3. DEFINITION
• The science concerned with the deformation of
the matter under the influence of stress which
may be applied perpendicularly to the surface of
the body.
• The term “Rheology” has been derived from two
Greek words “Rheo” which means “to flow” and
“Logos” means “science”
4. TYPES OF DEFORMATION
The deformation that results from the application of a
stress may be divided into two types:
• Elastic Deformation
• Plastic Deformation
ELASTIC DEFORMATION:
• elastic deformation refers to the reversible change
in shape or size of materials, such as excipients and
active pharmaceutical ingredients, when subjected
to applied force.
• This property is crucial in tablet and capsule
manufacturing, influencing the compaction
process and tablet integrity.
5. EXAMPLES OF ELASTIC DEFORMATION:
1. Rubber
2. Metal Spring
PLASTIC DEFORMATION:
• plastic deformation refers to the permanent change in the
shape or size of materials, such as during tablet
manufacturing.
• Controlling plastic deformation is essential in dosage form
development to ensure consistent product quality and
performance.
EXAMPLES OF PLASTIC DEFORMATION:
1. Paperclip
2. Clay
6. VISCOSITY
• In pharmaceutics, viscosity refers to a fluid's resistance to flow, impacting
manufacturing processes, administration, and stability of liquid
pharmaceuticals.
• Controlling viscosity is crucial for uniform mixing, proper dosing, and
patient acceptance.
• Viscosity is measured using viscometers, and formulation adjustments are
made to achieve desired flow characteristics.
UNIT:
The unit of viscosity is typically measured in:
• poise (P) 1 poise is equal to 0.1 Pa·s
• centipoise (cP) 1 centipoise is equal to 0.001 Pa·s
• Dynes/cm2
7. TYPES OF VISCOSITY
i. KINEMATIC VISCOSITY:
Kinematic viscosity is the ratio of dynamic viscosity to the fluid's density.
Kinematic viscosity = 𝜂/𝜌
ii. RELATIVE VISCOSITY:
Relative viscosity also known as viscosity ratio is the ratio of a solution to the
viscosity of the solvent used.
Relative viscosity = 𝜼𝒓 =
𝜼
𝜼𝒔
8. iii. SPECIFIC VISCOSITY:
Specific viscosity may be defined as the relative increase in the viscosity of the
dispersion over that of the alone solvent.
Specific viscosity = 𝜼𝒔𝒑 =
𝜼−𝜼𝒔
𝜼𝒔
iv. REDUCED VISCOSITY (OF A POLYMER):
Reduced viscosity (of a polymer) or viscosity number is defined as the ratio of
the specific viscosity to the mass concentration of the polymer (c).
Reduced viscosity =
𝜼𝒔𝒑
𝒄
9. DETERMINATION OF VISCOSITY
The various viscometers used for determining the viscosity of different
systems:
STANDARD LABORATORY VISCOMETERS FOR LIQUIDS
i. U-TUBE VISCOMETER (OSTWALD OR CAPILLARY VISCOMETER)
• An Ostwald viscometer, also known as an Ostwald capillary viscometer, is a laboratory
instrument used for measuring the viscosity of a fluid.
• Named after the German chemist Wilhelm Ostwald.
• This viscometer is based on the principle of capillary flow.
• The fluid whose viscosity is being measured is drawn into the capillary by suction or gravity.
• The time it takes for a certain volume of the fluid to flow through the capillary is measured.
• . An Ostwald viscometer is generally preferred to measure the viscosity of Newtonian fluids or
typically the fluids whose viscosity does not change with respect to the flow rate.
11. OTHER VISCOMETERS
• Falling Sphere Viscometer
The falling-sphere viscometer measures fluid viscosity by timing the descent
of a sphere through a fluid-filled tube under gravity, based on Stokes' law.
• Falling Piston Viscometer
The Norcross viscometer, also known as the falling piston viscometer, was
created by Austin Norcross. It works by lifting a piston to draw in the liquid,
then timing how long it takes for the piston to fall due to the liquid's
resistance, which helps determine viscosity.
• Oscillating Piston Viscometer
An oscillating piston viscometer is a device used to measure the viscosity of
fluids. It operates by oscillating a piston within the fluid, and the resistance
encountered by the piston's movement helps determine the viscosity of the
fluid.
12. FUNDAMENTAL CONCEPTS
i. NEWTONIAN FLUIDS (NEWTONS LAW OF VISCOUS
FLOW):
The higher the viscosity of a liquid, the greater is the force
per unit area (shearing stress) required to produce a certain
rate of shear.
According to this law:
“the rate of shear is directly proportional to the shearing
stress”
Rate of shear =
𝒅𝒗
𝒅𝒓
Where,
dv = velocity difference between two planes
dr = small distance between two planes
13. • The difference in the velocity gradient (dv) between two plan of liquid
separated by distance (dr) is the rate of shear (dv/dr) and it symbol is G.
• The force per unit area required to bring about flow is called the shearing
stress ant its given a symbol F
F α G F = η . G
Where, “η” is the viscosity of fluid
η =
𝑭
𝑮
14. ii. NON NEWTONIAN FLUIDS:
• A non-Newtonian fluid is a fluid whose flow properties are not described by a
single constant value of viscosity.
• In a non-Newtonian fluid, the relation between the shear stress and the strain
rate is nonlinear, and can even be time-dependent. Therefore a constant
coefficient of viscosity cannot be defined.
• Examples: ketchup, shampoo
THREE CLASSES OF NON NEWTONIAN FLOW:
• Plastic Flow
• Pseudo Plastic Flow
• Dilatant Flow
15. i. PLASTIC FLOW:
These materials are known as Bingham bodies “In materials science,
a Bingham plastic is a viscoelastic material that behaves as a rigid body at low
stresses but flows as a viscous fluid at high stress”.
Examples:
Whipping cream, suspension of zinc oxide in mineral oil, paints, Printing inks,
Jellies or Gel.
16. Where,
• F = Yield Value
definite shearing stress used to produce the flow of plastic material.
• Fl = lower yield value
indicates when actual flow begins.
• Fh = higher yield value
corresponds to shearing stress beyond which the flow curve becomes
linear.
SLOPE:
The slope of Rheogram is termed as mobility similar to the fluidity as in
case of Newtonian system and its reciprocal is known as plastic viscosity
or “U”.
U =
𝑭 −𝑭𝑩
𝑮
Where,
FB = Yield value in dynes/cm2
F = Shearing stress
G = Rate of shear
17. ii. PSEUDO PLASTIC FLOW:
Unlike the plastic material, the Pseudoplastic material exhibits no yield value but
these materials are characterized by a rheological curve which passes through the
origin of the graph as in the case of Newtonian liquids.
18. • The shear stress (F) doesn’t increase linearly with the shearing rate (G) and
therefore the viscosity doesn’t remain constant at different rate of shear.
• These systems are also known as the shear thinning system.
• The decrease in the viscosity with the increasing rate of shear is also exhibited
in the pseudoplastic system.
F N = η’ G
Where,
F = Shearing stress
η’ = Coefficient of viscosity
G = Rate of shear
N = An indicative of the Non-Newtonian behavior
Examples: Gums (e.g. agar, alginate), Solutions of the suspending agents e.g.
tragacanth, gelatin, carboxymethyl cellulose (CMC) , Other water soluble
Mucilage's
19. iii. DILATANT FLOW:
• Dilatancy is a phenomenon in which the material exhibits an increase in the
resistance to flow with increasing rate of shear.
• The material returns to a state of fluidity when the shear is removed, all the
agitation is stopped.
• This phenomenon is sometimes referred to as shear thickening system.
20. • As in the case of pseudo plasticity flow, the dilatant flow may also be
expressed by the exponential expression.
F N = η’ G
• Whereas in case of pseudo plasticity then N takes a value greater than 1, but
in Dilatancy N is always less than 1.
• As the N approaches the unity, the material approaches the Newtonian
behavior and the curve line becomes linear.
EXAMPLES:
• Suspension containing high concentration of very fine particles
• Deflocculation suspension
• Butter
21. THIXOTROPY
• So far for Newtonian and non-Newtonian behavior:
- observed behavior when the rate of shear was
progressively increased and plotted against the
resultant shear stress
• Assumed that if the rate of shear was reduced, the
down curve would be identical with and
superimposed on the upcurve.
• This is so with
- Newtonian systems
- some non-Newtonian material
22. • This is a characteristic exhibited by some thixotropic
systems
• A phenomenon where a sol forms a gel more readily
when gently shaken than when allowed to form the gel
while the material is kept at rest.
• The rocking motion provides a mild turbulence which
aids in returning derandomized particles to a random
orientation.
• The gel is the equilibrium form.
SOL GEL SOL
RHEOPEXY
23. Pharmaceutical
importance
1.Stability Assurance:
Rheology helps ensure that medicines maintain their desired characteristics
and effectiveness over time.
2. Injection Ease:
It contributes to creating injections that flow smoothly, making them easier
to administer.
3. Skin-Friendly Formulations: Rheology is used to develop creams and gels
that feel good on the skin and are absorbed effectively.
4. Tablet Optimization:
In tablet manufacturing, rheology is considered to ensure the pills are easy to
swallow and disintegrate properly.
5. Liquid Medication Flow:
Rheology plays a role in formulating liquid medicines, ensuring they pour
easily and are convenient for patients.
24. 6. Consistent Quality:
It serves as a tool for maintaining consistent quality in pharmaceutical
manufacturing processes.
7. Enhanced Solubility:
Rheological studies help improve the solubility of medicines, ensuring better
absorption in the body.
8. Manufacturing Efficiency:
By optimizing processes like blending and coating, rheology contributes to
efficient pharmaceutical manufacturing.
9. Bioavailability Improvement:
It aids in designing drug carriers that are compatible with the body's fluids,
enhancing the availability of the medicine.
10. Process Optimization:
Rheology assists in optimizing various manufacturing processes, contributing
to the overall efficiency of pharmaceutical production.
