definition,importance,types...................................................

1. RHEOLOGY
What is rheology?
Rheology is the science of flow and deformation of matter and describes the interrelation
between force, deformation and time. The term comes from Greek rheos meaning to flow.
Rheology is applicable to all materials, from gases to solids. The science of rheology is only
about 70 years of age.
 The study of viscosity is of true liquids, solutions, dilute and concentrated colloidal
systems.
 It is involved in the mixing and flow of materials, their packaging into containers, the
pouring from the bottle, extrusion from a tube or a passage of the liquid to a syringe
needle.
 It can affect the patient’s acceptability of the product, physical stability, biologic
availability, absorption rate of drugs in the gastrointestinal tract.
 It influence the choice of processing equipments in the pharmaceutical system.

2. Importance & fundamentals:
 Formulation of medicinal and cosmetic creams, pastes and lotions.
 Formulation of emulsions, suspensions, suppositories, and tablet coating.
 Fluidity of solutions for injection.
 In mixing and flow of materials, their packaging into the containers, their removal prior to
use, the pouring from the bottle.
 Extrusion of a paste from a tube .
 Passage of the liquid to a syringe needle.
 Influence the choice of processing equipments in the pharmaceutical system.
 Can affect the patient’s acceptability of the product, physical stability, biologic availability,
absorption rate of drugs in the gastrointestinal tract.
 Manufacturing of dosage forms: Materials undergo process such as mixing, flowing
through pipes, filling into the containers etc. Flow related changes influence the selection of
mixing equipment.
 Handling of drugs for administration: The syringibility of the medicines, the pouring of
the liquids from containers, extrusion of ointment from tubes, all depend on the changes in
flow behavior of dosage forms.
Two Categories of Flow & Deformation
Newtonian (Newtonian Law of Flow):
 “the higher the viscosity of a liquid, the greater is the force per unit area (shearing stress)
required to produce a certain rate of shear”

3.  Shear – is a stress which is applied parallel or tangential to a face of a material, as opposed to
a normal stress which is applied perpendicularly.
 Shear stress
 Measured in (SI unit): pascal
 Commonly used symbols: τ
 Expressed in other quantities: τ = F / A
A Newtonian fluid (named for Isaac Newton) is a fluid whose stress versus rate of strain curve is
linear and passes through the origin. The constant of proportionality is known as the viscosity.
 In common terms, this means the fluid continues to flow, regardless of the forces acting on it.
For example, water is Newtonian, because it continues to exemplify fluid properties no
matter how fast it is stirred or mixed.
A shear stress, is applied
to the top of the square
while the bottom is held in
place. This stress results in
a strain, or deformation,
changing the square into a
parallelogram.

4.  For a Newtonian fluid, the viscosity, by definition, depends only on temperature and pressure
(and also the chemical composition of the fluid if the fluid is not a pure substance), not on the
forces acting upon it.
Examples :Water, chloroform, Castor oil,ethyl Alcohol etc.
Viscosity:
It is defined as resistance to the flow. ῃ is the coefficient of viscosity. And is calculated as
ῃ=F/ G
Where F= Shearing stress
G= Rate of shear
Unit of viscosity is Poise or dyne.sec/cm2.
Non-Newtonian:
 A non-Newtonian fluid is a fluid whose flow properties are not described by a single constant
value of viscosity.
 Many polymer solutions and molten polymers are non-Newtonian fluids, as are many
commonly found substances such as ketchup, starch suspensions, paint, blood and shampoo.
 In a non-Newtonian fluid, the relation between the shear stress and the strain rate is
nonlinear, and can even be time-dependent. Therefore a constant coefficient of viscosity
cannot be defined.
Non-newtonian systems:
Three classes:
 Plastic Flow
 Pseudoplastic Flow
 Dilatant Flow

5. Plastic Flow:
• In which curve does not pass through the origin, the substance behaves initially
• Elastic body and it fails to flow when less amount of stress is applied.
• As increase the stress, leads to non-linear increase in shear rate but after that curve is linear.
• The linear portion extrapolated intersects the x axis at the point called as yield value
So, plastic flow shows Newtonian flow above the yield value.
• The curve represents plastic flow, such materials are called as Bingham bodies.
• Bingham bodies does not flow until the shearing stress is corresponding to yield Value
exceeded.
• So, yield value is important property of certain dispersions.
• The reciprocal of mobility is Plastic viscosity
 EXAMPLES: ZnO in mineral oil, certain pastes , paints and ointments.
Plastic flow explained by flocculated particles in concentrated suspensions, ointments, pastes
and gels.
Yeild velue increase stress
Flow
Curve:

6. The equation describing plastic flow is,
U = F – f / G
Where,
f = Yield value
F = Shearing stress
G = Rate of shear
Pseudo plastic flow:
Many P’ceutical products liquid dispersion of natural and synthetic gums shows pseudo plastic
flow.
eg. 1. Tragacanth in water
2. Sod. Alginate in water
3. Methyl cellulose in water
4. Sodium CMC in water
• With increase in the shearing stress the disarranged molecules orient themselves in the
direction of flow, thus reducing friction and allows a greater rate of shear at each shearing
stress.
• Some of the solvent associated will be released resulting in decreased viscosity.
• This type of flow behavior is also called as shear thinning system.
Graph for pseudo plastic flow is like this:

7.  In which curve is passing from origin (Zero shear stress), so no yield value is Obtained.
 As shear stress increases, shear rate increases but not linear.
The exponential equation shows this flow:
N = no. of given exponent
η = Viscosity coefficient
 In case of pseudo plastic flow, N > 1.
More N >1, the greater pseudo plastic flow of material.
 If N = 1, the flow is Newtonian.
Dilatant flow:
 Certain suspensions with high % of dispersed solids shows an increase in resistance to flow
with increasing rates of shear, such system increase in volume when sheared, such system
called as dilatant flow.
 Also, called as “ Shear thickening system” i.e. when stress is removed, dilatant system
return to its original position
Graph for dilatant flow is like this:
 In which curve is passing from origin (Zero shear stress), so no yield value is Obtained.

8.  Non-linear increase in rate of shear.
 Increase resistance to flow on increase rate of shear
The relationship of shear stress-stain for all fluids:
There are also fluids whose strain rate is a function of time. Fluids that require a gradually
increasing shear stress to maintain a constant strain rate are referred to as rheopectic. An
opposite case of this, is a fluid that thins out with time and requires a decreasing stress to
maintain a constant strain rate (thixotropic).
Thixotrophy (Gel-Sol-Gel):
It is defined as, isothermal and comparatively slow recovery on standing of material of a
consistency lost through shearing. It is shear thinning system, when agitated and kept aside it is
expected to return its original state of fluidity, but takes longer time to recover compared to the
time taken for agitation.
Thixotropic behavior can be shown by plastic and pseudo plastic system.

9. Anti-thixotrophy (-ve thixotrophy):
Anti-thixotrophy represents an increase in consistency (high viscosity) rather decrease in consistency in
the down curve.The increase in thickness or resistance to flow with increase time of shear
observed for (magnesia magma). Anti – thixotrophy is flocculated system containing low solid
content ( 1 – 10 %).
Dilatancy system is deflocculated system containing solid content ( > 50 %).
Rheopexy:
Rheopexy is phenomena in which a sol forms a gel more readily when shaken or sheared than
when allow to form the gel while the material is kept at rest.
e.g. Magnesia magma, Clay suspension
In rheopectic system, the gel is the equilibrium state.
In anti – thixotropic system, the sol is the equilibrium state.
Pharmaceutical Applications:
1. The viscosity of creams and lotions may affect the rate of absorption of the products by
the skin.
2. A greater release of active ingredients is generally possible from the softer, less viscous
bases.
3. The viscosity of semi-solid products may affect absorption of these topical products due
to the effect of viscosity on the rate of diffusion of the active ingredients.
4. The rate of absorption of an ordinary suspension differs from thixotropic suspension.
5. Thixotropy is useful in the formulation of pharmaceutical suspensions and emulsions.
They must be poured easily from containers (low viscosity)

10. Food rheology:
Food rheology is important in the manufacture and processing of food products, such as
cheese and gelato.
Thickening agents, or thickeners, are substances which, when added to an aqueous mixture,
increase its viscosity without substantially modifying its other properties, such as taste. They
provide body, increase stability, and improve suspension of added ingredients.
Thickening agents are often used as food additives and in cosmetics and personal hygiene
products. Some thickening agents are gelling agents, forming a gel. The agents are materials
used to thicken and stabilize liquid solutions, emulsions, and suspensions. They dissolve in the
liquid phase as a colloid mixture that forms a weakly cohesive internal structure. Food
thickeners frequently are based on either polysaccharides (starches, vegetable gums, and pectin),
or proteins.
Concrete rheology:
Concrete's and mortar's workability is related to the rheological properties of the
fresh cement paste. The mechanical properties of hardened concrete increase if less water is used
in the concrete mix design, however reducing the water-to-cement ratio may decrease the ease of
mixing and application. To avoid these undesired effects, superplasticizers are typically added to

11. decrease the apparent yield stress and the viscosity of the fresh paste. Their addition highly
improves concrete and mortar properties.
Physiology:
Physiology includes the study of many bodily fluids that have complex structure and
composition, and thus exhibit a wide range of viscoelastic flow characteristics. In particular there
is a specialist study of blood flow called hemorheology. This is the study of flow properties of
blood and its elements (plasma and formed elements, including red blood cells, white blood
cells and platelets). Blood viscosity is determined by plasma viscosity, hematocrit (volume
fraction of red blood cell, which constitute 99.9% of the cellular elements) and mechanical
behaviour of red blood cells. Therefore, red blood cell mechanics is the major determinant of
flow properties of blood.

12. References:
 Textbook of pharmaceutics
 Pharmaceutics The Science Of Dosage Form design
 www.merriam-webster.com
 www.unaab.edu.ng
 ciks.cbt.nist.gov
 sor.scitation.org
 www.rheosense.com
 www.msubbu.in/ln/fm/Unit-I/NonNewtonian.htm
 www.dictionary.com
 encyclopedia2.thefreedictionary.com/pseudoplastic
 physics.info/viscosity
 en.wikipedia.org