Physical Pharmaceutics II

1. Presentation
on
RHEOLOGY
Presented By: Ms. Sarika S. Suryawanshi
M. Pharm Pharmaceutics
Associate Professor
Ashokrao Mane College of Pharmacy, Peth
Vadgaon

2.  Introduction:
 Rheo – to flow
 logos – science
 Resistance to flow of liquid & deformation of
solid
 Used in simple liquids, ointments, cream,
pastes
 Change in flow behavior under stress
condition
1. Manufacturing of dosage forms: mixing,
flowing through pipes, filling into containers
II. Handling of Drugs for administration: pouring
from the bottle, extrusion from a tube or a
passage of the liquid to a syringe needle

3.  Concept of viscosity
 Shear stress: Force per unit area which
applied to bring the flow.
 Shear Stress F= F’/A
 Rate of shear: change in velocity, dv
with infinite change in distance, dr
 Rate of shear G = dv/dr
 Higher viscosity, greater force per unit
area required to produce rate of shear.
 F=nG
 n= coefficient of viscosity
 Centipoise, cp= 0.01 P
 CGS:dy.sec/cm2

4.  Factors:
 Intrinsic Factors
1. Molecular weight: Higher molecular
weight , higher viscosity
2. Large & irregular shape particles
more viscous than regular shape
3. Intermolecular interaction: stronger,
particles stick to each other
enhances viscosity

5.  Extrinsic Factors
 Pressure: enhances cohesive forces
 Addition of nonelectrolytes: increases
 Polymers
 Strong electrolytes: Decreases, alkali
metal
 Temperature: breaking of cohesive
forces leads to decrease viscosity

6.  Determination of flow properties
 Capillary Viscometer
 Falling sphere viscometer
At a Single rate of shear one point on the
curve
 Newtonian fluids
 Cup & Bob
 Cone & Plate
 Rate of shear many points on the curve
 Both

7. Ostwald Viscometer
n1 = ρ1t1/ ρ2t2 X n2
ρ1- density of unknown liquid
t1- time of flow of unknown
liquid
n2- viscosity of known liquid
Applications
1. Quality control purpose in
formulation & evaluation of
dispersed system
2. Evolution of liquid paraffin,
dextran 40 injection

9.  Based on Hoeppler vicometer
 Glass tube placed vertically
 Constant temperature jacket for water
circulation is arranged around glass tube
 Steel / glass ball dropped & allowed to reach
equilibrium with temp of outer jacket
 Invert tube with jacket
 Note time taken for the ball to fall between two
marks
 Newtonian liquids
 n1= t(Sb - Sf) B
 Sb & Sf- specific gravity of ball & fluid
 0.5-200000 poise
 NLT 30 sec

11.  Rotational viscometer
 Bob rotatory
 No. of rpm & torque represents rate of
shear & shearing stress rsp.
 Pseudoplastic system-
 n= Kv W/v
 W- shearing stress
 V- rpm
 n- viscosity
 Kv- contant

12.  Plug flow
 Bob exert pressure on inner wall of cup
 Use largest bob to reduce gap
 Increase speed of bob
Cone & Plate
 Plug flow can not be observed
 0.1- 0.2 ml sample
 Cleaning & filling easy
 Less time required

13. Newtonian flow
n= C T/v
T= torque
V= speed
Plastic flow
U= Cf T- Tf/v

14. Brookfield viscometer

15. Types of Flow
Newtonian (Newtonian Law of Flow)
 Liquid obeys Newton’s law
 F= nG
 Shear stress & rate of shear in the form
curve called rheogram or consistency curve
 Rheogram passes through origin & slope
gives the coefficient of viscosity
 In a Newtonian fluid, the relation between the
shear stress and the strain rate is linear
 Eg: water, glycerin, solution of syrup

17.  Non-Newtonian
 Does not fallows Newton's law
 Many polymer solutions and ketchup,
starch suspensions, paint, blood and
shampoo.
 Plastic
 Pseudoplastic
 Dilatant

18. Plastic
 Curve doesn't pass through the origin.
 Substance initially behaves like an elastic body &
fails to flow when less amount of stress is applied.
 Further increase in shear stress leads to nonlinear
portion get linear
 Linear portion extrapolated intersects the X axis at
the point called yield point
 flocculated particles in suspensions, butter, pastes,
gel
 Yield value represents stress required to break the
inter particle contact so particle behave individually.
 Once stress is increases with rate of shear

19.  Material shows plastic flow called Bingham
bodies
 Slope = mobility & reciprocal is called
plastic flow
 U= F-f/ G
 F= hear stress
 F= yield value
 G= rate of shear

21. Pseudoplastic Flow
 Curve begins at origin (nearly zero at lower shear
stress)
 Stress increase with rate of shear but it is nonlinear
 Polymers in water such as tragacanth, sodium
alginate, methylcellulose

23. Dilatant Flow
 Enhances resistance of flow with increasing rate of
shear
 Volume increases , so called dilatant
 Shear thickening
 Stress is removed system returns to initial state
 Suspension containing high-concentration of small
deflocculated particles
 Suspension of starch
 Zinc oxide 30% in water

24.  At rest, molecules are closely packed,
minimum void space, amount of vehicle is
sufficient to fill void space.
 When shear stress is applied particles are
open or expands / dilatants
 Increases void space, insufficient fluid,
particle not wetted, slows paste like
consistancy
 F N = nG
 N is higher than 1

26. THIXOTROPY

27.  Hysteresis loop
 Up & down curves of thixotropic system
 Region between curves for the increasing &
decreasing shear rate ramps
 The area of Hysteresis is a measurement of
thixotropic breakdown.
 Shearing rate of plastic thixotropic material is
increased at constant rate from point a –b then
decreased at same rate at point e.
 Connectivity gives formation of Hysteresis loop abe
 Staring from point a if sample on application of
shear taken to point b & if at this point shear rate is
held constant for some time , t1 sec then depending
upon extent of time of shear

28.  Rate of shear & degree of sample structure it
shows reduction in shearing stress & hence
consistency of material.
 Further decrease in shear rate results in
formation of Hysteresis loop abce
 If sample held at constant rate of shear at
point (b) for some extended time t2 sec, loop
abcde is observered
 So, rheogram of thixotropic material is not
unique but it depends upon sample &
material

29.  Rheological property depends on rate at
which shear is increased or decreased &
length of time for which material is subjected
to any rate of shear.
 Procaine penicillin suspension in water

30.  Bulges
 Swell in presence of water gives bulges
 Conc aq solution of bentonite gel
(magma) 10-15 % gives hysteresis loop
with bulge in up curve
 Due to formation of some specific str of
crystalline plates of bentonite which
leads to swelling of magma.
 Swelled 3D str responsible for bulge in
up curve

31.  Spurs
 Highly structured material such as
parenteral solution (Procaine penicillin
gel for injection in 2% cmc solution) ,
buldged curve develops like spur
 Due to sharp structural breakdown
when taken into syringe needle
 Complex str exhibits high yield value
called spur value in up curve
 This value represents sharp point of
structural breakdown at low shear rate

32.  Negative thixotropy
 Negative thixotropy Known as anti
thixotropy which represents time
dependant increasing apparent viscosity
rather than decrease on application of
shearing stress.
 This is called sol to gel
 At resting consists of large number of
individual particles and small size
floccules
 When the system is sheared the
molecules of dispersed phase colloids
 Increase in collision frequency of these

33.  At equilibrium state very small number of
large floccules exists therefore system
exhibits sol form.
 Again when system is at rest large size
floccules breakup and gradually returns
to original state of small size floccules
and individual particles.
 It contains 1-10% of solid, dilatant
system are deflocculated containing
more than 50%by volume of solid.

34.  Observed in magnesium magma
 It was observed that when magnesium
magma was sheared with alternatively
first by increasing then decreasing
shear rates, it continuously get
thickened.
 With continuation of cycle the extent
of thickening reduces gradually then
reaches to equilibrium State.

35.  Rheopexy
 Sol transforms to a gel more readily
after it has been deformed by gentle
shaking and regular rolling and rocking
movements.
 Rheopexy is analogous to rheopexy and
fluids behavior called rheopectic
 It provides a mild turbulence which helps
the dispersed particles to get
themselves in random alignment to re-
establish gel structure.
 Used in plastic and pseudo plastic
system

36.  Measurements of thixotropy
 Thixotropic measurements of plastic
and pseudo plastic system can be
achieved by use of hysteresis loop
formed during thixotropic breakdown
of the system.
 Degree of thixotropy obtained by..
 1. Structural breakdown with time at
constant rate of shear
 2. Structural breakdown with time at
two different rates of shear

37.  Importance
 Desirable property in emulsion,
suspension, cream, ointment, pastes,
parenteral suspension for depot therapy.
 On storage gel on shaking sol.so poured
out easily.
 Helpful in improving stability of
thixotropic pharmaceutical system.
 Greater thixotropy higher physical
stability.
 Speardability of cream and ointment can
be corrected with thixotropic property


38.  Pouring of lotions from container , shape
of cream in container, extrusion of paste
from tube shows high thixotropic
property.
 T agents: bentonite, kaolin
microcrystalline cellulose for viscosity
which obstruct sedimentation and
creaming.
 Degree of thixotropy may change over
period time. So plastic viscosity, spur
value yield value are important
parameters.

39.  Deformation of solid
 Deformation change in size and shape of
object changing dimensions
 Stress
 Force per unit area that applies on object to
deform it unit Nm-2 or Pa
 Types
 1. Direct stress
 It is produced under direct loading conditions
 1Tensile stress
 Tensile force acting per unit area of the body
 Extension or elongate dimensions of the body
 Ratio of change in length to original length

40.  2. Compressive stress
 Compressive force acting per unit area of the
body
 Forces applied is opposite to each other
 Compress the dimensions
 3. Shear stress
 Shear force acting per unit area of the body
 Due to this body develops some resistive
force which is parallel to each surface but
opposite to direction of force applied
 2. Indirect stress
 Due to torque produced in the body
 3.combined stress
 Combination of above two types of stress

41.  Strain
 Measure the amount of deformation
 If bar has original length L and load is
applied on bar length of bar will change
∆L
Strain =∆L/L
 Types
 1. Tensile strain
 Ratio of increase in length to original
length of bar
 2. Compressive strain: Ratio. Of
decrease in length to original length of
bar

42.  Elastic modulus
 Ratio of stress/strain
 The constant of proportionality depends on
the Material being deformed and nature of
the deformation.
 Determine amount of force required per unit
deformation.

43.  Hooke's Law
 In an elastic member stress is directly
proportional to strain within elastic limit
 N/m2
 Young's modulus used to identify how much
the Material is elastic
 Elastic limit maximum stress that Nan be
applied to the substance before it deforms
permanently.
 Initial strain strain curve is straight line.
 Stress increases, curve is no longer straight.

44.  Stress exceeds the elastic limit, object
is permanently distorted and does not
return to its original shape after stress
is removed.
 Hence shape of the object is
permanently changed.
 As stress increases even further
material ultimately breaks

45.  Heckel equationFrom tablet dosage form we can
understand deformation behaviour of individual
components
 Useful method for estimating the volume reduction under
the compression pressure in pharmacy.
 Plot can be affected by time of compression, degree of
lubrication and size of die.
 In [1/1D]=KP +A
 Kuentz and Leuenberger modified Rule which explain
transition between state of powder to state of tablet
 Hersey and Rees , York and Pilpel differentiate powders
into 3 types
 Types A
 Material comparatively soft, readily undergoes plastic
deformation. Sodium chloride
 Linear relationship observed with plots remaining parellel
at the applied pressure increases

46.  Type B
 Initial curve region followed by a straight line.
 Harder material having higher yield pressure
 Lactose
 Types C
 Initial steep linear region which become
superimposed and flattened out as applied pressure
is increased
 Significance
 Used to characterize single material and also for
powder
 Two regions of plot I type B material represents the
initial repacking stage and subsequent deformation
process
 Crushing strength of tablets is correlated with values
of K of plot