Rheology-Visco-elasticity Model


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MODEL FORVISCO-ELASTICITY Mr. Nitin M. Kadam M.Pharm [Pharmaceutics] [email protected]

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Non newtonian systems - contineous or steady shear rheometry. Oscilatory and creep measurement are also of considerable importance for investigating the properties of semisolid drug products, foods, cosmetics, etc. The materials are classified as a Viscoelastic materials... INTRODUCTION

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Materials Shows Viscoelastic Property Systems studied in pharmacy belong to this class are as follows, Creams, Lotions, ointments, suppositories, suspension, Colloidal dispersing, emulsifying, suspending agents , Biological Materials like Blood, sputum, and cervical fluid

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Deformation of these materials can be divided to two components: Elastic component – Hooke’s law Viscous component – Newton’s law Deformation of such materials  combination of Hooke’s law and Newton’s law.

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The behaviour of solids [elastic] were given by Hooke’s law: E= F/g E= Elastic modulus , g = strain h= viscosity, F= Shear stress G= Shear rate The behaviour of Newtonian fluids were given by Newton’s Law: Hooke’s law & Newton’s Law h = F/G Where,

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Methods that used to predict the behaviour of visco-elasticity. They consist of a combination of elastic behaviour and viscous behaviour. Two basic elements that been used in this model: Viscous dashpots with viscosity h which follows Newton’s law. Elastic spring with modulus which follows Hooke’s law Mechanical Model

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The common mechanical model that use to explain the viscoelastic phenomena are: Maxwell Spring and dashpot  align in series Voigt Spring and dashpot  align in parallel Standard linear solid Maxwell model and Voigt align in parallel.

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Maxwell model consist of spring and dashpot in series and developed to explain the mechanical behaviour. On the application of stress, the total strain is the sum of strain in spring & dashpot. Elastic spring Viscous dashpot Maxwell Model

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Can also known as the Kelvin model. It consists of a spring and dashpot in parallel. The compliance of a viscoelastic material following this model is given as a function of time, t, by, J= J∞ (1-e-t/T) Voigt Model Viscous dashpot Elastic spring Strain is expressed in as a deformation or compliance,J.

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Maxwell model can accurately predict the phenomenon stress relaxation Voigt Model can accurately predict the creep phenomenon Standard linear solid model was developed to combined the Maxwell and Voigt model  to describe both creep & stress relaxation e.g. Wool fat behaviour at 30°C. [Creep curve] Standard linear solid Model

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Oscillatory testing Does not disturb structure. Sample is subjected to oscillatory driving force in an apparatus known as Rheogoniometer. S.S produced results in SR praportional to the surface velocity of the material. The viscoelastic behaviour of materials obtained by oscillatory shear measurements can be analyzed by an extension of the Maxwell spring-and –dashpot model.

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Write a note on Viscoelasticity Gives a brief description how the chosen mechanical model can be used to estimate the creep or stress relaxation behavior for semisolid materials? Example of the exams question

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