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RHEOLOGY

- Roselyn Aperocho-Naranjo
- Faculty, College of Pharmacy
- USPF

Rheology

- rheo to flow
- logos science
- ology the study of
- is the study of the flow of materials that behave

in an interesting or unusual manner. - Unusual materials such as mayonnaise, peanut

butter, chocolate, bread dough, paints, inks,

road building aterials, cosmetics, dairy

products, etc.

Rheology

- The study of viscosity is of true liquids,

solutions, dilute and concentrated colloidal

systems is of much importance in this study - It is involved in the mixing and flow of

materials, their packaging into containers, the

pouring from the bottle, extrusion from a tube or

a passage of the liquid to a syringe needle.

Rheology

- Can affect the patients acceptability of the

product, physical stability, biologic

availability, absorption rate of drugs in the

gastrointestinal tract - Influence the choice of processing equipments in

the pharmaceutical system

Two Categories of Flow Deformation

- Newtonian (Newtonian Law of 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 - Shear as a stress which is applied parallel or

tangential to a face of a material, as opposed to

a normal stress which is applied perpendicularly.

- Shear stress
- Measured in (SI unit) pascal
- Commonly used symbols t
- Expressed in other quantities t F / A

A shear stress, is applied to the top of the

square while the bottom is held in place. This

stress results in a strain, or deformation,

changing the square into a parallelogram.

Two Categories of Flow Deformation

- Newtonian (Newtonian Law of Flow)
- A Newtonian fluid (named for Isaac Newton) is a

fluid whose stress versus rate of strain curve is

linear and passes through the origin. The

constant of proportionality is known as the

viscosity. - A simple equation to describe Newtonian fluid

behavior is where - t is the shear stress exerted by the fluid

("drag") Pa - µ is the fluid viscosity - a constant of

proportionality Pas - du is the velocity gradient perpendicular to

the direction - dy of shear s-1

Two Categories of Flow Deformation

- Newtonian (Newtonian Law of Flow)
- In common terms, this means the fluid continues

to flow, regardless of the forces acting on it.

For example, water is Newtonian, because it

continues to exemplify fluid properties no matter

how fast it is stirred or mixed. - For a Newtonian fluid, the viscosity, by

definition, depends only on temperature and

pressure (and also the chemical composition of

the fluid if the fluid is not a pure substance),

not on the forces acting upon it.

Two Categories of Flow Deformation

- Newtonian (Newtonian Law of Flow)
- For a Newtonian fluid, the viscosity, by

definition, depends only on temperature and

pressure (and also the chemical composition of

the fluid if the fluid is not a pure substance),

not on the forces acting upon it. - If the fluid is incompressible and viscosity is

constant across the fluid, the equation governing

the shear stress is expressed in the Cartesian

coordinate system

Two Categories of Flow Deformation

- Newtonian (Newtonian Law of Flow)
- Cartesian coordinate system
- where, by the convention of tensor notation,
- tij is the shear stress on the ith face of a

fluid element in the jth direction - ui is the velocity in the ith direction
- xj is the jth direction coordinate

Two Categories of Flow Deformation

- Newtonian (Newtonian Law of Flow)
- Cartesian coordinate system
- Tensor - are geometrical entities introduced into

mathematics and physics to extend the notion of

scalars, (geometric) vectors, and matrices - - Components of stress, a second-order tensor,

in - three

dimensions. The tensor in the image is the - row vector, of

the forces acting on the X, Y, and - Z faces of the

cube. Those forces are represented - by column

vectors. The row and column vectors - that make up

the tensor can be represented - together by a

matrix.

Two Categories of Flow Deformation

- Non-Newtonian
- A non-Newtonian fluid is a fluid whose flow

properties are not described by a single constant

value of viscosity. - Many polymer solutions and molten polymers are

non-Newtonian fluids, as are many commonly found

substances such as ketchup, starch suspensions,

paint, blood and shampoo. - In a Newtonian fluid, the relation between the

shear stress and the strain rate is linear (and

if one were to plot this relationship, it would

pass through the origin), the constant of

proportionality being the coefficient of

viscosity.

Two Categories of Flow Deformation

- Non-Newtonian
- A 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. - A ratio between shear stress and rate of strain

(or shear-dependent viscosity) can be defined,

this concept being more useful for fluids without

time-dependent behavior.

Two Categories of Flow Deformation

- Non-Newtonian Examples
- An inexpensive, non-toxic example of a

non-Newtonian fluid is a suspension of starch

(e.g. cornflour) in water, sometimes called

oobleck (uncooked imitation custard, being a

suspension of primarily cornflour, has the same

properties). - The sudden application of force for example
- by stabbing the surface with a finger, or rapidly

inverting the container holding it leads to the

fluid behaving like a solid rather than a liquid.

- This is the "shear thickening" property of this

non-Newtonian fluid. More gentle treatment, such

as slowly inserting a spoon, will leave it in its

liquid state. - Trying to jerk the spoon back out again, however,

will trigger the return of the temporary solid

state. - A person moving quickly and applying sufficient

force with their feet can literally walk across

such a liquid.

Two Categories of Flow Deformation

- Non-Newtonian Examples
- There are fluids which have a linear shear

stress/shear strain relationship which require a

finite yield stress before they begin to flow.

That is the shear stress, shear strain curve

doesn't pass through the origin. - These fluids are called
- 1. Bingham plastics.
- clay suspensions, drilling mud, toothpaste,

mayonnaise, chocolate, and mustard. The classic

case is ketchup which will not come out of the

bottle until you stress it by shaking.

Two Categories of Flow Deformation

- Non-Newtonian Examples
- These fluids are called
- 1. Pseudoplastic Flow
- Polymers in solutions such as tragacant, sodium

alginate, methylcellulose - Viscosity decreases with an increase in shear

thinning - Caused by the re-alignment of polymer and/or the

release of solvents associated with the polymers - 2. Dilatant Flow
- Volume increases when sheared
- Shear thickening
- Suspension containing high-concentration of small

deflocculated particles

Two Categories of Flow Deformation

- Non-Newtonian Examples
- There are also fluids whose strain rate is a

function of time. Fluids that require a gradually

increasing shear stress to maintain a constant

strain rate are referred to as rheopectic. - An opposite case of this, is a fluid that thins

out with time and requires a decreasing stress to

maintain a constant strain rate (thixotropic).

THIXOTROPY

- is the property of some non-Newtonian

pseudoplastic fluids to show a time-dependent

change in viscosity the longer the fluid

undergoes shear stress, the lower its viscosity. - A thixotropic fluid is a fluid which takes a

finite time to attain equilibrium viscosity when

introduced to a step change in shear rate. - the term is sometimes applied to pseudoplastic

fluids without a viscosity/time component. Many

gels and colloids are thixotropic materials,

exhibiting a stable form at rest but becoming

fluid when agitated.

THIXOTROPY

- pseudoplastic fluids
- Shear thinning is an effect where viscosity

decreases with increasing rate of shear stress.

Materials that exhibit shear thinning are called

pseudoplastic. - This property is found in certain complex

solutions, such as lava, ketchup, whipped cream,

blood, paint, and nail polish. - Pseudoplasticity can be demonstrated by the

manner in which shaking a bottle of ketchup

causes the contents to undergo an unpredictable

change in viscosity. The force causes it to go

from being thick like honey to flowing like

water. - thixotropic fluid viscosity decreases over time

at a constant shear rate.

THIXOTROPY

- The distinction between a thixotropic fluid and a

shear thinning fluid - A thixotropic fluid displays a decrease in

viscosity over time at a constant shear rate. - A shear thinning fluid displays decreasing

viscosity with increasing shear rate. - Some fluids are anti-thixotropic constant shear

stress for a time causes an increase in viscosity

or even solidification. Constant shear stress can

be applied by shaking or mixing. Fluids which

exhibit this property are usually called

rheopectic. They are much less common.