RHEOLOGY
Origin:
The term rheology, from the Greek
Rheo (to flow)
And Logos (science)
Bingham and Crawford to describe the flow of liquid and deformation of solids suggested it.
Definition:
“Rheology is the branch of physics,
Which describes the flow of liquids.”
Classification of Materials According to Flow and Deformation:
When classifying materials according to the types of flow and deformation, it is customary to place them in two categories.
(i) Newtonian system
(ii) Non-Newtonian system
Rheology For Physiologist:
Circulation of blood and lymph through capillaries and vessels flow of mucous, bending of bones streaming of cartilage contraction of muscles, while sitting stretching of gluteal regions.
Rheology For Physicians:
Fluidity of solutions to be injected with hypodermic syringes. Intravenous infusions flexibility of tubing used as catheters, extensibility of gut actions of fecal softener and strength of sutures and ligatures.
Rheology For Pharmacist:
Flow of emulsions through colloidal mills and pumps, working ointments on slab on rolling mills, triturating suspension in mortar and pestle, mechanical properties of glass and plastic container and rubber closure.
Rheology For Consumer:
Squeezing of toothpastes, lotion of skin, butter on slice, paint on surface. Writing with pen spray from atomizer or aerosol from a container, chews food all involve the rheology.
Classes of Materials:
Materials has two main classes
1- Newtonian Materials
2- Non-Newtonian Materials
1.NEWTONIAN MATERIALS:
Definition:
“Materials, which follow the Newton’s law of flow”
Newton’s Law of Viscous Flow:
“If the top plane of liquid is moved at a constant velocity,
Each lower layer will move with a velocity directly
Proportional to its distant from the stationary bottom.”
Explanation of Newton’s Law:
Consider a “ block” of liquid consisting of parallel plate of molecules, similar to a deck of card, as shown in figure (1). The bottom layer is considered to be fixed in place.
F’
dr
The difference between two planes of liquid separated by an infinitesimal distance dr is the velocity gradient or rate of shear, dv/dr. The force per unit area F’/A required to bring about flow is called the shearing stress and is given by symbol F.
Mathematical Representation:
The rate of shear is directly proportional to the shearing stress so mathematical expression is
F’ dv
=h
A dr
Where
Rate of shear = dv/dr =F
Shearing stress = F’/A = G
Graphical Representation:
A representative flow curve, or rheogram, obtained by plotting F versus G for Newtonian system. A straight line passing through origin is obtained.
Shearing stress
Liquids Obeying Newton’s Law:
Liquids obeying Newton’s Law are Castor oil, chlorophorm, Ethyl alcohol and etc.
Liquids Which Don’t Obey Newton’s Law:
Colloidal solutions, emulsions, liquid suspensions.
VISCOSITY (h):
Definition:
“The viscosity of the fluid is the internal
resistance or friction involved in the relative
motion of one layer of molecules with
respect to the next.”
Unit:
Unit of viscosity is poise.
Poise:
Definition:
“The shearing force required to produce
a velocity of 1cm/sec between two parallel
planes of liquid each 1cm in area
and separated by distance of 1cm.”
The cgs units are dynes sec cm or g/cm-sec.
F’ dr dynes´cm dyne sec
h= = =
A dv cm ´ cm/sec cm
Gives the result
Dyne sec g´ cm/sec g
= =
cm cm cm sec
Dependence Of Viscosity On Factors:
There are two main factors effecting the viscosity
a. Intermolecular Attraction
b. Temperature
a. Intermolecular Attraction:
· When there are strong attractions between the molecules of a liquid (van der Waals force, dipole interaction, etc) the viscosity will be high.
· When the attraction are weak the viscosity will be
low.
b.Temperature:
· At high temperature the mutual attraction between the molecules of liquid decreases.
· At high temperature the mutual attraction between the molecules of gas increases.
KINEMATIC VISCOSITY:
Definition:
“The absolute viscosity divided by the density
of a liquid at definite temperature.”
Mathematical Representation:
h
Kinematic viscosity =
r
Units:
Stoke(s) and centistokes (cs).
Temperature Dependence And Theory Of Viscosity:
· The viscosity of gas increases with temperature.
· The viscosity of a liquid deceases with temperature.
· The fluidity of a liquid increases with temperature.
Arrhenius Equation:
The dependence of viscosity on temperature is expressed or determined by Arrhenious equation.
Ev/RT
h=Ae
· In which ‘A’ is constant depending upon molecular weight & molar mass of a liquid.
· ‘Ev’ is an activation energy required to initiate flow between molecules.
Absolute Viscosity Of Some Newtonian Liquids At 20 °:
Liquid
Viscosity (cps)
Castor oil
1000
Chloroform
0.563
Ethyl alcohol
1.19
Glycerin, 93%
400
Olive oil
100
Water
1.0019
2.NON-NEWTONIAN MATERIALS:
Definition:
“Substances that fail to follow
Newton’s law or equation of flow.”
Examples:
Examples are liquid +solid heterogeneous dispersions such as
· Colloidal solutions
· Emulsions
· Liquid suspensions
· Ointments, etc.
.
Importance Of Non-Newtonian Materials:
Pharmacist most probably deals with non-Newtonian materials than with simple liquid. Many pharmaceutical formulations are non-Newtonian.
Types Of Non-Newtonian Materials:
There are three types Non-Newtonian flow
1. Plastic Flow
2. Pseudoplastic Flow
3. Dilatant Flow
1.Plastic Flow:
“ The material, which fails to flow until
a certain shearing stress has been applied.”
Bingham Bodies:
“The bodies, which follow the plastic flow,
are called as Bingham Bodies.”
They are named in the honor of the pioneer of modern rheology and first investigator to study plastic substance.
Example:
Examples of Bingham bodies are
· Concentrated suspensions of solids
· Ointments
· Gels
Graphical Representation:
The plastic flow curve does not pass through the origin but rather intersects the shearing stress axis.
Fffff
Shearing stress
Mathematical Relationship:
The equation describing the plastic flow is
(F - f)
U =
G
Where
Yield value / intercept on the shear axis = f
Shearing stress = F
Rate of shear = G
Deviation Of Rheogram, A weakness of Bingham Equation:
Deviation of Bingham Equation is due to reason that it don’t consider the van der Waals forces and frictional forces present in the system.
Graphical Deviation:
Graphical Deviation is due to the reason that yield value is present due forces of attraction so certain shearing stress is required for flow.
Yield Value:
“The point at which curve intercept the
axis of shearing stress is called yield value.”
A Bingham body does not begin to flow until a shearing stress, corresponding to the yield value exceeded.
Yield Value Importance:
Yield value is an indication of flocculation: the more flocculated suspension the higher will be the yield value.
Liquid:
“Material, which flow at the smallest shearing stress
and show no yield value is defined as liquid.”
Elasticity:
“ When shearing stress below the yield value
is applied the material shows elasticity.”
Plasticity:
“When shearing stress is equal to the
yield value material shows plasticity.”
Reasons Of Plastic Flow:
Reasons for plastic flow are
· Flocculated particles in conc. Suspensions
· Van der Waals forces between adjacent particles
· Friction force
Flocculate Particles In Conc. Suspensions:
Plastic flow is associated with the flocculated particles in conc. Suspensions.
More the flocculated particles more concentrated suspension and more yield value.
Frictional Force:
Frictional force can also contribute the yield value. Once the yield has been exceeded, any further increase in shearing stress (i.e., F- f) brings about a directly proportional increase in G, the rate of shear.
2.Pseudoplastic Flow:
Definition:
“ Flow show by the materials
having polymers in solutions.”
Examples:
Example are given as
· Cellulose ether
· Tragacanth
· Alginates
And, etc.
Graphical Representation:
There is no yield value so no part of the curve is linear.
Shearing stress
Viscosity:
The viscosity of the pseudoplastic materials
deceases with the increase in the rate of shear.
An apparent viscosity may obtain at any rate of shear from the slope of the tangent to the curve at the specific point.
Curved Rheogram:
The curved rheogram for pseudoplastic materials result from a shearing action on the long chain molecules of the materials such as linear polymers.
Mathematical Representation:
It is represented mathematically as follow
N
F = h’G
As the exponent N raises the flow becomes more non- Newtonian.
When N=1 the flow is Newtonian.
Shear Thinning System:
As the shearing stress is increased 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 and each successive shearing stress. Due which material becomes thin.
Difference With Bingham Bodies:
Pseudoplastic materials have following differences from Bingham bodies
· Polymers in solutions exhibit pseudoplastic flow while plastic flow is due to flocculate particles in suspensions.
· In pseudoplastic flow curve passes through origin while in plastic not.
· They have no yield value while plastic flow has yield value.
3.Dilatant Flow:
Recognition:
Certain suspension with a high percentage of dispersed solids exhibit an increase in resistance to flow with increasing rate of shear.
Definition:
“Increase in the volume of a system
when sheared is termed as Dilatant.
Shear Thickening System:
When shear is applied on deflocculated particles material is said to be dried out because of void spaces between particle and less vehicle due which resistance increases and suspension becomes thicker.
Examples:
Example is given as
· Starch in cold water
· Deflocculated particles
· Suspension having more in small, and etc.
Reasons:
Deflocculated particles have very small spaces between them and less amount of vehicle to move so when stress is applied distance between the particles increase and quantity of vehicle decrease between them i.e. system dilates.
Increasing rate of shear
Closed packed particles; open packed (dilated) particles;
Minimum void volume; increased void volume;
Sufficient vehicle; insufficient vehicle;
Relatively low consistency relatively high consistency
Facilitation:
When suspension dilates and the amount of vehicle is constant there are many particles that are no more lubricated with vehicle so resistance increase which facilitates the process.
Solidification:
When more stress applied the resistance increase and ultimate suspension is solidified called as claying or caking of suspension.
Mathematical Representation:
Mathematically it is represented by equation
N
F = h’G
Where for dilatant flow N is less than 1
and dilatancy increase with increase in its value
when N= 1 material Newtonian.
Graphical Representation:
Dilatant flow is represented graphical as follow
Shearing stress
Application In Pharmacy:
There are many applications of rheology in pharmacy, which are as follows.
· It is involved in formulation and analysis of pharmaceutical products as emulsions, paste, suppositories and tablet coatings.
· It is involved in manufacture of pastes medicines creams and ointments.
· It is also involved in mixing and flow of materials and there packing in containers.
· The poloxamers are block polymers and are used in dermatologic bases or topical ophthalmic preparations because of their low toxicity & ability to form clear water based gels.
· Also used in study of paints, inks, doughs, road building materials, cosmetics, dairy products & other materials.
