Rheology
Contents of
this chapter
• Definition and concept of rheology
• Concept of viscosity, kinematic and dynamic viscosity
• Newton’s law of flow and its relation to viscosity
• Factors influencing viscosity
• Types of flow of liquid
• Concept of Newtonian and nonNewtonian flow
• Plastic and Pseudoplastic systems of fluid
• Shear thinning and Shear thickening systems
• Dilatant systems – Reasons and characters
• Concept of Plasticity
• Thixotropy and its types
• Factors influencing and applications of thixotropy
• Measurement of viscosity
• Types of viscometers
• Single point and multi point viscometers
• Advantages and disadvantages of viscometers
• Concepts of Thixotropy
• Positive and negative Thixotropy
• Factors affecting rheological properties in pharmaceutical
products
• Rheopectic flow behaviour
• Pharmaceutical applications of rheology
Learning
Objectives
At the end of this
lecture, student will be able to
• Explain the concept of rheology and viscosity in pharmacy
• Discuss the applications of viscosity, kinematic and
dynamic viscosity in pharmaceuticals
• Discuss Newton’s law of flow and its relation to viscosity
• Discuss various factors influencing viscosity
• Explain the types of flow of pharmaceutical liquids
• Discuss the concepts of Newtonian and nonNewtonian flow
• Explain Plastic and Pseudoplastic systems of fluid with
examples
• Discuss Shear thinning and Shear thickening systems
• Explain the concept of dilatant systems
• Discuss the reasons for dilatancy
• Describe the characters of dilatancy
• Explain thixotropy and its applications in pharmacy
• Explain the concept of single point and multi point
viscometers
• Explain the method of determination of viscosity by
various types of viscometers
• Discuss the advantages and disadvantages of viscometers
• Explain the concept of thixotropy in pharmaceuticals
• Explain the Rheopectic flow behaviour of different liquids
• Describe the physical and chemical factors affecting
rheological
• Discuss the pharmaceutical applications of rheology
Rheology
• Rheo – to flow
• Logos – science
• Rheology is the study of the flow and deformation of
matter under stress.
• It is the study of the flow of materials that behave in an
interesting or unusual manner
Rheogram:
• The plot between the shearing stress and rate of shear.
Definition of Rheology
• The branch of physics, which deals with deformation and
flow of matter.
• Rheology governs the circulation of blood & lymph
through capillaries and large vessels, flow of mucus, bending of bones,
stretching of cartilage, and contraction of muscles.
• Fluidity of solutions to be injected with hypodermic
syringes or infused intravenously, flexibility of tubing used in catheters,
extensibility of gut.
Importance of Rheology
• 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.
• Can affect the patient’s acceptability of the product,
physical stability, biologic availability, absorption rate of drugs in the GIT
• Influence the choice of processing equipments in the
pharmaceutical system.
From the rheological viewpoint systems are:
• Solid if they preserve shape & volume.
• Liquid if they preserve their volume.
• Gaseous if neither shape nor volume remains constant when
forces are applied to them.
Mechanical Behaviour
• Elasticity Recoverable deformation
• Plasticity Permanent deformation
• Fracture Propagation of cracks in a material
• Fatigue Oscillatory loading
• Creep Elongation at constant load at High temperatures
Dynamic viscosity
• The unit of dynamic viscosity η is the “Pascal ⋅
second” [Pa⋅s].
• The unit “milliPascal ⋅ second”
[mPa⋅s]
is also often used.
– 1 Pa ⋅ s = 1000 mPa ⋅
s
• The previously used units of “centiPoise” [cP] for the
dynamic viscosity η are interchangeable with [mPa⋅s].
– 1 mPa⋅s = 1 cP
Typical viscosity
values at 20°C [mPa⋅s]
Kinematic viscosity
• When Newtonian liquids are tested by means of some capillary
viscometers, viscosity is determined in units of kinematic viscosity υ.
• The force of gravity acts as the force driving the liquid
sample through the capillary.
• The density of the sample is additional parameter.
• Kinematic viscosity υ and dynamic viscosity η are linked.
NEWTONS LAW
• According to Newtons law “higher the viscosity of a
liquid, the greater is the force per unit area (shearing stress F) required to
produce a certain rate of shear (G)”.
• Rate of shear α shearing stress
F= ῃ G
Where
F= F’/ A
G= dv/ dr
ῃ= viscosity
Newton Law of Flow
• Laminar or Stream
line: The bottom layer is considered to be fixed in place. If the top plane
of liquid is moved at a constant velocity, each lower layer will move with a velocity
∞ to its distance from the stationary layer.
• Velocity gradient
or rate of shear, dv / dr.
• The rate of shear indicates how fast the liquid flows when
a shear stress is applied to it. Its unit is sec1.
• The force per unit area (F'/A) required to bring about
flow is called the shearing stress and its unit is dyne/cm2.
F'/A = η dv / dr (1)
Where η is the viscosity.
Equation (1) is frequently written as:
η = F/G (2)
Where F = F'/A &
G = dv/dr.
For Newtonian System is shown in the figure. A straight line
passing through the origin is obtained.
Viscosity
• It is defined as resistance to the flow.
• Viscosity (h) is the resistance of a fluid material to
flow under stress. The higher the viscosity, the greater the resistance
• 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.
Units of Absolute Viscosity
The Poise (p), is
the shearing force required to produce a velocity of 1 cm/sec. between two
parallel planes of liquid each 1 cm2 in area & separated by a distance of 1
cm.
• The Centipois (cp), 1 cp = 0.01 poise.
• Fluidity (f) is the reciprocal of viscosity:
(f) = 1/η
(3)
• Kinematic viscosity: is the absolute viscosity divided by
the density of the liquid
η/ρ (4)
The units of kinematic viscosity are the stoke (s) & the
centistoke (cs).
Effect of Temperature on Viscosity
• Viscosity of a gas increases with the increase of
temperature.
• Viscosity of liquid decreases as the temperature is raised
& the fluidity of a liquid, increases with temperature.
FACTORS AFFECTING VISCOSITY
• Intrinsic Factors:
a. Molecular weight – Heavier the molecular weight, higher
the viscosity
b. Shape of particles; Large and irregularly shaped
particles will be more viscous spheroid colloids is less viscous than linear
colloids, as the latter tend to form a network within the dispersion medium.
c. Higher the intermolecular interactions, viscosity will
increase
• Extrinsic factors:
a. Pressure – Increase in pressure increases
• Added substances influences viscosity. Eg. Nonelectrolytes
like sucrose glycerin increases viscosity, storng electrolytes like metal and
ammonium ions decerases visocity.
• Temperature; inverse relationship
Types of Flow
The choice depends on whether or not their flow properties
are in accordance to Newton's law of flow.
1. Newtonian
2. Non  Newtonian
Newtonian flow
• A Newtonian fluid (named for Isaac Newton) is a fluid
whose stress versus rate of shear curve is linear and passes through the
origin. The constant of proportionality is known as the viscosity.
• Examples:
1. Water
2. Chloroform
3. Castor oil
4. Ethyl Alcohol
• Shows constant viscosity regardless of shear rates applied
at a given temperature.
• Exhibit true viscosity
• Obeys newtons law
• Ex: water, dilute suspensions,
• True solutions.
• Curve pass through origin
• Slope gives coefficient of viscosity
NonNewtonian flow
• A nonNewtonian flow is defined as one for which the
relation between F and S is not linear.
• In other words when the shear rate is varied, the shear
stress is not varied in the same proportion. The viscosity of such a system
thus varies as the shearing stress varies.
• It can be seen in liquids and in solid heterogeneous
dispersions such as emulsions, suspensions, colloids and ointments.
Non newtonian systems
Three classes:
• Plastic flow
• Pseudoplastic Flow
• Dilatent Flow
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 nonlinear 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.
• The plastic flow curve it intersects the shearing stress
axis (or will if the straight part of the curve is extrapolated to the axis) at
a particular point referred to as yield value. (f)
• 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. A Bingham body does not begin to flow
until a shearing stress, corresponding to the yield value, is exceeded.
• The reciprocal of mobility is Plastic viscosity
• EXAMPLES: ZnO in mineral oil, certain pastes, paints and
ointments.
• The slope of the rheogram = mobility, (fluidity in
Newtonian systems).
• Its reciprocal is known as the plastic viscosity.
U = (Ff) / G
Where,
f = Yield value
F = Shearing stress
G = Rate of shear
U = Plastic viscosity
• Plastic flow is associated with the presence of flocculated
particles in concentrated suspensions.
• Yield value represents the stress required to break the interparticle
contracts so that particles behaves individually.
• The yield value is present due to contacts between
adjacent particles (brought about by Van der Waal's forces).
• Consequently, the yield value is an indication of the force
of flocculation, the more flocculated the suspension, the higher will be the
yield value.
• Frictional forces between moving particles can also
contribute to the yield value.
• Once the yield value has been exceeded, any increase in
shearing stress (i.e. Ff) brings about a directly proportional increase in G,
the rate of shear.
• Aplastic system resembles a Newtonian system at shear stresses
> the yield value.
• Plastic flow explained by flocculated particles in concentrated
suspensions, ointments, pastes and gels.
Pseudo plastic flow
• Many Pharmaceutical products liquid dispersion of natural
and synthetic gums shows pseudo plastic flow.
• eg. Tragacanth in water, Sodium Alginate in water, Methyl
cellulose 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.
• 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.
• Pseudo plastic flow can be explained by Long chain
molecules of polymer.
• In storage condition, arrange randomly in dispersion
• On applying F/A, shearing stress molecules (water & polymer)
arrange long axis in the direction of force applied.
• This stress reduces internal resistance & solvent
molecules released form polymer molecules.
• Then reduce the concentration and size of molecules with
decrease in viscosity.
• The exponential equation shows this flow
FN = η G
N = no. of given exponent
η = Viscosity coefficient
• In case of pseudo plastic flow, N > 1.
i.e. More N >1, the greater pseudo plastic flow of
material.
• If N = 1, the flow is Newtonian.
Taking Log on both sides,
N log F = log η + log G
On rearrangement, we get
log G = N log F  log η
This equation gives straight line,
• As the shearing stress á
the normally disarranged molecules begin to align their long axes in the
direction of flow. This orientation reduces the internal resistance of the
material and allows a greater rate of shear at each successive shearing stress.
• In addition, some of the solvent associated with the
molecules may be released, resulting in an effective lowering of the
concentration and size of dispersed molecules.
• An equilibrium exists between the shear induced changes
and random coiling tendency caused by Brownian motion which entraps water
inside the coils. The rate of entanglement and randomization by Brownian motion
is constant, while the rate of disentanglement and alignment increases with
increasing shear stress.
• The viscosity diminishes as the shear is increased, so
they are known as “shear thinning systems”.
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.
• Nonlinear increase in rate of shear.
• Increase resistance to flow on increase rate of shear
• In which, particles are closely packed with less voids
spaces, also amount of vehicle is sufficient to fill the void volume.
• This leads particles to move relative to one another at
low rate of shear.
• So therefore, dilatant suspension can be poured from
bottle boz in these condition it is fluid.
• When stress is increased, the particles shows the open
packing and bulk of system (void volume is increase) is increased.
• But the amount of vehicle is insufficient to fill this
void space.
• Thus particles are not wetted or lubricated and develop
resistance to flow.
• Finally system show the paste like consistency.
• Because of this type of behavior, the dilatant suspension
can be process by high speed mixers, blenders or mills.
The exponential equation shows this flow
F^{N} = η G
N = no. of given exponent
η = Viscosity coefficient
In which N < 1, and decrease as the dilatancy
Increase
If N = 1, the system is Newtonian flow
Reasons for Dilatency
• At rest particles are closely packed with minimal
interparticle volume (void), so the amount of vehicle is enough to fill in voids
and permits particles to move at low rate of shear.
• Increase shear stress, the bulk of the system expand
(dilate), and the particles take an open form of packing.
• The vehicle becomes insufficient to fill the voids between
particles. Accordingly, particles are no longer completely wetted (lubricated)
by the vehicle.
• Finally, the suspension will set up as a firm paste.
• This process is reversible.
Characters of Dilatant System
Resting 
Sheared 
Closed pack particles 
Open packed particles 
Minimum void volume

Increased void volume 
Insufficient vehicle 
Sufficient 
• It describes pseudoplastic liquids which additionally
feature a yield point.
• They are mostly dispersions which at rest can build up an
intermolecular/interparticle network of binding forces (polar forces, van der
Waals forces, etc.).
• These forces restrict positional change of volume elements
and give the substance a solid character with an infinitely high viscosity.
• Forces acting from outside, if smaller than those forming
the network, will deform the shape of this solid substance elastically.
• Only when the outside forces are strong enough to overcome
the network forces  surpass the threshold shear stress called the “yield
point”  does the network collapse.
• Volume elements can now change position irreversibly: the
solid turns into a flowing liquid.
• Typical substances showing yield points include oil well
drilling muds, greases, lipstick masses, toothpastes and natural rubber
polymers.
• Plastic liquids have flow curves which intercept the
ordinate not at the origin, but at the yield point level of τ0.
Time Dependent Behaviour
Newtonian systems
• If the rate of shear was reduced once the desired maximum
rate had been reached, the down curve would be identical with &
superimposed on the upcurve.
Non Newtonian systems
With shearthinning systems (i.e., plastic & pseudoplastic),
the down  curve is frequently displaced to the left of the upcurve. This
means that the material has a lower consistency at any one of shear on the
downcurve than it had one rate of shear on the downcurve than it had on the
up curve.
Pharmaceutical and Biological Applications of Rheology
1 Prolongation of
Drug Action
• The rate of absorption of an ordinary suspension differs
from thixotropic suspension.
• Example: procaine penicillin G, a form of penicillin, of
relatively low water solubility. Aqueous suspensions containing between 40 and
70% w/v of milled or micronized procaine penicillin G + small amount of sodium
citrate & polysorbate 80 are thixotropic pastes & are of depot effect
when injected intramuscularly.
2 Effect on Drug
Absorption
• The viscosity of creams and lotions may affect the rate of
absorption of the products by the skin.
• A greater release of active ingredients is generally
possible from the softer, less viscous bases.
• The viscosity of semisolid products may affect absorption
of these topical products due to the effect of viscosity on the rate of
diffusion of the active ingredients.
(3) Thixotropy in
Suspension and Emulsion Formulation
• Thixotropy is
useful in the formulation of pharmaceutical suspensions and emulsions. They
must be poured easily from containers (low viscosity)
• Disadvantages of
Low viscosity:
– Rapid settling of solid particles in suspensions and rapid
creaming of emulsions.
– Solid particles, which have settled out stick together,
producing sediment difficult to redisperse ("caking or claying").
– Creaming in emulsions is a first step towards coalescence.
(Break down of emulsion)
• A thixotropic agent such as sodium bentonite magma,
colloidal silicon dioxide, is incorporated into the suspensions or emulsions to
confer a high apparent viscosity or even a yield value.
• At rest:
• High viscosities retard sedimentation & creaming.
• Yield values prevent them altogether; since there is no
flow below the yield stress, the apparent viscosity at low shear becomes
infinite
• Pouring the suspension or emulsion from its container:
• Shaking at shear stresses above the yield value
• The agitation breaks down the thixotropic structure so
reducing the yield value to zero & lowering the apparent viscosity. This
facilitates pouring.
• Back on the shelf, the viscosity slowly increases again
and the yield value is restored as Brownian motion rebuilds the houseof cards
structure of bentonite.
Determination of rheologic (flow) properties
Selection of viscometer
Type 
Single point Viscometer 
Multi point viscometer 
Example 
Ostwald viscometer Falling sphere viscometer 
Cup and bob viscometer Cone and plate viscometer 
Principle 
Stress α rate of shear Equipment works at Single rate of shear 
Viscosity det. at several rates of shear to get consistency curves 
Application 
Newtonian flow 
non Newtonian flow Newtonian flow 
Single point viscometers
Ostwald viscometer (Capillary)
• The Ostwald viscometer is used to determine the viscosity
of Newtonian fluid.
• The viscosity of Newtonian fluid is determined by
measuring time required for the fluid to pass between two marks.
Principle: When a
liquid flows by gravity, the time required for the liquid to pass between two
marks ( A & B) through the vertical capillary tube. the time of flow of the
liquid under test is compared with time required for a liquid of known
viscosity (Water).
• Therefore, the viscosity of unknown liquid (η1) can be
determined by using following equation:
ρ_{1t1}
η_{ 1 }=
η_{2} Eq.1
ρ_{2}t_{2}
Where,
ρ1 = density of unknown liquid
ρ2 = density of known liquid
t1 = time of flow for unknown liquid
t2 = time of flow for known liquid
η2 = viscosity of known liquid
Eq. 1 is based on the Poiseuille’s law express the following
relationship for the flow of l
η = П r2 t Δ P / 8 l V Eq:2
Where,
r = radius of capillary, t = time of flow, Δ P = pressure
head dyne/cm2 ,
l = length of capillary cm, V = volume of liquid flowing,
cm3
For a given Ostwald viscometers, the r, V and l are combine
into constant (K), then eq. 2 can be written as,
η = KtΔP
Eq.3
In which,
• The pressure head ΔP ( shear stress) depends on the
density of liquid being measured, acceleration due to gravity (g) and
difference in heights of liquid in viscometers.
• Acceleration of gravity is constant, & if the levels
in capillary are kept constant for all liquids,
• If these constants are incorporate into the eq. 3 then,
viscosity of liquids may be expressed as:
η1 = K’ t1 ρ1 eq. 4
η2 = K’ t2 ρ2 eq. 5
• On division of eq. 4 and 5 gives the eq .1, which is given
in the principle,
ρ_{1t1}
η_{ 1 }=
η_{2} Eq.6
ρ_{2}t_{2}
• Equation.6, may be used to determine the relative an
absolute viscosity of liquid.
• This viscometer, gives only mean value of viscosity because
one value of pressure head is possible.
• Ostwald viscometer is used for highly viscous fluid i.e.
Methyl cellulose dispersions
Applications of Ostwald Viscometer
• It is used in the formulation and evaluation of
Pharmaceutical dispersions system such as colloids, suspensions, emulsions etc.
• It is official in IP for the evaluation of liquid
paraffin, light liquid paraffin and dextran 40 injection.
Falling sphere viscometers
• It is called as Hoeppler falling sphere viscometers.
Principle: A
glass or ball rolls down in vertical glass tube containing the test liquid at a
known constant temprature. The rate at which the ball of particular density and
diameter falls is an inverse function of viscosity of sample.
Construction:
• Glass tube position vertically.
• Constant temprature jacket with water circulation around
glass tube
Working: A glass
or steel ball is dropped into the liquid & allowed to reach equilibrium
with temprature of outer jacket. The tube with jacket is then inverted so that,
ball at top of the inner glass tube. the time taken by the ball to fall between two
marks is measured,
repeated process for several times to get concurrent results.
For better results select ball which takes NLT 30 sec. to fall between two
marks.
η = t ( Sb – Sf ) B
Where,
t = time in sec.for ball to fall between two marks
Sb & Sf = Specific gravities of ball and fluid under examination.
B = Constant for particular ball.
Multi point viscometers (Rotational)
Cup and Bob
• Various instruments are available, differ mainly whether torque
results from rotation of cup or bob.
• Couette type
viscometers: Cup is rotated, the viscous drag on the bob due to sample
causes to turn. The torque is proportional to viscosity of sample
Ex. McMichael viscometer
• Searle type
viscometers: Bob is rotated, the torque resulting from the viscous drag of
the system under examination is measured by spring or sensor in the drive to
the bob.
Ex. Stormer viscometer
Working: The test
sample is place in space between cup and bob & allow to reach temprature
equilibrium. A weight is place in hanger and record the time to make 100
rotation by bob, convert this data to rpm. This value represent the shear rate,
same procedure repeated by increasing weight.
So then plotted the rheogram rpm Vs weights the rpm values
converted to actual rate of shear and weight converted into units of shear
stress, dy/cm2 by using appropriate constants.
Mathematical treatment:
For, rotational viscometers, the relationship can be
expressed as,
η = Kv w/v
Where
v, rpm generated by
Weight w, in gm
Kv is obtained by analyzing material of known viscosity in
poise.
Plug Flow
• Sample is placed between cup and bob
• During motion, when bob rotates, amount of stress exerted
near the wall of bob is high compared to stress at inner wall of cup
• Stress will be varying at different areas
• So this may not represent the stress of entire sample
• This can be minimized by
 Using large bob
 Increasing RPM of bob
• Plug flow is undesirable for flow of dispersion systems
but suitable for flow of paste
Cone and plate viscometer (Rotational viscometer)
Principle: The
sample is placed on at the center of the plate, which is raised into the
position under the cone. The cone is driven by variable speed motor and sample
is sheared in the narrow gap between stationary plate and rotate Rate of shear
in rpm is increased & decrease by selector dial and viscous traction or
torque (shearing stress) produced on the cone.
• The sample is placed at the center of the plate which is
then raised into position under the cone.
• The cone is driven by a variable speed motor & the
sample is sheared in the narrow gap between the stationary plate and the
rotating cone.
• The rate of shear in rev. /min. is increased &
decreased by a selector dial & the torque (shearing stress) produced on the
cone is read on the indicator scale.
• A plot of rpm or rate of shear versus scale reading or shearing
stress may be plotted
C is an instrumental constant.
T is torque reading.
V is speed in revolution / minute.
η = C T / V
U = C (T  T_{ f})
/ V
f = T f x C f Cf is constant Plastic materials
Advantages
• The rate of shear is constant throughout the entire sample
being sheared. As a result, any change in plug flow is avoided.
• Time saved in cleaning & filling.
• Temperature stabilization of the sample during a run.
• The cone and plate viscometer requires a sample volume of
0.l to 0.2 ml. This instrument could be used for the rheological evaluation of
some pharmaceutical semisolids.
Brookefield Viscometer
Viscosity for Newtonian system can be estimated by,
Where,
η = C T/V eq.1
C = Instrument constant,
T = Torque reading & V = Speed of the cone (rpm)
Plastic viscosity determined by,
U = Cf T – Tf / v eq.2
Yield value (f) = Cf × Tf
Tf = Torque at shearing stress axis (extrapolate from linear
portion of curve).
Cf = Instrument constant
THIXOTROPHY
Definition
• It is a isothermal and comparatively slow recovery, on
standing of a material which lost its consistency through shearing."
• Thixotropy is only applied to shearthinning systems. This
indicates a breakdown of structure (shearthinning), which does not reform
immediately when the stress is removed or reduced.
At rest  Gel
structure
Asymmetric particles, many points of contact, network
structure, small Low viscosity & rigid structure.
â Shearing
stress
Sol State
Breakdown of structure, flow starts, particles are aligned
and Particle collisions, contacts are more,transform to sol (shear thinning)
â Removal of Shearing stress
Gel structure
Rebuild of the gel structure by brownian motion, flocs
contacts break,
Individual particles, low consistency, (time is not defined)
• Thixotropic systems usually contain asymmetric particles
which, possess numerous points of contact & set up a loose
threedimensional network.
• At rest, this structure confers some degree of rigidity on
the system & it resembles a gel.
• As shear is applied & flow starts, this structure
begins to break down. Points of contact are disrupted & the particles
become aligned.
• The material undergoes a geltosol transformation &
exhibits shear thinning. Eg. aqueous dispersion of 8% w /w sodium
• Upon removal of the stress, the structure starts to
reform. This process is not immediate.
• It is a progressive restoration of consistency as the
asymmetric particles come into contact with each other by undergoing random
brownian movement.
• The rheograms obtained with thixotropic materials are
dependent on:
1 The rate at which shear is increased or decreased.
2 The time for which a sample is subjected to any one rate of shear.
• The Figure describes thixotropy in graphical form.
• In the flow curve the “upcurve” is no longer directly
underneath the “downcurve”.
• The hysteresis now encountered between these two curves
surrounds an area “A” that defines the magnitude of this property called
thixotropy.
• This area has the dimension of “energy” related to the
volume of the sample sheared which indicates that energy is required to break
down the thixotropic structure
The distinction
between a thixotropic fluid and a shear thinning fluid:
• A thixotropic fluid displays a decrease in viscosity over
time at a constant shear rate.
• A shear thinning fluid displays decreasing viscosity with
increasing shear rate.
• Some fluids are antithixotropic: constant shear stress
for a time causes an increase in viscosity or even solidification. Constant
shear stress can be applied by shaking or mixing.
Rheopexy
• It is a phenomenon in which a sol transforms to a gel more
readily rather than keeping a sol at rest.
• Shaking or low rate of shear is sufficient to transform a
sol into gel.
• At equilibrium the system is in gel state, while
antithixotrophy exhibits sol form
An increase in apparent viscosity with time under constant
shear rate or shear stress, followed by a gradual recovery when the stress or
shear rate is removed.
Bulges
• The curve produces a Hystersis loop with a characteristic
bulge in the upcurve
• This is due to the crystalline plates that form a “Cage
like Structure” that causes swelling in the formulation
• Eg: Bentonite Magma
Spur
• This is a rheogram wherein the bulged curve may develop
into a
•“spur like protrusion”
• Spur value represents a sharp point of structural
breakdown at a low shear rate in the upcurve
• But spur value will be missed out if shear stress at low
shear rate are not recorded properly
• Eg. Procaine Penicillin gel
Negative Thixotrophy
• Also called as anti thixotrophy
• It represents increase in consistency of the down curve.
Eg. Magnesia Magma
• Magnesia magma exhibits enhanced resistance to flow with
increased time of shear compared to resting state
• When magma is sheared alternatively with increasing and
decreasing rate of shear, magma thickens
• As this continues, extent of thickening decreases
gradually and reaches equilibrium and there will be no change in the
consistency curves on further cycles of shear rate
• Its molecular mechanism is explained as following:
At rest – On storage 
Gel structure
Low viscositysmall flocs individual particles are in large
number
â Shearing stress
Equilibrium – Sol state
Particle collisions, contacts are more, large flocs are in
small number
High consistency
â Removal
of Shearing stress
Set Aside
Flocs contacts break individual particles – Low consistency
Measurement of Thixotrophy
Method 1:
a. In a thixotrophic system the hysteresis loop is formed by
the up and down curves of the rheogram
b. The area of hysteresis loop represents thixotropic
breakdown c. It can be obtained with the help of planimeter
Method 2:
a. The nature of rheogram largely depends on the rate at which
shear is increased or decreased
b. Suppose shear rate is increased at a constant rate on the
system upto point ‘b’ and then decreased
c. when the results are plotted ’abe’ rheogram is obtained.
If the shear rate is maintained at ‘b’ for time ‘t1’ secs and then decreased,
‘abce’
d. Similarly at point ‘b’ if the shear rate is maintained
for time ‘t2’ seconds and then decreased, ‘abde’ curve is obtained
e. Structural breakdown w.r.t time at constant rate of shear
gives the rheogram
f. Based on these rheograms, Thixotropic Coefficient “B” is
calculated by the equation
(U1U2)
B= 
ln(t2/t1)
U1 and U2 = Plastic viscosities of two down curves
g. Thixotropic Coefficient “B” represents the rate of break
down
Method 3:
a. The system is subjected to different rates of shear (say
V1 and V2) and the rheogram is obtained, which shows TWO hysteresis loops.
b. The Thixotropic Coefficient “M” is calculated by the
equation
2(U1U2)
M =

ln(V2Vt1)2
U1 and U2 = Plastic viscosities of two down curves having
shear rates V1 and V2
c. Thixotropic Coefficient “M” represents the loss in
shearing stress per unit increase in shear rate
Applications:
• Thixotrophy is desirable in emulsions, suspensions and
creams
• Greater the thixotrophy, higher the physical stability of
suspension
• Degree of thixotrophy is related to specific surface Eg:
Procaine Penicillin G parentreal suspension – while injecting structural break
down takes place and product pass through hypodermic needle – after injection
original structure of gel will rebuilt – this leads to Depot of penicillin at
the site of injection in the muscle from which it is slowly released providing
sustained action
Factors Affecting Rheological Properties in Pharmaceuticals
Chemical
Factors
(a) Degree of
Polymerization
• Suspending agents, and emulsion stabilizers act in low
concentrations to produce viscous solutions (high molecular weight).
• Lower concentrations of the high molecular weight grades
of synthetic & modified natural gums are used to obtain the desired
viscosity.
(b) Extent of Polymer
Hydration
• In hydrophilic polymer solution the molecules are
completely surrounded by immobilized water molecules forming a solvent layer.
Such hydration of hydrophilic polymers gives rise to an increased viscosity.
• The solvate layer is strongly bound to the macromolecule viscosity
will be insensitive to pH changes or low concentrations of electrolytes.
• Loose solvate around the macromolecules, pH &
electrolytes will produce
(c) Impurities, Trace
Ions and Electrolytes
• Changing the viscosity of natural polymers, e.g. in sodium
alginate solution, the viscosity á
to the gelling point â the formation of
calcium alginate.
• At á
concentrations, electrolytes do not change the viscosity of natural colloids in
aqueous solution.
• Concentrations, the salts compete for the adsorbed water
molecules, surrounding the hydrated polymer, due to the affinity of the salt
ions for water.
• As the polymer molecules become dehydrated, their
dispersions decrease in viscosity & precipitation occurs
(d) Effect of pH
• Changes in pH greatly affect the viscosity & stability
of the hydrophilic natural & synthetic gums.
• The natural gums usually have a relatively stable
viscosity plateau extending over 5 or 4 pH units. Above and below this stable
pH range viscosity decreases sharply.
• Sodium salts polymers are unstable in acid medium due to
the separation of the acid form of the polymer, e.g. sodium alginate.
(E) Sequestering Agents
and Buffers
• Sequestering agents have a stabilizing effect on viscosity
in some polymer solutions, which are decomposed by traces of metals. Ex:
Calcium ions á the
viscosity of sodium alginate. Addition of sequestering agents i.e. EDTA or
hexametaphosphate will viscosity in sodium alginate solutions.
Physical
Factors
(a) Aeration
• Aerated products usually result from high shear milling.
Aerated samples are more viscous or have more viscous creamed layer than
nonaerated samples.
• Some aerated emulsions will be less viscous & less
stable than unaerated samples due to concentration of the surfactant or
emulsion stabilizer at the air liquid interface & thus deletion of the
stabilizer at the oil – water interface.
Deaeration is done:
• Mechanically by roll milling, which squeezes out the air.
• Heat the aerated system.
(b) The Degree of
Dispersion and Flocculation
• In concentrated suspensions of 3% solids & higher, a
decrease in particle size of the solid phase, produce an increase in the
viscosity of the system.
• This viscosity increase to immobilization of the vehicle
with an increase in the fraction of the suspension volume effectively occupied
by the solid.
• The addition of insoluble solids to a Newtonian vehicle
nonNewtonian flow properties in system.
• The smaller the particle size of the dispersed solid
phase, the lower the concentration of the solids required to produce
nonnewtonian flow
(c) Light
• Various hydrocolloids in aqueous solutions are reported to
be sensitive to light. These colloids include carbopol, sodium alginate &
sodium carboxymethyl cellulose.
• To protect
photosensitive hydrocolloids from decomposition:
• The use of
lightresistant containers,
• Screening agents,
antioxidants.
Summary
• Rheology is the study of the flow and deformation of
matter under stress.
• It is the study of the flow of materials that behave in an
interesting or unusual manner
• Rheogram is the plot between the shearing stress and rate
of shear.
• Kinematic viscosity  When Newtonian liquids are tested by
means of some capillary viscometers, viscosity is determined in units of
kinematic viscosity υ.
• The force of gravity acts as the force driving the liquid
sample through the capillary.
• According to Newtons law “higher the viscosity of a
liquid, the greater is the force per unit area (shearing stress F) required to
produce a certain rate of shear (G)”.
• Newtonian fluid is a fluid whose stress versus rate of
shear curve is linear and passes through the origin. The constant of
proportionality is known as the viscosity.
• A nonnewtonian flow is defined as one for which the
relation between F and S is not linear.
• When the shear rate is varied, the shear stress is not
varied in the same proportion. The viscosity of such a system thus varies as
the shearing stress varies.
• In pseudoplastic system, as the shearing stress is
increased, disarranged molecules orient themselves to the direction of flow
which reduces internal friction and resistance of the molecules and allows a
greater rate of shear at each shear stress.
• 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.
• Characters of dilatant system include both Resting and
Sheared
• At rest particles are closely packed with minimal
interparticle volume (void), so the amount of vehicle is enough to fill in
voids and permits particles to move at low rate of shear.
• Viscometers are used in the formulation and evaluation of
Pharmaceutical dispersions system such as colloids, suspensions, emulsions etc.
and they are official in IP for the evaluation of liquid paraffin, light liquid
paraffin and dextran 40 injection.
• The rate of shear is constant throughout the entire sample
being sheared. As a result, any change in plug flow is avoided.
• Time saved in cleaning & filling.
• Temperature stabilization of the sample during a run.
• Thixotropy is the property of some non newtonian
pseudoplastic fluids to show a timedependent change in viscosity; the longer
the fluid undergoes shear stress, the lower its viscosity.
• A thixotropic fluid is a fluid which takes a finite time
to attain equilibrium viscosity when introduced to a step change in shear rate.
• Rheopexy is an increase in apparent viscosity with time under constant shear rate or shear stress, followed by a gradual recovery when the stress or shear rate is removed.
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