1. RHEOLOGY
Presented by
(Dr) Kahnu Charan Panigrahi
Asst. Professor, Research Scholar,
Roland Institute of Pharmaceutical Sciences,
(Affiliated to BPUT)
Web of Science Researcher ID: AAK-3095-2020
2. INTRODUCTION
• The branch of physics, which deals with deformation
and flow of liquid.
To the pharmacist:
• Flow of emulsions through colloid mills,
• Working of ointments on slabs or roller mills.
• Trituration of suspensions in mortar and pestle.
• Fluidity of solutions to be injected with hypodermic
syringes or infused intravenously, flexibility of tubing
used in catheters, extensibility of gut.
To the consumer:
• Squeezing toothpaste from a collapsible tube.
• Spreading lotion on his skin.
• Spraying liquids from atomizers or aerosol cans.
3. Types of Flow:
The choice depends on whether or not their
flow properties are in accordance to Newton's
law of flow.
Newtonian Non - Newtonian
5. Newton Law of Flow:
• The rate of shear indicates how fast the liquid flows
when a shear stress is applied to it. Its unit is sec-1.
• The force per unit area (F'/A) required to bring about
flow is called the shearing stress and its unit is
dyne/cm2.
• Newton Law of Flow :
The rate of shear α shearing stress
6. F'/A = η dv / dr
Where η is the viscosity .
η
=
F/G
Where F = F'/A & G = dv/dr.
Unit: Poise (p) Centipois (cp)
Fluidity () is the reciprocal of viscosity:
() = 1/η
Kinematic viscosity : is the absolute viscosity divided
by the density of the liquid
Kinematic viscosity = η/p
The units of kinematic viscosity are the stoke (s) and the
centistoke (cs).
7. 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.
Non - Newtonian bodies are those substances, which fail
to Newton's law of follow. They are classified into three
types of flow:
• Plastic
• Pseudo-plastic
• Dilatant
Non-Newtonian Systems:
8. Plastic Flow:
Materials exhibiting plastic flow are known as Bingham bodies.
The plastic flow curve does not pass
through the origin & 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)
A Bingham body does not begin to
flow until a shearing stress, corresponding
to the yield value, is exceeded.
9. • The slope of the rheogram = mobility, ( fluidity in
Newtonian systems).
• Its reciprocal is known as the plastic viscosity .
U = ( F-f ) / G (5)
• where f is the yield value, or (intercept, on the
shear stress axis in dynes cm-2).
• U is the plastic viscosity.
10. • Plastic flow is associated with the presence of
flocculated particles in concentrated suspensions. eg:
butter, ointment, paste ,gel
• 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.
• Once the yield value has been exceeded, further increase
in shearing stress (i.e. F-f ) brings about a directly
proportional increase in G, the rate of shear.
• A plastic system resembles a Newtonian system at
shear stres > the yield value.
11. Pseudoplastic Flow:
The rheogram is a non liniear curve for a pseudoplastic material which
begins at the origin.
In contrast to Bingham bodies, there is no yield value. No part of the
curve is linear, one cannot express the viscosity of a pseudoplastic
material by any single value.
Polymers dispersion exhibit this type of flow. e.g. liquid dispersions of
tragacanth, sodium alginate, methylcellulose.
12. Rate of shear increases with increase in shear stress. After
applying stress polymer molecule arranged in the direction of
force and also solvent molecule released which decrease internal
resistance.
•The viscosity diminishes as the shear is increased, so they known
as “shear thinning systems”.
FN = η' G
When N = 1, the flow is Newtonian. F= η' G.
For pseudoplastic N is greater than 1
The term η' represent apparent viscosity.
Following rearrangement, equation may be written in the
logarithmic form:
log G = N log F - log η'
This is an equation for a straight line.
13. Dilatant Flow:
• Certain suspensions with a
high percentage of dispersed
solids exhibit an increase in
resistance to flow with
increasing rates of shear.
• Such systems actually increase
in volume when sheared & are
called dilatant.
• Dilatant materials "shear
thickening systems."
• When the stress is removed, a
dilatant system returns to its
original state of fluidity.
14. FN = η' G
• N is always < 1 and decreases as the degree of
dilatancy increases.
• As N approaches 1, the system becomes
increasingly Newtonian in behavior.
• Substances possessing dilatant flow properties are
suspensions containing a high concentration (about
50 % or greater) of small, deflocculated particles.
eg: starch suspension, kaolin suspension, ZnO
suspension
15. At rest:
Particles are closely packed with small void space . The amount of
vehicle in the suspension is enough to fill this volume. The system
exhibit low consistency.
Applying shear stress
particles take an open form of packing (dilate). The amount of
vehicle in the suspension is constant & becomes insufficient to fill
the inter-particles voids. The resistance to flow increases, the
particles are no longer completely wetted or lubricated by the
vehicle. Eventually, the suspension will set up as a firm paste
16. 1. Flocculated suspensions exhibit the flow of a type:
a) Dilatant
b) Newtonian
c) Plastic
d) Pseudoplastic
Ans: c
2. In general, Newtonian fluids are expressed in terms of
viscosity. A corresponding expression in non-Newtonian
fluids (in terms of viscosity) is:
a) Apparent
b) Dynamic
c) Intrinsic
d) kinematic
Ans: a
17. 3. Fluidity is a term associated with Newtonian fluids. An equivalent
term in plastic flow fluids is:
a) apparent viscosity
b) flexibility
c) mobility
d) plastic viscosity
Ans: c
4. Deflocculated suspension with high concentration of the dispersed
so Ids exhibi e flow of type:
a) Dilatant
b) Newtonian
c) Plastic
d) Pseudoplastic
e) Ans: a
18. Thixotropy:
• 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 up-curve.
• Non Newtonian systems:
With shear-thinning systems (i.e., plastic &
pseudoplastic), the down - curve is frequently displaced
to the left of the up-curve. This means that the material
has a lower consistency at any one rate of shear on the
down-curve than it had on the up curve. This
phenomenon is known as Thixotropy.
19. Definition:
It is a comparatively slow
recovery, on standing of a
material which lost its
consistency through shearing."
Thixotropy is only applied to
shear-thinning systems. This
indicates a breakdown of
structure (shear-thinning),
which does not reform
immediately when the stress is
removed or reduced .
20. Gel state
Asymmetric particles, many points of contact ,
network structure & rigid structure.
Sol state
Breakdown of structure, flow starts, particles are
aligned and transform to sol (shear thinning)
Removal of Shearing stress
Shearing stress
Gel state
Rebuild of the gel structure by brownian motion
21. • A concentrated aqueous bentonite gel, 10% to 15%
by weight, produces a hysteresis loop with a
characteristic bulge in the up-curve. It is presumed
that the crystalline plates of bentonite form a house
of cards structure that causes the swelling of
bentonite magmas.
• In still more highly structured systems, such as a
procaine penicillin gel for intramuscular injection, the
bulged curve may actually develop into a spur like
protrusion.
22. Antithixotropy
• Negative thixotropy or antithixotropy, which represents
an increase rather than a decrease in consistency on
the down curve.
• Magnesium magma exhibit an enhanced resistance to
flow with increased time of shear compered to resting
stage.
• When magnesia magma was alternately sheared at
increasing and then at decreasing rates of shear, the
magma continuously thickened.
• Finally reached an equilibrium state in which further
cycles of increasing–decreasing shear rates no longer
increased the consistency of the material.
• Antithixotropy results from an increased collision
frequency of polymer molecules in floculated
suspension but system exhibit sol form at equlibrium.
23.
24. • Antithixotropy or negative thixotropy should not be
confused with dilatancy or rheopexy.
• Dilatant systems are deflocculated and ordinarily contain
greater than 50% by volume of solid dispersed phase.
• Whereas antithixotropic systems have low solids content
(1%–10%) and are flocculated.
• Rheopexy is a phenomenon in which a sol transform to a
gel more readily when gently shaken or otherwise sheared
than when allowed to form the gel while the material is
kept at rest.
• In a rheopectic system, the gel is the equilibrium form,
whereas in antithixotropy, the equilibrium state is the sol.
25. Choice of Viscometer
“One point" instruments :
• Used for Newtonian fluids. Since the rate of shear is
directly proportional to the shearing stress.
• The capillary and falling sphere are for use only with
Newtonian materials
“Multi-point" instruments:
• Used with non-Newtonian systems
• The instrumentation used must be able to operate at
a variety of rates of shear.
• Cup and Bob , Cone and Plate viscometers may be
used with both types of flow system
26. CAPILLARY VISCOMETER (OSTWALD VISCOMETER)
• The viscosity of a Newtonian liquid can be
determined measuring the time required for the
liquid to pass between two marks as it flows by
gravity through a vertical capillary tube known as an
Ostwald viscometer.
• If 𝜂1 and 𝜂2 are the viscosities of the unknown and
the standard liquids respectively, 𝑝1and 𝑝2 are the
respective densities of the liquids, and 𝑡1 and 𝑡2 are
the respective flow times in seconds, the absolute
viscosity of the unknown liquid,𝜂1 , is determined by
substituting the experimental value in the equation
𝜼𝟏
𝜼𝟐
=
𝝆𝟏𝒕𝟏
𝝆𝟐𝒕𝟐
27. :Falling Sphere Viscometer
The sample & ball are placed in the inner
glass tube & allowed to reach temperature
equilibrium with the water in the
surrounding constant temperature jacket.
The tube & jacket are then inverted, which
effectively places the ball at the top of the
inner glass tube.
The time for the ball to fall between two
marks is accurately measured & repeated
several times.
For newtonian liquids:
B
(
Sb – Sf
)
η = t
28. • t = the time interval in sec.
• Sb & Sf are the specific gravities of the ball &
fluid under examination at the temperature
being used.
• B is a constant for a particular ball and is
supplied by the manufacturer.
• The instrument can be used over the range 0.5
to 200,000 poise.
29. CUP AND BOB VISCOMETER:
• In cup-and bob viscometer, the sample is sheared
in the space between the outer wall of a bob and
the inner wall of a cup into which the bob fits.
• The torque resulting from the viscous drag of the
system under examination is generally measured
by a spring or sensor in the drive to the bob.
• Following equation is used for calculating
apparent viscosity for a non-Newtonian fluid
η =𝑘𝑣
𝑤
𝑣
30. :Cone and Plate Viscometer
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.
31. C is an instrumental
constant.
T is torque reading.
V is speed in revolution /
minute.
C T / V
=
η
U = C (T - T f ) / V
f = T f x C f C f is constant
Plastic materials
32. 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.
33. Factors Affecting Rheological Properties in
Pharmaceutical Products:
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.
34. (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 variations in viscosity.
35. (c) Impurities, Trace Ions and Electrolytes
• Changing the viscosity of natural polymers, e.g. in
sodium alginate solution, the viscosity to the
gelling point traces of calcium are present
the formation of calcium alginate.
• At 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
36. )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.
• Methyl cellulose has a stable pH range of 3 to 12.
• Sodium salts polymers are unstable in acid medium due
to the separation of the acid form of the polymer, e.g.
sodium alginate.
37. (E) Sequestering Agents and Buffers:
• Sequestering agents have a stabilizing effect on
viscosity in some polymer solutions, which are
decomposed by traces of metals.
Examples:
• Calcium ions the viscosity of sodium alginate.
Addition of
sequestering agents i.e. EDTA or hexameta-
phosphate will
viscosity in sodium alginate solutions.
• Tragacanth solution also suffers a rapid loss in
viscosity,
regardless of the pH, in systems, which bind
calcium ions,
i.e citrate buffers.
38. Physical Factors:
(a) Aeration:
• Aerated products usually result from high shear milling.
Aerated samples are more viscous or have more viscous
creamed layer than non-aerated samples.
• Some aerated emulsions will be less viscous & less stable
than un-aerated 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.
De-aeration is done:
• Mechanically by roll milling, which squeezes out the air.
• Heat the aerated system.
39. (b) 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 light-resistant containers,
• Screening agents, antioxidants.
• In the case of carbopol, the use of sequestering
agents.
40. (c) 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
non-Newtonian flow properties in system.
• The smaller the particle size of the dispersed solid phase,
the lower the concentration of the solids required to
produce non-newtonian flow and thixotropy.
41. Flocculation of a suspension system:
• Flocculation viscosity & thixotropy.
• The flocs or aggregates are held weakly together and
are capable of forming extended networks which give
the flocculated suspension its structural properties.
• Immobilization of a portion of the dispersing media
in the network & between the flocs viscosity.
42. 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.
43. Ordinary suspension of
pencillin G
Thixotropy suspension of
pencillin G
I.M injection
Forms no depot, fast dispersion &
absorption so maintain therapeutic
Level for short time
Forms spherical deposits at site of
injection which resists disintegration
by tissue fluids& Small surface area
( absorption) so maintain therapeutic
Level for longer time
The formation of depot depends on: a- high yield value
b-fast thixotropic recovery after injection.
44. (
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 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.
45. (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)
46. • 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
47. 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 house-of-
cards structure of bentonite.
