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Title: Slide 1 Author: test Last modified by: amal Created Date: 5/15/2007 3:18:17 AM Document presentation format: On-screen Show (4:3) Other titles – PowerPoint PPT presentation

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Title: Rheology

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Rheology is the branch of physics which deals
with deformation and flow of matter.
The term rheology, from Greek rheo (to flow) and
logy (science). This term was used to describe
the flow of liquids and the deformation of solids.
Viscosity is an expression of the resistance of
a fluid to flow the higher the viscosity, the
greater the resistance.
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Rheology may be defined as the science that
concerned with the deformation of matter under
the influence of stress, which may be applied
perpendicularly to the surface of a body (tensile
stress), tangentially to the surface of a body
(shearing stress), or at any other angle to the
surface of the body.
  • There are two extremes of rheological behavior
  • Most viscoelastic materials lie between elastic
    and viscous behavior.
  • i) Elastic behaviorwhich refers to the ability
    of a formulation to restore its original shape
    when the external force is removed.
  • It is a spontaneous and reversible deformation.
  • Exhibited by elastic bodies.
  • ii) Viscous (or plastic) behavior, which is known
    as a property of ideal Newtonian liquids, where
    any deformation ceases when the applied force is
  • It is a permanent or irreversible deformation.
  • Plastic deformation is exhibited by viscous

Rheology is important in pharmaceutical
science. The flow properties influence each step
of the pharmaceutical development processes, such
  1. Filling.
  2. Mixing.
  3. Packing.
  4. Removal of a substance from the container,
    extrusion from a tube, or passage through a
    syringe needle.

  • The rheology of a particular product (which can
    range in consistency from fluid to semisolid to
    solid) can affect
  • patient acceptability of the product.
  • physical stability of the product
  • biological availability of the product .

  • Let us consider a block of liquid consisting of
    parallel plates of molecules as shown in the
    figure (1).
  • The bottom layer is considered to be fixed in
  • 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

Figure (1) Representation of shearing force
acting on a block of material
  • Rate of Shear, G dv/dr
  • It is the velocity difference dv between two
    planes of liquid separated by an infinite
    distance dr.
  • It is Indicates how fast a liquid flows when a
    stress is applied on it.
  • The Shearing Stress, F F'/A
  • It is the force per unit area required to cause

  • Rheogram
  • The graph representing the flow property of a
    material is termed as Rheogram.
  • To express the rheological properties of liquids
    graphs showing the variation of shear rate with
    shear stress (obtained by plotting shear rate, G,
    versus shear stress, F)
  • Assessment of Rheological Property of Materials
  • There are a number of ways to quantify the
    thixotropic behaviour of materials. Among these
    methods, viscometers or rheometers is considered
    one of the best instruments used to assess
    rheological behaviour at varying shear stress and
    shear rates.

Classification of Systems
  • A system is considered as either a Newtonian flow
    or non-Newtonian flow depending on whether
    viscosity is correlated with the shear rate or

Classification of Systems
  • The liquids that follow Newtonian flow include
    water, ethanol, benzene, ethyl ether, glycerin
    and castor oil.
  • Whereas the liquids that follow non-Newtonian
    flow include ointments, creams, gels, pastes, and

  • Newtonian systems
  • A system is said to have Newtonian flow behavior
    when its viscosity is independent of shear rate
    and dependent upon the composition of the liquid,
    temperature and pressure.
  • It is observed that viscosity decreases as the
    temperature increases, whereas it increases with
    an increase in pressure.

  • The graphs representing the flow properties are
    termed as Rheograms.
  • In case of Newtonian system the flow curve (shear
    stress vs. shear rate) is straight line passing
    through origin, indicating that shear stress (t)
    or the force per unit area (F/A) varies directly
    with the shear rate as described in the following

  • 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 a dv/dr
  • F'/A ? dv/dr ..(1)
  •  where ? is a constant known as
  • Viscosity
  • ? F / G...(2)
  •  F ? G..(3)
  • The slope of the line represents viscosity, which
    is defined as the resistance to the relative
    motion of adjacent liquid layers.

  • 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 cm2 in area and separated by a
    distance of 1 cm.
  • Centipoise (cp) 0.01 Poise.
  • Newtonian systems like water, simple organic
    liquids, true solutions and dilute suspensions
    and emulsionssolutions and dilute suspensions and

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  • 2. Non-Newtonian systems
  • As the name implies, there is a deviation from
    Newton's relation between shear stress and the
    rate of shear.
  • The viscosity of non-Newtonian fluids changes
    according to the rate of shear, thus
    non-Newtonian systems have no constant viscosity.
  • non-Newtonian systems can be of three general
    types, such as plastic, pseudo plastic and

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  • In case of plastic materials, it is observed that
    there is no flow until it reaches the yield value
    as shown in the following figure

When stress above the yield value is applied,
they exhibit free flowing liquid nature.
Materials exhibiting this type of flow property
are also termed as Bingham Bodies.
Dilatant systems (also called as shear thickening
agent) are systems whose viscosity increases with
an increase in the rate of shear, as shown in the
following Figure
This property is exhibited by dispersions
containing high percentage of small,
deflocculated particles, for example clays,
slurries, suspensions of starch in water, aqueous
glycerine or ethylene glycol.
Fig.  Effect of shear rate on the viscosity of
(A) Newtonian liquids, (B) shear-thinning systems
and (C) shear thickening
  • The viscosity of the fluid varies with the shear
    stress and the consistency depends upon
  • The rate of shear.
  • The duration of shear (Time)

Time dependant behaviour
  • Thixotropy is a term to describe an isothermal
    system in which the apparent viscosity decreases
    under shear stress, followed by a gradual
    recovery when the stress is removed.
  • The opposite of thixotropic materials are
    rheopectic materials which are materials that
    become more viscous as the duration of applied
    force increases.
  • Thixotropy and rheopexy profiles
    (viscosity vs. time).

  • Because of the wide range applications of the
    thixotropic properties in the field of pharmacy,
    formulations in particular, it is essential to
    understand this complex phenomenon utilized in
    the advanced formulations.

The factors affecting thixotropic property
  • The phenomenon of thixotropy is influenced by
    several factors like
  • pH,
  • temperature,
  • polymer concentrations,
  • polymer modification or combinations,
  • addition of cations or anions and excipients,
    such as lecithin, sodium chloride and glycerol.

  • pH
  • Due to wide variations in the pH value of the
    physiological fluids, solution to gel (solgel)
    conversion induced by pH changes seems to be an
    ideal approach for enhancement of the
    pharmacological efficacies of the topical drug
    delivery, especially ophthalmic and intravaginal

  • pH
  • One of the most widely used polymers with
    thixotropical property is polyacrylic acid (PAA)
    (Carbopol polymers), whose aqueous solutions were
    less viscouse and acidic in nature, and were
    transformed into gels upon increasing the pH.

  • 2. Temperature
  • Thermo-reversible gels can be used as a delivery
    system which requires a solgel transition at
    body temperature.
  • For example, the viscosity of Poloxamer-407
    (Pluronic F127) is increased with temperature.

Poloxamers (Pluronics) are hydrophilic non-ionic
polymeric surfactant (triblock copolymer)
consisting of a central hydrophobic block of
polypropylene glycol flanked by two hydrophilic
blocks of polyethylene glycol.
  • 3. Concentrations of the polymer(s)
  • The Poloxamer based system developed for the
    ophthalmic drug delivery showed the strong
    concentration dependence of solgelsol
  • A rheological property of binary hydroalcoholic
    gels made of Carbopol and hyaluronic acid varied
    as a function of the polymer concentration.

  • 4. Polymer combinations
  • An aqueous mixture of 1.5 HPMC and 0.3 PAA
    exhibited the rheological characteristics that
    were similar to those of 2.0 PAA solution.

  • 5. Addition of cat/anions
  • An addition of cat/anions significantly affected
    the viscosity of the thixotropic formulations.
  • An incorporation of positively charged ions, such
    as Ca, into sodium alginate (SA) dispersions
    enhanced the viscosity and shifted the flow type
    from Newtonian to a pseudo plastic flow with
    thixotropic properties.

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  • 6. Addition of excipients
  • An addition of excipients, such as lecithin,
    sodium chloride and glycerol, to the gel system
    significantly affected its viscosity, producing
    viscous thixotropic gels with enhanced stability
    of the system.
  • Lecithin, a permeation enhancer, induced solgel
    conversions of Poloxamer 407 gels through the
    formation of micellar structures and affected in
    vitro permeation rate of triamcinolone acetonide.

Pharmaceutical applications of thixotropy
  • The time-dependent change in viscous nature of
    thixotropy finds its major applications in
    pharmaceutical formulations including
  • Hydrogel,
  • Ointment,
  • Suspensions, and
  • Emulsions.
  • Through various routes including
  • Oral,
  • Topical,
  • Ophthalmic, and
  • Mucosal administration.

  • 1. Ophthalmic formulations
  • The conventional ocular drug delivery systems
    like solutions, suspensions and ointments showed
  • such as
  • Increased pre-corneal elimination,
  • High variability in efficiency
  • Blurred vision.

Various formulations including gels and
nanoparticles with thixotropic property have been
developed as an ophthalmic drug delivery system
to address these drawbacks.
  • It was reported that aqueous PAA gels
    administered into rabbit eyes could be retained
    for 46 h and resulted in a longer duration and
    greater activity of incorporated pilocarpine
    compared with viscous drug solutions.

  • 2. Parenteral formulations
  • It is of strong interest for biomedical field to
    develop a hydrogel which is able to pass through
    a needle without losing its structure.
  • An ideal thixotropic liquid should have high
    consistency under the storage conditions yet
    being removed easily.

  • 2. Parenteral formulations
  • In one study, 50 hydrogels made of hyaluronane
    and alginate were formulated, their thixotrophic
    behavior was verified, and their mechanical
    properties were determined before and after the
    passage through the needle.
  • The unique property of these gels to flow like a
    liquid with thixotropic behavior allowed them to
    be used as an injectable hydrogel drug delivery
    system for various bioactive agents (drugs,
    proteins, vaccines or plasmid DNAs).

  • 3. Nasal formulations
  • The major limiting factor for drug delivery to
    the nasal mucosa has been the mucociliary
  • It was reported that thixotropic solutions
    containing methylcellulose derivatives lowered
    the clearance rate and enhanced the
    bioavailability of the drugs administered through
    the nose.
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