Pseudoplastic systems

Rheology Rheology: : has been derived from Greek words logos
“science” and Rheo “to flow”.
Viscosity is an expression of the resistance of a fluid to flow.
The higher the viscosity, the greater the resistance.
Rheology may thus be defined as:
The science concerned with the deformation of matter under
the influence of stress, which may be applied perpendicularly
to the surface of a body (tensile stress) or tangentially to the
surface of a body (a shearing stress) or at any other angle to
the surface of the body.

The deformation that results from the application of
a stress may be divided into two types:
i) Elastic Deformation :
It is a spontaneous and reversible deformation.
Exhibited by elastic bodies
ii) Plastic Deformation :
It is a permanent or irreversible deformation.
Plastic deformation is exhibited by viscous bodies.

NEWTONIAN’
S LOW OF FLOW
Let us consider a block of liquid consisting of parallel
plates of molecules as shown in the figure.
The bottom layer is considered to be fixed in place.
If the top plane of liquid is moved at constant velocity,
each lower layer will move with a velocity directly
proportional to its distance from the stationary bottom layer
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Representation of shearing force
acting on a block of material

☻Rate of Shear dv/dr = G
Is the velocity difference dv between two planes of liquid
separated by an infinite distance dr.
Indicates how fast a liquid flows when a stress is applied on it
☻The Shearing Stress F'/A = F
Is the force per unit area required to cause flow.
☻ Newton recognized that:
The higher the viscosity of a liquid, the greater the force per
unit area (shearing stress) required to produce a certain rate
of shear.Thus, the rate of shear is directly proportional to the
shearing stress .
F'/A α dv/dr
F'/A = η dv/dr (1)
where η is a constant known as viscosity
η = F / G. (2)
The unit of viscosity is poise or dyne.sec.cm -2 .

Poise
Is the shearing force required to produce a velocity of 1
cm/sec between two parallel planes of liquid each 1 cm 2 in
area and separated by a distance of 1 cm.
Centipoise
= 0.01 Poise.
(cp)
Rheogram
To express the rheological properties of liquids
Graphs showing the variation of shear rate with shear stress
(obtained by plotting F versus G)

Newtonian systems
► These systems have constant viscosity wh where
► η = F / G.
► When we plot a rheogram of G against F then we
become a strait line passing through the origin, the
slope of which is equal to the reciprocal of viscosity, a
value referred to as the fluidity Φ, Φ = 1 / η
► Newtonian systems like water, simple organic liquids,
true solutions and dilute suspensions and emulsions
Shearing
rate
Slope = Φ = 1 / η
Shearing stress
Rheogram of a
Newtonian liquid

The linear curve for newtonian liquids passing through
the origin
1. The passage through the origin indicates that even a mild
force can induce flow in these systems.
2. The linear nature of the curve shows that the viscosity (η) of
a newtonian liquid is a constant unaffected by the value of
the rate of shear.
Thus a single determination of viscosity from the shear stress
at any given shear rate is sufficient to characterize the flow
properties of a Newtonian liquid.
viscosity
Shear rate

Kinematic Viscosity
The kinematic viscosity of a liquid is its absolute viscosity
divided by the density at a definite temperature.
kinematic viscosity ( (s) ) = η η /ρ /ρ
The units of kinematic viscosity are the stoke ( (s) and the
centistoke ( (cs cs)
Relative Viscosity
Relative Viscosity (ηη r) is the ratio of
solution viscosity (ηη) ) to the viscosity of the solvent (ηη o )
(in pharmaceutical products it is often water).

FLOW CHARACTERISTICS OF NON NON-NEWTONIAN NEWTONIAN SYSTEMS
Do not follow the simple Newtonian relationship
i.e. when f is plotted against G the rheogram is not a
straight line passing through the origin
i.e. viscosity is not a constant value.
Such as colloidal dispersions, emulsions, suspensions
and ointments, etc.
There rheograms represents three types of flow:
-plastic plastic
-pseudoplastic
pseudoplastic
-dilatant. dilatant.

1. . Plastic Flow
Such materials are called Bingham bodies
Shear strain
Rheogram for a plastic material
• The curve is linear over most of its length corresponding to
that of a Newtonian fluid.
• However, the curve does not pass through the origin but rather
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 the Yield value or Bingham Yield value
f
Shear stress

§ Contrary to a Newtonian liquid that flows under the
slightest force, a Bingham body does not flow until a
definite shearing stress equal to the yield value is
applied. Below the yield value the system acts as an
elastic material.
plastic systems resembles Newtonian systems at
shear stresses above the yield value.
§ The slope of the rheogram is termed mobility,
analogous to fluidity in Newtonian systems and its
reciprocal is known as the Plastic viscosity viscosity, , U.
U = (F F - f)
G
§ Plastic systems are shear shear-thinning thinning systems

1. . Plastic Flow

Explanation of Plasticity:
► Flocculated particles in a concentrated suspensions
usually show plastic flow
► The yield value is because the van der Waals forces
between adjacent particles, which must be broken first
before flow can occur
Shear
§ The more flocculated the suspension the higher
will be the yield value

2-Pseudoplastic Pseudoplastic Flow
►A A large number of pharmaceutical products, including
natural and synthetic gums, e.g. liquid dispersions of
tragacanth, sodium alginate, methyl cellulose, and
Na Na-carboxymethylcellulose carboxymethylcellulose show pseudoplastic flow.
►As As a general rule pseudoplastic flow is exhibited by
polymers in solution solution, , in contrast to plastic systems
which are composed of flocculated particles in
suspension.

Shear strain
Rheogram of a
pseudoplastic system
Shear stress
►Curve Curve for a pseudoplastic material begins at the origin
consequently, in contrast to Bingham bodies, there is no yield
value. Since no part of the curve is linear, one can not express
the viscosity of a pseudoplastic material by any single value.
►The The viscosity of a pseudoplastic substance decreases with
increasing rate of shear. (shear (shear-thinning thinning systems)
►As As the shearing stress is increased, the normally normally-disarranged
disarranged
molecules begin to align their long axes in the direction of flow.

►Newtonian Newtonian system is completely described by η, the
viscosity.
►Plastic Plastic system is described by the yield value and the plastic
viscosity.
►Pseudoplastic Pseudoplastic systems which can not be described by a
single value are expressed by:
FN = η’ G
ŁWhen When N = 1, , equation the flow is Newtonian
ŁAs As N rises the flow becomes increasingly non Newtonian.
ŁThe The term η’ is a viscosity coefficient.
The logarithmic form is a straight line equation
log G =N N log F – log η’
A straight line is obtained when log G is plotted against log F

3-Dilatant Dilatant Flow

3-Dilatant Dilatant Flow
q Dilatant systems exhibit an increase in resistance to
flow (viscosity) with increasing rates of shear. “ shear
thickening systems”.
q Such systems actually increase in volume when
sheared and are hence termed dilatant. When the
stress is removed, a dilatant system returns to its
original state of fluidity

Shear strain
Rheogram of a dilatant
system
Shear stress
Dilatant flow is the reverse of that possessed by pseudoplastic
systems.
The equation: F FN = η’ G
can be used to describe dilatancy in quantitative terms. In this
case, N is always less than 1 and decreases as the degree of
dilatancy increases.
as N approaches 1, , the system becomes increasingly Newtonian
in behaviour

Substances possessing dilatant flow properties are invariably
suspensions containing a high concentration (about 50 percent
or greater) of small small, , deflocculated particles particles.
As discussed previously, particulate systems of this type which
are flocculated would be expected to possess plastic, rather
than dilatant flow characteristics.
Dilatant behavior may be explained as follows:
►At At rest, the particles are closely packed with the interparticle
volume, or voids, being at a minimum.
►The The amount of vehicle in the suspension is sufficient,
however, to fill this volume and permits the particles to move
relative to one another at low rates of shear.
►Thus, Thus, one may pour a dilatant suspension from a bottle since
under these conditions it is reasonably fluid.

§As As the shear stress is increased, the bulk of the system expands
or dilates, hence the term dilatant. The particles, in an attempt to
move quickly past each other, take on an open form of packing.
Such an arrangement leads to a significant increase in the
interparticle void volume.
§The The amount of vehicle remains constant and at some point,
becomes insufficient to fill the increased voids between the
particles. Accordingly, the resistance to flow increases because
the particles are no longer completely wetted or lubricated by
the vehicle. Thus, the suspension will set up as a firm paste

Time dependant behaviour
Thixotropy
When shear stress is reduced after it reaches its maximum
desired value:
Newtonian flow: The down-curve is identical with the up curve
Pseudoplastic and plastic flow: down curve is displaced to the
left of the up curve
ŁThe breakdown of the structure does not reform immediately
when the stress is reduced or removed
Shear strain
Shear stress

Thixotropy, may be defined as:
an isothermal and comparatively slow-recovery, on standing of
a material, of a consistency lost through shearing”. As so
defined, thixotropy may only be applied to shear-thinning
systems.
Antithixotropy
Is the slow loss of consistency that was gained by shearing. It is
associated with dilatant systems
Measurement of Thixotropy
The most apparent characteristic of a thixotropic system is the
hysteresis loop, formed by the up- and down-curves of the
rheogram.
This area of hysteresis has been proposed as a measure of
thixotropic breakdown; it may be obtained readily by means of a
planimeter

2- Physical Factors
A- Temperature
A temperature increase usually produces a rapid viscosity
decrease, with the exception of certain synthetic polymers such as
methyl cellulose,
B-Aeration
Aerated products usually result from high shear milling. Aerated
samples appear to be more viscous or have more viscous
creamed layer than non-aerated samples
(c) Light
Various hydrocolloids in aqueous solutions are reported to be
sensitive to light. These colloids include carbopol, Na alginate,
and Na CMC. To protect photosensitive hydrocolloids from
decomposition and resultant viscosity change use light-resistant
containers