Rheology PDF

Rheology By MARY-ANN ARCHER Outline Definision Importance of rheology in pharmacy and its application Viscosity Newtonian laws Thixotropy Determination of viscosity Factors affecting rheological properties Pharmaceutical applications of rheology objectives At the end of...

Rheology By MARY-ANN ARCHER Outline Definision Importance of rheology in pharmacy and its application Viscosity Newtonian laws Thixotropy Determination of viscosity Factors affecting rheological properties Pharmaceutical applications of rheology objectives At the end of this lesson, students should be able to: Define rheology Define and understand Newton’s law of flow and its application Differentiate flow properties and corresponding rheogram between Newtonian and Non-Newtonian materials Appreciate the fundamentals of determination of rheological properties Recognise different factors affecting rheological properties of materials Provide examples of fluid pharmaceutical products exhibiting various rheological behaviours Definition The term rheology was suggested by Bingham and Crawford in 1910 Rheology is derived from the Greek word, rheo which means flow and logos which also means science Rheology is defined as the science which deals with flow (liquids and gases) and deformation (solids) of materials under the influence of stress Introduction The deformation of any pharmaceutical system can be divided into 2 type: a. Elastic flow : it is a spontaneous and reversible deformation b. Plastic deformation: it is a permanent and irreversible deformation c. Plastic deformation flow is exhibited by viscous bodies. it has great importance in liquid dosage forms Ideal solids deform elastically and ideal fluids deform plastically as they flow Recoverable versus Permanent The deformation is recoverable if the material returns to its Deformation is permanent if the initial shape when the stress is material remains deformed removed. when the stress is removed Elastic deformation is Plastic deformation is recoverable permanent Definition of Fundamental terminologies Shear stress : is the stress applied per unit area to make liquid flow (force/area) Shear : is the movement of material relative to parallel layer Shear rate : is the rate at which a progressive shearing deformation is applied to some materials Rheogram: plot of rate of shear as a function of shear stress Viscogram: plot of rate of shear as a function of viscosity  The deformation of matter under influence of force or stress can de described by 2 components; a. Elasticity: Elasticity is achieved if the shape of the body is restored once the force is withdrawn b. Viscosity: is defined as the property of fluid which offers resistance to the movements of one layer of fluid over adjacent layer. - it is denoted by Tau - in a fluid, the top layer causes a shear stress on the adjacent lower layer and the layer causes shear on adjacent top layer Classification of rheological systems According to the type of flow, materials can be classified into; a. Newtonian systems - Fluids which obey the Newtonian law of viscous flow b. Non-Newtonian systems - Fluids which do not obey the Newtonian law of flow Newtonian and Non-Newtonian Flow Newtonian systems A Newtonian fluid whose stress rate of shear curve is linear and passes through the origin This means that, the fluid continues to flow regardless of the forces acting on it. The constant of proportionality is known as viscosity Example: water, chloroform, castor oil, syrup Non-Newtonian fluids Time independent Time dependent 1. Plastic Thixotropic 2. Pseudoplastic Antithixotropic 3. Dilatant Rheopexy Negative rheopexy Non-Newtonian fluids A non-Newtonian flow is defined as one for which the relation between shear of stress and shear of rate are not linear Thus, varying shear rate does not result to a corresponding varying of shear stress in the same proportion The viscosity of such a system, thus varies as the shearing stress varies With time dependent non-Newtonian fluids, the viscosity of the fluid is dependent on the temperature, shear rate and time For time independent non-Newtonian fluids, they instantaneously adapt to changing shear stress Non-Newtonian fluids 1. Plastic materials or Bingham plastics require an initial finite force called yield value, before any rheological flow can start Examples: clay suspension, toothpaste, tomato paste, chocoloate, etc.