Rheology 2 PDF

2. Pseudoplastic Flow Viscosity decreases with an increase in shear stress (shear thinning) Cause by the re-alignment of polymer and/or the release of solvents associated with the polymer Examples: Polymers in solutions such as tragacanth, sodium alginate, methylcellulose,...

2. Pseudoplastic Flow Viscosity decreases with an increase in shear stress (shear thinning) Cause by the re-alignment of polymer and/or the release of solvents associated with the polymer Examples: Polymers in solutions such as tragacanth, sodium alginate, methylcellulose, emulsions 3. Dilatant flow Volume increases when sheared Shear thickening Eg. Suspension containing; a. high-concentration of small deflocculated particles b. More than 50%w/v of solids At rest, the particles are closely packed with a miniu interparticle volume, or voids (the vehicle is sufficient to fill this volume) As the shear stress is increased, the bulk of the system expands or dilates - Results in increase in the inter-particular void volume (vehicle becomes insufficient to fill the increase voids) - There is resistance to flow - The process is reversible Dilatant The effect is reversible and removal of the shear stress results in the re- establishment of the fluid nature Dilatancy can be a problem during the processing of dispersions and granulation of tablet masses when high-speed blenders and mills are employed Thus, dilatant material being processed could overload and damage the motor due to resultant solidification 4. Rheopectic fluids These are fluids whose strain rate is a function of time. They require a gradually increasing shear stress to maintain a constant strain rate Rheopexy is the conversion of solids to gel, up on the application of shearing stress Substances with low melting points exhibit rheopectic property Eg. Magnesia magma, clay suspension 5. Thixotropy These are fluids which thin out with time and require a decreasing stress to maintain a constant strain rate Thixotropic fluids take finite time to attain equilibrium viscosity when introduced to a step change in shear rate It is the property of some non-Newtonian pseudoplastic fluids to show a time-dependent change in viscosity; the longer the fluid undergoes shear stress, the lower its viscosity The most apparent characteristic of the Examples; gel, bentonite thixotropic system is the Hysteresis loop magma, paint formed by up curve and down curve of the rheogram 6. Antithixotropic Is believed to result from an increased collision frequency of the dispersed particles in suspensions which results in an increased inter-particle bonding with time This changes the original state of the system from a network of a large number of individual particles and small floccules to an equilibrium state consisting of a small number of relatively large floccules This helps the dispersed particles to acquire a random orientation and the network is established At rest, the large floccules break up and gradually return to the original state of small floccules and individual particles Determination of the rheological properties Viscometers are used to evaluate the rheological properties of fluids and semisolids Successful determination and evaluation of rheologic properties of any particular system depends on choosing the correct instruments For Newtinian systems - Single point instruments can be used ( thus those which operates by a single shear For non-Newtonian system - Multipoint instruments should be used Measurement of viscosity Single/one point Multipoint At a single rate of shear, one Several rates of shear, many point on the curve points on the curve Equipment Equipment 1. Ostwald viscometer 1. cup and bob 2. Falling sphere viscometer 2. Cone and plate Application Applications Newtonian fluids Non-Newtonian fluids Newtonian fluids Measurement of viscosity 1. Based on the material to be analysed and / or type of program obtained 2. Based on the principle of measuring viscosity 1. Falling sphere viscometer 2. Ostwald viscometer Factors affecting the rheological properties 1. Chemical factors Extent of polymer hydration Impurities, trace ions and electrolytes 2. Physical factors Temperature Concentration