Rheology

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Matter

Rheology

study of the deformation and flow of matter, The study of deformation and flow of matter under applied stress.

To the Pharmacist Rheology means

Flow of emulsion through colloid mill and pump
Working ointment on slab or mills
Titrating suspensions in mortar and pestle
Mechanical properties of glass or plastic container and rubber closures.

RheologyPharmaceutic to the consumers

Sqeezing tooth paste from collapse tube
Spreading lotion on his skin.
Paint on a surface.
Spreading butter on bread
Write with pen
Spray liquid from automizer or aerosol cans

Rheology viewpoint of solid

If they preserve shape and volume

Rheology viewpoint of liquid

If they preserve their volume

Rheology viewpoint of gaseous

If neither shape nor volume is preserve when force is applied.

When stress is applied to solid

It gets deformed but return to it original shape when stress is released.

Elasticity

Is the ability of a solid to regain its shape after stresses is release from them.

Liquid when stresses is applied

It compresses to smaller volume but assumes it original volume when stress is release.

Types of flow

Plastic
Pseudo plastic
Dilatant
Thixotropic

Plastic flow

flow that take place inresponse to pressure deformation without facturing
it requires minimum stress to commence which is referred to as yield stress or yield point.

Yield stress

Point at which plastic deformation occurs. It occurs from particle to particle bonds that exist when the suspension is at rest.

The forces required is called

Yield stress

Bingham flow

Is the term that described the flow after the yield stress has been surpassed, and the flow is Newtonian.

Fluidity

the ability to flow

Viscosity

a liquid's resistance to flow,
a measure of the resistance of a liquid to flow, which is affected by the size and shape of particles, and generally increases as the temperature decreases and as intermolecular forces increase.

Viscosity

Is expressed as the ratio between shear stress and shear rate.

Shear stress

Is the ratio of the force used to move one layer of fluid past another compared with the area of the material in contact. Unit express as Dynes per square centimeter
N= force(dynes)/ area( cm2)

Dilatant flow

increase in flow resistance with increasing shear rate (shear thickening); occurs in suspensions containing small, deflocculated particles which are originally closely packed with minimal, but sufficient vehicle volume between them - when shear stress increased molecules become open and unpacked so that the overall volume increases and the vehicle amount is no longer sufficient and molecules are not wetted as well causing increase in resistance; graphed as log G vs log F where N is the slope and less than 1

Pseudoplastic flow

- sheer thinning (thick at rest, thins when shaken)
- NO yield value
- "IDEAL solutions" because you can pour them after shaken, and particles will stay suspended when resting)
- slope curves down (if rate on bottom)

Thixotropic flow

Use widely in pharmaceutical and cosmetic, food and industrial areas.

Features of thixotropic flow are

Matrials posses a yield point.
Reduction in viscosity on shearing with time
Rebuilding viscosity on standing
Has both features of plastic and pseudo plastic flow
There is a time dependency common to other types of flow.

What is the contradicting characteristic of pseudo plastic and dilatant flow that must be considered

Undesirable in suspension, cause physical instability to the product.

Suspension that exhibit plastic and thixotropic Rheology.

Has good physical stability. They prevent sedimentation , aggregation and caking by having high yield value at rest, yet reducing viscosity.

Shear rate

G- difference in velocity between 2 layers of moving liquid- reflects flow rate
D= speed( cm/sec) /distance( cm).

Viscosity

Ratio of shear stress to shear ratio.
N( viscosity)= shear stress (^) / shear ratio(D).

Category of materials

Newtonian
Non-Newtonian

Newtonian Flow

constant viscosity, regardless of shear stress
, a fluid whose stress versus strain rate curve is linear; the constant of proportionality is the coefficient of viscosity,

Non-Newtonian flow

Change in viscosity, with increase shear rate.
, Viscosity changes with applied shear stress
, Emulsions, suspensions and ointments undergo ___________

Example of non-Newtonian flow

Plastic
Pseudo plastic
Dilatant

Characteristic of Newtonian fluid graph.

Will plott as a straight line or linear

What is the name given to the straight line in Newtonian graph

Viscosity line.

What is the unit of viscosity

Poise

Non Newtonian substances.

Failed to follow newton's equation of flow.
Eg. Colloid, emulsion, liquid suspension, ointment .

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