# RHEOLOGY

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```					Rheology

1
Classification of liquids
1.   Newtonian materials
   Viscosity is constant
e.g., water, true solutions and diluted suspension
2.   Non- Newtonian
   Do not follow Newton’ law
   Viscosity changes with changing the shear force
E,g. colloidal dispersion, emulsions and gels
A.   time independent materials
   Shear thinning
   Shear thickening
B.   time dependent materials
   Thixotropic
Newtonian systems
   These systems have constant viscosity
where
   η = F / G.
   When we plot a rheogram of G against
F then we become a straight 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

3
Newtonian systems

   Newtonian systems like water, simple

organic liquids, true solutions and

dilute suspensions.

4
Newtonian Viscosities of Common Liquids

Material      Temperature    Viscosities
(C)           (poise)
Water         20             0.0100
50             0.0055
99             0.0028
Ethanol       20             0.0120
50             0.0070
Benzene       20             0.0065
50             0.0044
Glycerin      20             15
Castor oil    20             10.3
Newtonian flow


G

G
F
F = Shear stress; G = Shear rate;  = Viscosity Coefficient
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Characteristics of Newtonian flow

1.   The   passage   through   the   origin

indicates that even a mild force can

induce flow in these systems.

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Characteristics of Newtonian flow
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.                           8
FLOW CHARACTERISTICS OF NON-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.
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FLOW CHARACTERISTICS OF NON-NEWTONIAN
SYSTEMS

   There rheograms represents three
types of flow:

- Plastic

- Pseudoplastic

- Dilatant.

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1. Non-Newtonian flow: plastic

G                     

G
Yield value
F
(f)
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1. Plastic Flow
   Such     materials   are   called   Bingham

bodies

   The curve is linear over most of its length

corresponding to that of a Newtonian fluid.

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1. Plastic Flow
   However, the curve does not pass
through the origin but rather intersects
the shearing stress axis at a particular
point referred to as the Yield value or
Bingham Yield value

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1. Plastic Flow
   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.
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1. Plastic Flow
   The slope of the rheogram is termed
mobility,    analogous    to    fluidity in
Newtonian systems and its reciprocal is
known as the Plastic viscosity, U.
   U = (F - f)
G
   Plastic systems are shear-thinning systems

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1. Plastic Flow

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Explanation of Plasticity:
   Flocculated particles in a concentrated
suspensions usually show plastic flow

   The yield value is because the van der
which must be broken first before flow
can occur

   The more flocculated the suspension the
higher will be the yield value          17
Explanation of Plasticity:

18
2-Pseudoplastic Flow
   A   large   number     of   pharmaceutical
products,     including      natural     and
synthetic gums, e.g. liquid dispersions of
tragacanth,   sodium      alginate,    methyl
cellulose and Na-carboxymethylcellulose
show pseudoplastic flow.

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2-Pseudoplastic Flow

   As a general rule pseudoplastic flow
is exhibited by polymers in solution,
in contrast to plastic systems which
are composed of flocculated particles
in suspension.

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Non-Newtonian flow: pseudoplastic
Shear-thinning

G                    

G
F
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2-Pseudoplastic Flow

   Curve for a pseudoplastic material

begins at the origin consequently, in

contrast to Bingham bodies, there is

no yield value and no part of the curve

is linear.
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2-Pseudoplastic Flow

   The    viscosity   of   a   pseudoplastic

substance decreases with increasing

rate of shear. (shear-thinning systems)

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2-Pseudoplastic Flow
►Newtonian     system      is   completely
described by η, the viscosity.
►Plastic system is described by the yield
value and U, the plastic viscosity.
►Pseudoplastic systems which can not
be described by a single value are
expressed by:
FN = η’ G

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2-Pseudoplastic Flow
*When N = 1, equation the flow is Newtonian
*As N rises the flow becomes increasingly non
Newtonian.
*The term η’ is a viscosity coefficient.
The logarithmic form is a straight line
equation
log G = N log F – log η’
A straight line is obtained when log G is plotted
against log F
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Explanation of pseudo-plasticity

   Most of the pseudoplastic materials
consist of long chain molecules which
are disarranged at rest, but As the
shearing stress is increased, the
normally-disarranged molecules begin
to align their long axes in the direction
of flow.
3-Dilatant Flow

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3-Dilatant Flow
   Dilatant   systems     exhibit   an   increase        in

resistance to flow (viscosity) with increasing

rates of shear. “ shear thickening systems”.

   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        28
Non-Newtonian flow: dilatant
Shear-thickening

G                     

G
F
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3-Dilatant Flow
   Dilatant flow is the reverse of that possessed
by pseudoplastic systems.
   The equation: 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
30
3-Dilatant Flow
   Substances     possessing   dilatant    flow
properties are invariably suspensions
50   percent    or   greater)   of     small,
deflocculated particles.

   Flocculated    would   be   expected      to
possess plastic, rather than dilatant flow
characteristics.                          31
Explanation of dilatancy

   At rest, the particles are closely packed
with the interparticle volume, or voids.
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.
32
Explanation of dilatancy
   As the shear stress is increased, the bulk of the system
expands or dilates. Such expansion leads to a
significant increase in the interparticle void volume
and the amount of vehicle remains constant and
become 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
Explanation of dilatancy
Examples of non-Newtonian
fluids
   Shear thinning: Ketchup, toothpaste,
blood, paint, nail polish, whipped cream,
and face-cream work the opposite way.
They start off relatively thick and viscous
but become more runny if you subject
them to forces.
   Shear thickening: Cornstarch, custard,
Some slurries and pastes thicken up
when you subject them to forces.

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 views: 507 posted: 4/4/2011 language: English pages: 35