Rheology 1 + 2 + 3

Question 1

What is rheology? How is it used in pharmacy?

Answer 1

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

Question 2

Define fluid viscosity and elasticity

Answer 2

• 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

Question 3

What are the types of flow? Describe them

Answer 3

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

Question 4

What is newton’s flow model? Compare Newtonian vs non-newtonian behavior

Answer 4

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

Question 5

Provide a graph of newtonian flow.

Answer 5

Question 6

How can viscosity be described? What is kinematic viscosity, relative viscosity and specific viscocity?

Answer 6

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 (n_r ) 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)

Question 7

How to calculate viscosity for non-newtonian?

Answer 7

• 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)

Question 8

What are some applications and behavior of plastic flow?

Answer 8

• 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

Question 9

What are some applications and behavior of pseudoplastic flow?

Answer 9

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

Question 10

What are some applications and behavior of dilatant flow?

Answer 10

• 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

Question 11

What are some applications and behavior of dynamic viscosity?

Answer 11

• 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

Question 12

What is thixotropy? Provide some examples

Answer 12

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)

Question 13

What is a thixotropic loop?

Answer 13

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

Question 14

What are some applications and behavior of thixotropy/rheology?

Answer 14

E.g. 2% sodium carboxymethylcellulose (suspending agent – large macromolecules)

Overall, applications include the following:

1. Prolonged (depot) injections e.g. penicillin G-benzathine
2. Suspensions: stable/gel structure and when shaken, flows out of the bottle
3. 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.

Question 15

How to measure the viscosity of newtonian fluids?

Answer 15

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

Question 16

How to measure the viscosity of non-newtonian fluids?

Answer 16

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

Question 17

What is viscoelasticity?

Answer 17

• 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

Question 18

For viscoelasticity, what is the difference between viscous and elastic materials?

Answer 18

• 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

Question 19

What are the five important ways in which the mechanical behavior of a polymer may deviate from expected?

Answer 19

The polymer may exhibit:

1. Time-dependence of response
2. Non-recovery of strain on removal of stress
3. Non-linearity of response, which does not imply non-recovery
4. Large strains without fracture
5. Anisotropy (directionally dependent) of response (as opposed to isotropy)

Question 20

What is the Boltzmann superposition principle (viscoelasticity)?

Answer 20

Boltzmann extended the idea of linearity in viscoelastic behavior to take account of the time dependence. He assumed that, in a creep experiment;

1. The strain observed at any time depends on the entire stress history up to that time
2. 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

Question 21

What are viscoelastic materials affected by?

Answer 21

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’

Question 22

How do semi-solids demonstrate solid and liquid behavior (viscoelasticity)?

Answer 22

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

Question 23

Compare solid (elastic) behavior of a polymer and liquid (viscous) behavior of a polymer

Answer 23

Question 24

What is a creep test? How can it be measured?

Answer 24

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.

Question 25

What rheological components can the creep test and Kevin/Voight model provide?

Answer 25

Can provide rheological components including the elastic measurement, the retardation (slow/recovery) measurement and the Newtonian measurement

Question 26

What are some pharmaceutical applications of rheology

Answer 26

• Flow figures
• Thixotropy

Question 27

Why are rheological measurements performed in pharmaceutics?

Answer 27

• Application and acceptance of pharmaceuticals and cosmetics are also dependent on the flow properties of the final product
• Rheological measurements are important to reveal the flow and deformation behaviours
• These measurements can be used to improve productivity, formulation and quality of formulations produced
• Study of rheology is used to characterize and evaluate various excipients in drug formulations and preparations

Question 28

What are the reasons that rheological measurements are performed on pharmaceutical and cosmetic materials?

Answer 28

1. To understand the fundamental nature of a system
2. For quality control of raw materials, final products, and manufacturing processes such as mixing, pumping, packaging, and filling
3. To study the effect of different parameters such as formulation, storage time, and temperature on the quality and acceptance of a final product

Question 29

What are flow figures?

Answer 29

Flow figures are used to characterize various pharmaceutical and cosmetic excipients to determine their ability to maintain desirable shape and shelf-life properties, as well as their suitability to various drug formulations

• flow figure of viscosity versus shear rate, across a wide range of shear rates, can provide important information about storage stability; optimal conditions for mixing and pumping
• also provides important information regarding the ways in which the structure changes to comply with the applied shear in different conditions, such as storage, processing, and application
• The rheological behavior of the material may change as a result of these forces. If the shear rate changes during an application, the internal structure of the sample will change and the change in stress or viscosity can then be seen

Question 30

Describe how newtonian flow, pseudoplastic flow, dilatant or shear-thickening flow is applied to flow figures/curves

Answer 30

Newtonian flow (A curve) is the simplest type, displaying a shear-independent viscosity while the material is under stress

• Water and some low-molecular-weight mineral oils are typical examples of Newtonian fluids

Pseudoplastic or shear thinning fluids (B curve) display viscosity reduction while the shear rate increases

• Typical examples of these are colloidal systems. The colloidal structure breaks down while shear rate increases, displaying reduced viscosity

Dilatant or shear thickening flow (C curve), in which viscosity increases with shear rate, is seldom encountered in the pharmaceutical and cosmetics fields

Question 31

What can flow curves be used for (see previous questions for flow curves)? How does any deviation from the initial flow curve affect the use of the final product?

Answer 31

Flow curves can be used to determine the long term stability of emulsions and creams

Any deviation from the initial flow curve could affect the use of the final product:

• A cream has to have pseudoplastic flow so that when it is applied to the skin (shear rate is increased) it flows like a liquid
• However upon the removal of the shear rate (rubbing motion) it returns to its original viscosity so that it remains on the skin

Question 32

How are flow figures also useful during manufacturing processes?

Answer 32

• If a semisolid mix has pseudoplastic flow properties, homogeneity will be achieved at higher mixing speeds as the shear rate will be increased and thus viscosity lowered
• For Newtonian mixtures, mixing speed will have no effect on viscosity e.g. with mixing water

Question 33

What is thixotropy? What are some of its properties?

Answer 33

• Time-dependent flow measures the increase or decrease in viscosity with time, while a constant shear is applied
• The flow is called thixotropic if viscosity decreases with time while a constant shear is applied, or rheopetic if it increases while a constant shear is applied
• The most desirable type of flow behaviour encountered in pharmaceutical or cosmetic products is thixotropic flow

Question 34

What are some behaviors of thixotropy?

Answer 34

Thixotropic behaviour describes a degradation of the structure during the loaded phase; thus, a reduction in viscosity with time occurs when shear is applied

• During the relieved phase, the original structure is recoverable. The extent of structural recovery is dependent on the time allowed for the recovery
• Therefore, a thixotropic material will have a shear thinning behaviour when a gradually increasing shear is applied
• This is because the orientation of the structure’s molecules or particles will change to align with the flow direction
• However, its original orientation can be restored over a period of time after the external force is removed
• There is a delay in time for the structure to recover completely

Question 35

What is a thixotropic loop? What does it represent

Answer 35

The region between curves for the increasing and decreasing shear rate ramps

• Represents the deformation history of a material and provides qualitative information about its time dependence
• The loop area indicates how fast the sample structure will recover after the load is removed
• The loop area is dependent on the sample nature and on the length of time that passes after the load is removed

Question 36

Why is a thixotropy loop test performed? What information obtained from a loop test is valuable?

Answer 36

• This test is related to the kinetics of structural change as encountered in aggregated colloidal dispersions
• If the structures in the material are broken apart by shearing and cannot reform completely during the ramp-down period, a thixotropic loop can be seen –> viscosity (or stress) during the ramp-down period will be higher than that in the ramp-up shearing period
• Shear-thinning measures how easily the structure can be broken and the loop area indicates the recovery extent of that broken structure during the experimental time

Information obtained from this loop test is valuable in many situations:

• A suspension formulation must have proper fluidity during application and preparation; a certain consistency is required so that the particles can be dispersed effectively in the container
• Therefore, it is important for the disturbed structures to rebuild at a suitable rate upon resting in order to have proper film formation and sedimentation resistance

Question 37

What is yield stress?

Answer 37

Yield stress measurement is crucial for pharmaceutical products in determining not only their shelf life but also the ease of application for the end-user

• Yield stress (tg) is defined as the minimum shear stress required to initiate flow
• Yield stress can be measured using a stress ramp experiment
• Yield stress can also be defined as the stress below which a material will not exhibit a fluid like behaviour
• This means that subjecting a material to stresses less than the yield stress will lead to a nonpermanent deformation or a slow creeping motion over the time scale of the experiment

Question 38

Using the image below

A) What is the exact yield stress

Answer 38

The exact yield stress for this material (a commercial children’s cough syrup) is 1.262 Pa

Question 39

Why is the minimum shear stresses required to cause flow an approximation (concept of yield stress)?

Answer 39

This stress value is experimental time-dependent i.e. it may vary throughout the experiment/stress

• Almost all substances will eventually flow if the time scale for imposing a shear stress (however small) is long enough

Question 40

What does the static value provide? What is the magnitude of static yield used for? (yield stress)

Answer 40

• The higher the static yield value, the more readily a medium will maintain particles in suspension with minimal sedimentation
• The magnitude of the static yield value may be used as one of the criteria for controlling sedimentation during storage and the ease of using or processing a product

Question 41

What instrumentation is used for rheology practical measurements?

Answer 41

Rotational and oscillatory rheometers, and measuring geometries

• Wide range of new and more advanced systems have been developed and refined for the measurements of wide range of parameters
• Main assessed parameters include shear force, viscosity, shear stress, torque, flow rate and others

> contraves viscometer, brookfield viscometer, bob viscometer, visco 88 cup

Visco 88

• Viscosity is based on the formulae for Newtonian liquids
• The dimensions of the inner and outer cylinders are fixed

Question 42

What are some properties of cone and plate viscometers?

Answer 42

1. Small sample volume
2. Ease cleaning
3. Temperature control
4. Constant rate of shear