Rheology 1 + 2 + 3 Flashcards
What is rheology? How is it used in pharmacy?
Rheology is the study of the mechanics between force, deformation and flow, in various pharmaceutical applications
- Rheology is widely used in pharmacy, categorizing solids, semisolids and fluids
- Carried out prior to manufacturing process
- Analyzed ingredients include low, medium and high viscosity fluids, semisolids, gels as well as solids
- Mechanical properties and behavior of materials in the pharmaceutical formulation is essential in the design and manufacturing of flow processes for production and quality control, in predicting storage stability and in understanding and controlling texture of final products
Define fluid viscosity and elasticity
- Fluids viscocity is its ability to resist flow
- Elasticity is the ability of a body to return to its original shape and size after deformation
What are the types of flow? Describe them
Newtonian and Non-Newtonian
- In Newtonian fluids, the rate of flow is directly proportional to the applied force, while in NonNewtonian fluids, there are many types of flow but with non-linear relationship between stress and shear rate
What is newton’s flow model? Compare Newtonian vs non-newtonian behavior
Newtons flow model: The difference in velocity dv (ms-1 ) separated by an infinitesimal distance dr (m)
- Water is a Newtonian liquid. Its viscosity does not change no matter how much stress is placed on it
- Ketchup is a non-Newtonian pseudoplastic (sheer thinning) liquid. Its viscosity decreases as the stress placed on it (shaking the bottle) increases.
- Corn starch is a non-Newtonian dilatant (sheer thickening) liquid. Its viscosity increases as the stress placed on it increases
- Toothpaste is a non-Newtonian plastic fluid. It behaves as a rigid body at low stress and flows as a viscous fluid at high stress

Provide a graph of newtonian flow.

How can viscosity be described? What is kinematic viscosity, relative viscosity and specific viscocity?
Viscosity can be best described as a hypothetical cube of fluid made up of thin layers (laminae) which slide over one another like a pack of playing cards
- Kinematic viscosity is related to the dynamic viscosity and density of the fluid (v = n/p) –> SI = m2/s-1
- RELATIVE VISCOSITY (nr ) is the viscosity ratio which is the viscosity of a solution to the viscosity of the solvent used e.g. when preparing mucilages (the relative viscosity can be changed so the produced dispersion has higher/lower viscosity than the solvent e.g. H2O)!
- Specific viscosity (see attached image)

How to calculate viscosity for non-newtonian?
- For Non-Newtonians, no single value of viscosity can be considered characteristic (at all times and in all formulations) so viscosity may be calculated from the slope of a tangent drawn to the curve at a specific point
- These viscosities are known as apparent viscosities and are only useful if quoted with the shear rate at which the determination was made
- When there is a dispersed phase where materials do not flow evenly
> Plastic flow (most common eg suspensions or concentrated emulsions)

What are some applications and behavior of plastic flow?
- Plastic materials behave as a rigid body at low stress and flow as a viscous fluid at high stress
- They have a linear stress versus strain (rate of shear) rate curve only after a particular stress (yield stress) has been exceeded
- The stress versus strain (rate of sheer) rate curve does not pass through the origin

What are some applications and behavior of pseudoplastic flow?
Examples include macromolecules such as sodium carboxymethyl cellulose etc
- the viscosity of pseudoplastic or shear-thinning materials decreases as the rate of shear (strain) increases
- As no yield stress exists the material will flow as soon as enough stress is applied to it

What are some applications and behavior of dilatant flow?
- Non-newtonian
- Dilatant behaviour occurs when particles are closely packed and the inter-particulate voids are at a minimum so at low sheer rates there is sufficiently solvent to fill these voids and lubricate the flow of the particles
- As the particles become displaced from their even distribution, they form clumps which result in larger voids being created into which the solvent drains so that resistance to flow is increased which increases the viscosity
- Removal of sheer results in a return to the fluid nature
- Dilatancy can result in problems during processing of dispersions and the granulation of tablet masses when high-speed blenders and mills are employed
- The increase in viscosity and resultant solidification could overload or damage the equipment
What are some applications and behavior of dynamic viscosity?
- When a force is applied to the upper layer it is assumed that each subsequent layer moves at decreasing velocity while the bottom layer remains stationary
- A velocity gradient exists, which is equal to the velocity of the upper layer in ms-1 divided by the height of the cube in metres
- A Newtonian fluid is a fluid whose viscosity does not change no matter what stress is applied to it
- The viscosity of a Newtonian fluid only depends on the temperature and pressure and not the forces acting upon it
What is thixotropy? Provide some examples
Thixotropy: change by touch
- An isothermal and comparative slow recovery on standing of a material whose consistency has been lost by shearing
- Examples include: natural clays and water soluble gums
- Thixotropy results from deformation of structure. Thus, starting with a structured-system and after experiencing shearing stress, forms individual components which recover to the original structure over time!
A simple example of thixotropy phenomenon is tomato sauce (see attached image)

What is a thixotropic loop?
A thixotropic loop represents the deformation history of a material (dilatant, plastic or pseudoplastic) and provides qualitative information over a time period

What are some applications and behavior of thixotropy/rheology?
E.g. 2% sodium carboxymethylcellulose (suspending agent – large macromolecules)
Overall, applications include the following:
- Prolonged (depot) injections e.g. penicillin G-benzathine
- Suspensions: stable/gel structure and when shaken, flows out of the bottle
- Topical products: e.g. steroid ointments
- Thixotropic behavior is the property of certain gels or fluids that are thick (viscous) under normal conditions, but flow (become less viscous) over time when shaken or agitated (stressed)
- Non-Newtonian fluids may show a time-dependent change in viscosity; the longer the fluid undergoes shear stress, the lower its viscosity.
How to measure the viscosity of newtonian fluids?
Capillary (Ostwald) viscometer
- Capillary viscometry was the first and most widely used method to measure the viscosity of fluids
- This procedure is based on the relationship between pressure drop and flow rate
- Capillary viscometry can be used provided the fluid is Newtonian and the flow is streamlined e.g. glycerol admixtures and Dextran Dispersions
How to measure the viscosity of non-newtonian fluids?
Rotational (Brookfield) viscometers
- These instruments rely on the viscous drag exerted on a body when it is rotated in the fluid to determine the viscosity of the fluid
- The major advantages of such instruments are that wide ranges of shear rate (including some of the non-newtonians) can be measured, and often a program of shear rates can be produced automatically e.g. mucilages
What is viscoelasticity?
- In an intermediate temperature range, called the ‘glass transition range’ the polymer is neither glass or rubber like
- Whole range of behaviour can be displayed by a single polymer, depending on the temperature and the strain-rate, i.e. how fast the deformation is performed, and whether tensile or compressive stress is used
- Synthetic polymers, wood, and human tissue as well as metals at high temperature display significant viscoelastic effects
Viscoelasticity is a property of materials that exhibit both viscous (liquid) and elastic (solid) characteristics when undergoing deformation
> factor that governs such behavior is time and thus viscoelasticity is used to characterize both, the viscosity and elasticity of various materials
> whole spectrum of viscoelastic behavior exists, from materials which are predominantly liquid to those that are predominantly solid
Our aim will focus on studying the rheological behavior of semi-solids
For viscoelasticity, what is the difference between viscous and elastic materials?
- Viscous materials resist shear flow and strain linearly with time when a stress is applied
- Elastic materials strain instantaneously when stretched and just as quickly return to their original state once the stress is removed
Under constant stress, viscoelastic materials will dissipate some of the energy in viscous flow and store the remainder, which will be recovered when the stress is removed
What are the five important ways in which the mechanical behavior of a polymer may deviate from expected?
The polymer may exhibit:
- Time-dependence of response
- Non-recovery of strain on removal of stress
- Non-linearity of response, which does not imply non-recovery
- Large strains without fracture
- Anisotropy (directionally dependent) of response (as opposed to isotropy)
What is the Boltzmann superposition principle (viscoelasticity)?
Boltzmann extended the idea of linearity in viscoelastic behavior to take account of the time dependence. He assumed that, in a creep experiment;
- The strain observed at any time depends on the entire stress history up to that time
- Each step-change in stress makes an independent contribution to the strain at any time and these contributions add to give the total observed strain
What are viscoelastic materials affected by?
Viscoelastic materials can be affected by heat:
- At low temperatures they tend to be brittle or ‘glass like’,
- At high temperatures they are flexible or ‘rubber like’
How do semi-solids demonstrate solid and liquid behavior (viscoelasticity)?
Elastic solid: obeys Hooke’s Law which demonstrates that: Stress-strain and independent of the rate of strain.
Perfect liquids: obeys Newton’s Law which demonstrates that: rate of shear (strain) a shearing stress.
- Where such anomalies occur they reflect both liquid-like and solid-like properties in a material which is termed VISCOELASTIC
- Both stress and time anomalies usually coexist –> the stress anomalies can be eliminated by using small values of stress. When only time anomalies exist the material is termed LINEAR VISCOELASTIC
stress/strain = function of time and not the stress magnitude
Compare solid (elastic) behavior of a polymer and liquid (viscous) behavior of a polymer

What is a creep test? How can it be measured?
Small stress is applied and maintained constant and the resulting strain monitored over time.
It can be measured by Kelvin or Voigt model (two types – see below).
> This model consists of a spring and dashpot in parallel
- If a fixed stress is suddenly applied, the dashpot cannot be displaced instantaneously, so that the spring does not change in length and carries none of the stress
- The dashpot is then displaced at a decreasing rate as the spring strains and takes up some of the stress
- Eventually, the dashpot and spring have both been displaced far enough for the spring to take the whole load. This is thus a model for creep.






