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RHEOLOGY

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RHEOLOGY C is an instrumental constant. T is torque reading. V is speed in revolution / minute. C T / V = U = C (T - T f ) / V ... – PowerPoint PPT presentation

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Title: RHEOLOGY


1
RHEOLOGY
2
Definition
  • 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,
    contraction of muscles.
  • Fluidity of solutions to be injected with
    hypodermic syringes or infused intravenously,
    flexibility of tubing used in catheters,
    extensibility of gut.

3
  • 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.

4
  • To the pharmacist
  • Flow of emulsions through colloid mills,
  • Working of ointments on slabs or roller mills.
  • Trituration of suspensions in mortar and pestle.
  • Mechanical properties of glass or plastic
    containers of rubber closures.
  • To the consumer
  • Squeezing toothpaste from a collapsible tube.
  • Spreading lotion on his skin.
  • Spraying liquids from atomizers or aerosol cans.

5
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
6
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7
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
    8 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 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.

8
F'/A ? dv / dr (1)
Where ? is the viscosity . Equation (1)
is frequently written as ? 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.
9
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 (?) is the reciprocal of
    viscosity
  • (?) 1/? (3)
  • Kinematic viscosity is the absolute viscosity
    divided
  • by the density of the liquid
  • Kinematic viscosity ?/? (4)
  • The units of kinematic viscosity are the stoke
    (s)
  • the centistoke (cs).

10
  • 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.

11
Non-Newtonian Systems
  • Non - Newtonian bodies are those substances,
    which fail to follow Newton's law i.e. liquid
    solid , heterogeneous dispersions such as
    colloidal solutions, emulsions, liquid
    suspensions and ointments.
  • They are classified into 3 types of flow
  • Plastic.
  • Pseudoplastic.
  • Dilatant.

12
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.

13
  • 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.

14
  • Plastic flow is associated with the presence
    of flocculated particles in concentrated
    suspensions.

  • continuous structure is set up.
  • 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
    in shearing stress
  • (i.e. F-f ) brings about a directly
    proportional increase in G, the rate of shear.
  • Aplastic system resembles a Newtonian system at
    shear
  • stresses gt the yield value.

15
Pseudoplastic Flow
Polymers in solution, natural synthetic gums,
e.g. liquid dispersions of tragacanth, sodium
alginate, methylcellulose. The curve for a
pseudoplastic material begins at the origin (or
at least approaches it at low rates of shear).
The curved rheogram for pseudoplastic materials
is due to shearing action on the long chain
molecules of materials such as linear polymers.
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.
16
  • FN ?' G (6)
  • The exponent N rises as the flow becomes
    increasingly non-Newtonian.
  • When N 1, equation (6) reduces to equation (2)
    the flow is Newtonian. F ?' G
  • The term ?' is a viscosity coefficient.
  • Following rearrangement, equation (6) may be
    written in the logarithmic form
  • log G N log F - log ?'
    (7)
  • This is an equation for a straight line. Many
    pseudoplastic systems fit this equation when log
    G is plotted as a function of log F.

17
Shearing stress
Coiling entanglement
Alignment disentanglement
Due to
Random Brownian motion in fluids
Shear stress applied to the fluid
18
The curved rheogram for pseudoplastic materials
is due to shearing action on the long chain
molecules of materials such as linear polymers.
The shearing stress 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. Therefore, the
viscosity diminishes as the shear is increased
  • As the shearing stress 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 known as shear thinning
    systems.

19
  • FN ?' G (6)
  • The exponent N rises as the flow becomes
    increasingly non-Newtonian.
  • When N 1, equation (6) reduces to equation (2)
    and the flow is Newtonian. The term ?' is a
    viscosity coefficient.
  • Following rearrangement, equation (6) may be
    written in the logarithmic form
  • log G N log F - log ?'
    (7)
  • This is an equation for a straight line. Many
    pseudoplastic systems fit this equation when log
    G is plotted as a function of log F.

20
Dilatant Flow
  • Certain suspensions with a high percentage of
    dispersed solids exhibit an 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.

21
  • FN ?' G (6)
  • N is always lt 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 invariably suspensions containing a high
    concentration (about 50 or greater) of small,
    deflocculated particles.

22
At rest Particles are closely packed with small
interparticle volume. The amount of vehicle in
the suspension is enough to fill this volume.
The particles move relative to one another at
low rates of shear.
23
Applying shear stress Particle ,s arrangement is
expanded, 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.
24
Time-Dependent BehaviourThixotropy
  • 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.

25
  • 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
    .

26
Gel structure Asymmetric
particles, many points of contact , network
structure rigid structure.

Shearing stress
Sol structure Breakdown
of structure, flow starts, particles are
aligned and transform to sol (shear thinning)
Removal of Shearing stress
Gel structure Rebuild
of the gel structure by brownian motion (time is
not defined)
27
  • An aqueous dispersion of 8 w /w sodium bentonite
    sets to gel within an hour or two after
    preparation when undisturbed, but flows can be
    poured within many minutes after it had been
    stirred above the yield value. After prolonged
    rest it reverts to a gel.
  • Thixotropic systems usually contain asymmetric
    particles which, possess numerous points of
    contact set up a loose three-dimensional
    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 a gel-to-sol
    transformation exhibits shear thinning.

28
  • 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.

29
Choice of Viscometer
  • One point" instruments
  • provide a single point on the rheogram.
  • Extrapolation of a line through this point to the
    origin will result in the complete rheogram.
  • 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

30
  • 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

31
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 ) ? t

32
  • 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.

33
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.

34
C is an instrumental constant. T is torque
reading. V is speed in revolution /
minute. C T / V ?
U C (T - T f ) / V
f T f x C f
C f is constant
Plastic materials
35
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.

36
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.

37
  • (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.

38
(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

39
)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.

40
  • (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.

41
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.

42
(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.

43
(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.

44
  • 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.

45
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.

46
Thixotropy suspension of pencillin G
Ordinary 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.
47
(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.

48
(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)

49
  • 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

50
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.
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