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RHEOLOGY

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					Rheology

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                  Classification of liquids
1.   Newtonian materials
        Follow Newton’s law
        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
                                                                  6
 Characteristics of Newtonian flow


1.   The   passage   through   the   origin

 indicates that even a mild force can

 induce flow in these systems.




                                          7
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.
                                               9
FLOW CHARACTERISTICS OF NON-NEWTONIAN
SYSTEMS


    There rheograms represents three
     types of flow:

 - Plastic

 - Pseudoplastic

 - Dilatant.

                                    10
1. Non-Newtonian flow: plastic


  G                     




                                 G
      Yield value
                    F
         (f)
                                     11
               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.


                                                  12
             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



                                         13
             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.
                                        14
             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

                                             15
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
    Waals forces between adjacent particles,
    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.


                                              19
       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.

                                        20
Non-Newtonian flow: pseudoplastic
                           Shear-thinning



  G                    




                                            G
               F
                                                21
        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.
                                         22
          2-Pseudoplastic Flow


   The    viscosity   of   a   pseudoplastic

    substance decreases with increasing

    rate of shear. (shear-thinning systems)




                                           23
      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


                                       24
       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
                                              25
       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




                  27
               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
                                                   29
             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
    containing a high concentration (about
    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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