1. Hindu College of Pharmacy
RHEOLOGY
Dept .of Pharmaceutics
Hindu College of Pharmacy

2. CONTENTS : Introduction Importance of Rheology
Types of Rheological systems
Thixotropy
Measurement of thixotropy
Rheopexy
Measurement of viscosity by different types of viscometers.

3. INTRODUCTION
The term rheology was derived from two Greek words namely rheo (to flow) and logos(science) was suggested by BINGHAM
Rheology is the science that deals with deformation of matter under the influences of stresses.
Rheology describes the flow of liquids and deformation of solids.
Viscosity is defined as frictional force(F) exerted unit area(A) between successive layers of liquid and seperated by unit distance(dx) possessing variable velocity(dv)
Fα A.dv/dx
F=η A dv/dx
η=F/A . dv/dx
Reciprocal of Viscosity called Fluidity .

4. Units : stoke and centistokes. Ex : lotions having viscosity - 300 cp
viscometer is an instrument used to measure viscosity. viscosity which vary with flow conditions, an instrument called Rheometer is used.
Viscosity play major role in pouring from the bottles, extrusion from tubes, passage through syringe needle
Units of viscosity is
Kinematic viscosity is defined as viscosity/density (η/ρ)
Units : stoke and centistokes.
Ex : lotions having viscosity cp
paste or ointments viscosity – 1200 poise

5. IMPORTANCE OF RHEOLOGY :
Rheological properties like viscosity are more important in liquid solutions, dilute and concentrated colloidal systems.
They are important in manufacturing of emulsions, suppositories, paste, tablet coating.
Rheology is involved in mixing and flow of materials their packing into containers and their removal prior to use.
Rheology is important in ink industries, paint industries, road welding materials, manufacturing of cosmetics and dairy materials.
Viscosity plays a major role in pouring from the bottle, extrusion from the tube, passage through the syringe nee

6. RELATION BETWEEN SHEAR STRESS AND RATE OF SHEAR
SHEAR STRESS: It is defined as unit force require to induce unit area
RATE OF SHEAR: It is the change in velocity with respect to change in time.
G=dv/dx
The relation between the F&G is G F

7. Types of Rheological systems:
It broadly classified into two types:
Newtonian system
Non Newtonian system
Newtonian Flow Of System:
Liquid that obeys the newtons law of flow called Newtonian fluids.
The system which follows a linearity relationship are called Newtonian fluids
Ex: water, glycerin, chloroform, solutions of syrup and very dilute colloidal solutions.

8. Shear stress-shear rate relationship for Newtonian flow
G slope= fluidity
Rate of
shear
shear stress F

9. Non Newtonian flow of system:
The substance which does not follow Newton's law are said to be Non Newtonian flow system.
Generally they analyzed in rational viscometer
This is classified into three types:
Plastic flow
Pseudo plastic flow
Dilatent flow

10. PLASTIC FLOW :
The material which exhibit plastic flow is called BINGHAM BODIES
A plastic body behaves as elastic bodies and fails to flow when less amount of stress is applied, after increases the shear stress leads to non linear increase in shear rate which progressively get linearised, it gives yield value(f)
Therefore plastic flow resembles to Newtonian flow above yield value
Plastic flow express in terms of yield value

11. Slope of plastic flow gives mobility of particular system or fluidity
Reciprocal of fluidity will give about plastic viscosity(U)
Ex: Floculated particles in concentrated suspensions, ointments, pastes, gels

12. PSEUDO PLASTIC FLOW :
As the shear stress increase progressively, shear rate also increases, viscosity of pseudo plastic material decreases. and curve is not linear
Viscosity of pseudo plastic system cannot be expressed by single value
The consistency curve for pseudo plastic material begins at the origin. there fore no yield value like plastic system, because no part of curve is linear
Ex; system containing natural and synthetic gums.
system containing dispersing agents like tragacanth,methyl cellulose,sodium cmc.

13. It is exponentially given by
where N= 1 system ready to flow
N>1 MORE PSEUDO PLASTIC
N<1 DILATANT FLOW SYSTEM
N log F = log η + log G
logη = N log F –log G
All thixotropic system may claimed as- pseudo plastic system But all pseudo plastic systems are not –thexotropic system
It is also called shear thinning system

14. DILATANT FLOW: It is also called shear thickening system
The system exhibit enhance resistant to flow with increasing rate of shear
Actually increasing in volume when sheared hence termed as dilatant system
The flow is inverse to pseudo plastic flow
Ex: starch powder in water, Concentrated deflocculated suspension
Here, the molecules are not completely wetted by the solvent as the stress applied the bulk of system expand or dilate. If shear stress is removed, the dilatent system returns to its original state of fluidity

15. This equation can be used to describe dilatancy in this case
N is always less than 1 and decreases as degree of dilatency increases as an N approaches 1 the system becomes increasingly Newtonian in behavior

16. THIXOTROPY
It is an isothermal and comparatively slow recovery on standing
of material, of a consistency loss through shearing
Assume that rate of shearing was reduced, in Newtonian flow the down curve would be identical with super imposable up curve but in non Newtonian flow down curve, will displaces relatively the up curve

17. In pseudo plastic system the down curve is displaced to the left of up curve showing that the material has a lower consistency, this indicates the break down of structure that does not reform immediately even when stress is removed
This phenomenon is said to be thixotrophy
Ex: gel-sol-gel
sol-gel-sol
. This process is not instantaneous rather it is progressive restoration of consistency as asymmetric particles come into contact with one another by under going random Brownian moment

18. NEGATIVE THIXOTROPY :
The increase in thickness of the resistance to flow with increased time of shear is said to be negative thixotrophy.
Ex: rheological analysis of magnesia magma, clay
In this rheogram the down curve shifts to right side of up curve

20. BULGES
Ex:Conc.aqueous magma of bentonite.
It produces hysteresis loop

21. SPURS A sharp point of structural breakdown at low
shear rate in up curve called spur value.
Ex: procaine penicillin gel

22. At single rate of shear MEASUREMENT OF THIXOTROPHY
The structural break down with respective time at constant rate of shear .
Thixotropic coefficient B=(U1-U2)/ln (t2/t1)

23. At a different rates of shear
The structural breakdown with respect to time having different shear rates. In this plastic viscosities (U1&U2) having shear rates of (V1&V2)
Thixotropic coefficient M=2(U1-U2)/ln (v2/v1)2

24. RHEOPEXY
The phenomenon in which sol transforms into gel state more readily
The time period to regain its original viscosity may be reduced by applying rolling and tumbling motion.
It is two types
positive rheopexy
Negative rheopexy

25. INSTRUMENTATION viscometer Single point viscometer
Falling sphere viscometer
capillary viscometer
Multi point
Cone and plate viscometer
Cup and bob viscometer

26. It follows poiseuille’s law
Ostwald’s viscometer
It follows poiseuille’s law
It is also called
capillary visco meter

27. Principle;-viscosity of unknown liquid determined by equation.
η1 = ρ1t1/ρ2t2 .η2
η=k’ t1 ρ1
Applications
Used for formulation and evaluation of dispersion system such as colloids , suspensions , emulsions.
Study of flow of liquids through capillary tube throw light upon the circulation of blood

28. FALLING SPHERE VISCOMETER

29. It follow’s stroke’s law.
The instrument can be used over a range of to poise.
Viscosity drag on sphear = force responsible for downward moment due to gravity.
The viscosity of Newtonian liquid calculated by
η = t ( Sb- Sf) B

30. ROTATIONAL VISCOMETERS
ADVANTAGES
It is possible to vary the shear rates over a wide range of values.
Measurement can be made continuously for extended period at given shear rate.
Viscosity of non Newtonian system exhibited by this at with accurate temperature control
Examples:
cup and bob
cone and plate

31. CUP AND BOB VISCOMETER This is Multi point viscometer.
Couette type : cup rotate –Mac Michael viscometer
Searle type : Bob rotate - stormer viscometer.
For systems of viscosity more than 20 centi poise
A rheogram obtained by plotting Rpm vs wt,added
Viscosity of pseudo plastic system calculated by
η = kv w/v
Viscosity of plastic system calculate by
U= kv w-wf/v
Yield value of Bingham body expression
f = kf.wf

32. CUP AND BOB VISCOMETER

33. DIS ADVANTAGES No uniform rate of shear.
Frictional heating may developed at high rate of shear
Cleaning is difficult.
Plug flow development
minimized by – decreasing gap between cup and bob
Increasing the shear rate.

34. CONE AND PLATE VISCOMETER
The cone to plate angle is less than
10 centigrade
sample required is small 0.1 to 0.2 ml
Principle:
pseudo plastic Viscosity estimated by
η = C. T/v
plastic viscosity estimated by
U = T –T f/v.

35. CONE AND PLATE VISCOMETER

36. ADVANTAGES Rate of shear is constant,no chance for plug flow.
Temperature stabilization is good.
It needs to vary little samples for the study
sample volume of 0.1 to 0.2 ml
Valuble aid for det.of hysteresis loops obtained in thixotropic system,and det. The thixotropic coefficient.

37. RHEOLOGY OF SUSPENTIONS
The flow property of suspension depend upon their rheological characters
The rheological properties of suspension decide the pourability, easy of injection, sedimentation, its redispersibility.
The viscosity of flocculated suspension greater than deflocculated particles in same suspension.
The flocculated suspension has yield value and behave like a plastic or pseudo plastic system.
Example: conc.parenteral suspension contain % w/v of procain pencillin G

38. RHEOLOGY OF EMULSIONS
Dilute emulsions shows Newtonian type flow
Highly conc.emulsions and creams shows plastic flow.
The important properties influencing the emulsions :
Aggregation of particles
Viscosity of continuous phase
Globule size and size distribution
Nature and proportion of emulsifying agent

39. RHEOLOGY OF OINTMENTS AND CREAMS
Ointments and creams show plastic and thixotropic behaviour.
Determination of rheological behavior and thixotropic coefficient of a standard preparation compare with subsequent batches serves as guide for quality control measurement, for stability, spreadability ,performance and elegance of the preparation