : U or ηpl = F-fB --------du/dx Where fB is Bingham yield value. RHEOGRAM SIMPLE PLASTIC FLOW The system behaving like a solid flows when such a stress is applied to it, exhibiting elastic deformation that are reversible when these small stresses are removed. Materials showing plastic behavior are often termed as Bingham bodies after the name of the scientist. EXAMPLES OF PLASTIC FLOW 1. Bingham plastics. clay suspensions, toothpaste, Mayonnaise The classic case is ketchup which will not come out of the bottle until you stress it by shaking Semi solids Thank you Any questions ?? For reference: PHYSICAL PHARMACY BY AGARWAL Pg:76-106 For further reference: Cooper and Gunn’s Tutorial Pharmacy Edited by S. J. Carter, 6th edition, 2005 (Pages: 115-127) Remington, The science and practice of Pharmacy, 21st edition, vol. 1, 2005 (Pages: 338-356) PSEUDOPLASTIC FLOW It can be seen from the graph that the curve arises from the origin and no yield value exists, hence the flow begins immediately on application of a shearing stress. The slope of the curve gradually increases until it reaches a maximum value. PSEUDOPLASTIC FLOW The decrease in apparent viscosity with increasing rates of shear in plastic and pseudoplastic systems results from the breakdown of structures (i.e. aggregates of dispersed particles) in the system under the influence of shear,. PSEUDOPLASTIC FLOW Greater breakdown occurs at higher rates, although beyond certain high shear rates no further structural breakdown can occur and the apparent viscosity then becomes constant. Contd.. When the shear stress is reduced or removed, reformation of the structures in these systems occurs immediately and the flow curve obtained from measurements at decreasing shear rate is superimposable on that obtained from measurements taken at increasing shear rates. Contd.. The occurrence of a yield value in plastic systems indicates that stronger forces than those in pseudoplastic systems must first be overcome before flow can occur. To distinguish between plastic and pseudoplastic systems it is necessary to obtain measurements at low shear rates. Contd.. Extrapolation of linear portions of flow curves obtained at high rates of shear should not be carried out to provide possible yield values since the system may in fact be a pseudoplastic one. Hence differentiation between these two behaviors often depends upon the sensitivity of the method of measurement at low shear rates. PSEUDOPLASTIC FLOW Also known as Shear-thinning non-Newtonian Liquid. examples tragacanth Gelatin Cmc Mucilages & gum RHEOGRAM DILATANT FLOW It can be seen from the graph that the slope of curve gradually decreases to a constant value, which indicates that the apparent viscosity must increase with increase in shear rate up to a maximum value. Dilatancy is usually exhibited by concentrated dispersions of deflocculated particles. DILATANT FLOW It is suggested that in these systems the particles are arranged in a state of close packing and the small amount of liquid present is sufficient to fill the narrow spaces between adjacent particles. DILATANT FLOW These thin liquid films allow the system to flow like a liquid when the rate of shear is low. However at high shear rates, the particles will become displaced from their close packed arrangement which results in the formation of larger void spaces in the system. DILATANT FLOW The liquid continuous medium is now insufficient to fill all the spaces between particles, hence the movement of the latter relative to each other involves a greater amount of friction and the apparent viscosity therefore increases. DILATANT FLOW This effect may be troublesome in high speed milling processes since the viscosity of dilatant suspensions may increase so much that the high rates of shear involved in the operation of these mills that overloading of the motors may occur. SHEAR-THICKENING NON-NEWTONIAN LIQUID Shear-thickening: The increase of viscosity with increasing rate of shear in a steady shear flow. Cream is a shear-thickening fluid. From IDF 1982 The consistency curve for a Pseudoplastic fluid. Thankyou Any questions ?? For reference: PHYSICAL PHARMACY BY AGARWAL Pg:76-106 For further reference: Cooper and Gunn’s Tutorial Pharmacy Edited by S. J. Carter, 6th edition, 2005 (Pages: 115-127) Remington, The science and practice of Pharmacy, 21st edition, vol. 1, 2005 (Pages: 338-356)
