34 terms

# Rheology

Matter
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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
Paint on a surface.
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 .