Rheology

  • Created by: LBCW0502
  • Created on: 14-02-18 14:34
What is viscosity?
The internal friction of a fluid, caused by molecular attraction, which makes it resist a tendency to flow
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What is rheology?
The study of the change in form and the flow of matter, embracing elasticity, viscosity and plasticity to describe the study of fluids flow and deformation of solids (field of knowledge/treats all kinds of materials)
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Is viscosity a property limited to fluids?
Yes
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Why is rheology important in Pharmacy?
Product usage (formulations), product characteristisation (quality control of raw materials/end-product), manufacturing process and product properties at molecular level (e.g. stability)
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Describe the concept of stress
Able to deform a solid material by applying stress. If the material is purely liquid, it will flow under stress. Stress is a force per unit area (sigma/N per m^2)
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Describe the concept of strain
Deformation caused by stress
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What are the different ways of applying stress
Tensile stress and shear stress
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When does shearing occur?
Whenever fluids are physically made to move e.g. pouring, spreading, mixing
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What is shear rate?
V/H. A measure of the change in speed of intermediate layers. It describes the shearing the material experiences (s^-1)
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What is shear stress?
F/A. The force to produce the shearing action (Pa)
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What is shear strain?
D/H. The resulting displacement per unit height
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Describe features of the definition of viscosity (Newton)
Shear stress (F/A) is proportional to the shear rate (V/H). The proportionality constant is called viscosity (F/A = n V/H)
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How many centipoise are in 1 poise?
100 (1 poise = 100 centipoise)
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What is 1 Pa.s equal to?
10 poise (1mPa.s = 1 centipoise)
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What is the viscosity of water (20.2 degrees Celsius)?
1 cp or 1mPa.s
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What is kinematic viscosity?
The ratio of absolute viscosity or dynamic viscosity to density (n/p)
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What are the factors which affect viscosity?
-
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What does a flow curve show?
Stress vs shear rate (diagonal) or viscosity vs shear rate (horizontal) - for Newtonian fluids
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Describe features of Newtonian fluids
Viscosity remains constant. Simple fluids like water/oils. Viscosity is not affected by the measuring method
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Describe features of non-Newtonian fluids
Shear stress/shear rate is not constant (viscosity can only be measured at a specific shear rate (apparent viscosity). Several types - pseudoplastic/shear-thinning, plastic, shear thickening, thixotropy
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What are the two ways to measure viscosity?
Capillary viscometer and falling ball viscometer
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Describe features of a capillary viscometer
Ostwald viscometer. Capillary tube, measure flow from start mark to the stop mark. Newtonian fluids used/streamlined flow. Viscosity determined by measuring the time taken to pass between two marks as fluid follows gravity through capillary
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Which concepts apply to capillary viscometer?
Poiseuille's law (n = KtP), n1/n2 = p1t1/p2t2 (also automated versions)
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Describe features of the falling ball viscometer
Applies to Newtonian fluids only. Based on Stoke's law. Steel/glass/gold ball rolls down glass tube containing liquid under test. The speed at which the ball of a given density/diameter rolls down the tube is inversely related to viscosity of liquid
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Which concepts apply to the falling ball viscometer?
When the ball falls through the liquid, it experiences first an acceleration, then moves at a uniform terminal velocity when gravity is balanced by the viscous drag (e.g. Anton Parr, Thermo Scientific)
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Describe features of Laminar and Turbulent flow
Reynold's apparatus. Movement of one layer of fluid past another with no transfer of matter from one to another (certain speed). Above speed, mass transfer occurs between layers (turbulence). Molecules in fluid jump between layers (dissipate energy)
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Which concepts apply to Laminar and Turbulent flow?
A larger amount of energy is required to maintain turbulent flow than laminar flow. Reynolds number
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What is the difference between Laminar flow and Turbulent flow?
Laminar flow is the mass transfer which only occurs by molecular diffusion. Turbulent flow is the rapid interchange across the tube
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What does Re < 2000 indicate?
Laminar (streamline flow)
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What does 2000 < Re < 4000 indicate?
Transitional flow
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What does Re > 4000 indicate?
Turbulent flow
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What is relative viscosity?
Viscosity of the solute compared to the solvent
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What is specific viscosity?
nsp = nr - 1
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What is the reduced viscosity?
nsp / C
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What is the intrinsic viscosity?
nsp/C = k (intercept on graph with reduced viscosity against concentration)
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What is the Mark-Houwink equation?
Estimate of molecular weight and shape given the intrinsic viscosity (spherical 0, rods > 1, random coil ~ 0.5)
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Describe the graphs for non-Newtonian fluids (shear stress against shear rate)
Newtonian (diagonal), shear-thinning (lower curve), shear thickening (curve upwards), plastic (upper curve)
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Describe the graphs for non-Newtonian fluids (viscosity against shear rate)
Newtonian (horizontal), shear thinning (lower sigmoid curve), shear thickening (curve upwards), plastic (lower sigmoid curve)
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Describe features of shear thinning
Viscosity decreases with increases shear rate (common behaviour e.g. emulsions). Need to choose right combination for high viscosity/stability
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Why does shear thinning occur?
Molecules align/compress (e.g. oil droplet)
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Describe features of shear thickening
Viscosity increases with increasing shear rate (systems with high solids content).
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Why does shear thickening occur?
Characteristic of highly concentrated deflocculated suspensions. Particles at low shear are lubricated by fluid. Apply shear/resistance to flow increases due to decreased lubrication by fluid. High volume fraction/high shear stress/higher viscosity
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Describe features of plastic behaviour
Under conditions of no shear, materials behave like solids. A certain amount of force is required to induce flow (yield value in shear stress) e.g. concentrated suspensions, ketchup, mayonnaise, creams
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Describe features of thixotropy (time dependent) behaviour
Display a change in viscosity with time under conditions of constant shear rate e.g. greases, printing links, syrup (viscosity may increase/decrease with time under constant shear stress
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Describe features of thixotropy behaviour when shear stress is applied and released?
Shear rate increased and decreased back to starting point (curves don't coincide). Viscosity decreases with increasing time of shear. When left to rest, the viscosity increases back (e.g. syrups, paints)
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Which factors influence rheology of dispersions?
High volume fraction, particle size, particle distribution, electrostatic interactions, particle shape, steric hindrance
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Give examples of non-Newtonian fluid behaviour
Toothpaste (shear thickening), non-drip paints (plastic), printing inks (shear thinning) , molten polymers (plastic), starch paste/wet cement (shear/plastic)
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What are the two main ways of measuring rheological properties
Concentric cylinders (cup and bob) and cone/plate or parallel plates
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What are the five main components of rheometer design principles
Air bearing (near frictionless rotation), motor (smooth application of torque), optical encoder (measures rotation), temperature control (accuracy), geometry (connect rheometer to sample)
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Describe features of rotational rheometer designs
Controlled strain (separate motor/transducer), measured torque/stress. Applied strain/rotation (direct drive motor). Controlled stress (combined motor/transducer). Displacement sensor/measured strain/rotation, non-contact drag cup motor, torque, stat
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Describe the principles for the cup and bob (couette) set uo
Two concentric cylinders (outer/cup and inner/bob). Test fluid in annular gap. Velocity, deform, force in liquid transferred to other cylinder. Torque
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Describe the principles for the cone and plate set up
Flat horizontal plate and inverted cone. Apex of cone touches platen. Angle alpha between cone and plate very small. Sample in gap between plate and cones. Parallel plates set-up (flat plates, 0.5-2 mm gap)
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What are the two extreme for material behaviour?
Solid (elastic/deform/store energy/Hooke's law) and liquid (viscous/flow/dissipate energy/Newton law)
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Describe the time characteristic for viscoelasticity
Viscosity and elasticity properties are highly dependent on time of testing. Stress high for short time with no flow leads to a solid. Small stress over a long time with flow leads to a liquid
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Describe the energy characteristic for viscoelasticity
Energy into material (deform) is recoverable for elastic solid. Energy into material is dissipated as heat for a viscous substance
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Describe the mechanical models for viscoelasticity
Spring - purely elastic response, Hookean solid. Dashpot - purely viscous response, Newto
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What are the two types of experiments used to test viscoelasticity?
Creep test and oscillatory measurements
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Describe features of the creep test
Application of a small constant stress for a determined amount of time and study of the resulting displacement (strain). Graphs. Sedimentation rate. Syrups. Creams
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Describe features of an oscillatory measurement
The sample is wobbled backwards and forwards by very small deformations, applied in a sinusoidal manner. The resulting deformation is small enough so that the sample is not destroyed (linear region) - elastic (delta 0), viscous (delta 90 degrees)
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What is G’ storage/elastic modulus?
Relates to the amount of energy stored = solid-like component
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What is G’’ loss/viscous modulus?
Relates to the amount of energy lost = liquid-like component
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What is G* complex modulus?
Defines the overall stiffness of the sample
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What is the phase angle (delta)?
Difference of phase between stress and strain. Tan delta = G''/G' (>1 liquid,
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What are the two steps in oscillatory measurements?
Amplitude sweep: determine the linear viscoelastic region (LVR). Frequency sweep: at fixed amplitude (strain or stress), determine variation of G’ and G’’ with frequency
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Describe features of frequency dependence (rheological fingerprint)
Behaviour at extremes of time. Information on structure. Information on storage/stability. Resistance to stress
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