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Ceutics Exam 1
Rheology
62
Pharmacology
Professional
01/22/2012

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Cards

Term
Define Rheology
Definition
The study of the flow of matter, primarily in the liquid state, but also as 'soft solids' or solids under conditions in which they respond with plastic flow rather than deforming elastically in response to an applied force
Term
Define Newtonian classification of fluid
Definition
Newtonian fluids can be characterized by a single coefficient of viscosity for a specific temperature.
Term
Define non-Newtonian classification of fluid
Definition
Fluids with strain rate dependent viscosity
Term
Define thixotropic
Definition
A fluid characteristic by becoming more viscous when shaken or agitated
Term
Define and describe viscosity
Definition
-The resistance to flow
-Internal friction of a system
-Strength that a layer exerts on adjacent layers
Term
In the Newtonian law of flow, what is dr and what is dv?
Definition

dr = thickness of imaginary infinite layers of the cube of liquid

 

dv = velocity

Term
Give the equation for the rate of shear for Newtonian flow
Definition
G=dv/dr
Term
What is the Newtonian law of flow?
Definition
F=ηG

F=shearing stress=Force/a
G=rate of shear
η=coefficient of viscosity
Term
If the relationship between F (shearing stress) and G (rate of shear) is direct, what kind of flow does the fluid demonstrate?
Definition
Newtonian
Term
Give three examples of Non-Newtonian flow
Definition
Plastic, pseudoplastic, and dilatant
Term
Define fluidity, Φ
Definition
The reciprocal of viscosity (1/η)

The slope of shearing stress (F) vs rate of sheer (G), if Newtonian
Term
Give the equation for kinematic viscosity
Definition
Viscosity/Density

(measured in centistokes)
Term
Define poise and what units it is measured in
Definition
The force required to move two adjacent layers of liquid across each other, if they are 1 cm apart, at a speed of 1 cm/s

units= g/(cm x sec)
Term
Define Bingham bodies
Definition
Substances characterized by plastic flow
Term
Define mobility
Definition
Like fluidity, but for Non-Newtonian flow
Term
Define yield value (f)
Definition
The shearing stress required for the substance to yield to force.

Below f, substance acts as a solid, elastic material.
Term
Define plastic flow
Definition

A material that, when F is greater than f, it is Newtonian.

 

f=yield value, F=shearing stress

Term
Give the equation for plastic viscosity. What units is it measured in?
Definition

The inverse of mobility

 

Plastic viscosity, U= (F-f)/G

 

Units are in poise

Term
If a suspension is more flocculated, is the yield value increased or decreased?
Definition
The yield value is increased. (flocculation ∝ yield value)

Yield value increases with van der Waals forces and interparticle friction
Term
Give an example of a plastic material
Definition
A flocculated suspension
Term
Define pseudoplastic flow
Definition
A fluid that is not completely plastic, but show some plasticity
Term
How is apparent velocity calculated using a graph of pseudoplastic flow?
Definition
Apparent viscosity at a given F is determined by a slope of tangent at that point
Term
Using the formula FN=η'•G , what does it mean when N=1?
Definition
Flow is Newtonian
Term
Give a pharmaceutical example of pseudoplastic material
Definition
Natural and synthetic gums (tragacanth, Na-alginate, methylcellulosa, Na-CMC)

Or even cornstarch in water
Term
Why is the rheogram of a pseudoplastic material curved?
Definition
-Shear force aligns polymers along their long axis, reducing internal resistance to flow

-Uncoiling releases entrapped solvent
Term
Define dilatant flow
Definition
Increasing shearing force increases resistance to flow
Term
Give an example of a pharmaceutical use of dilatant flow
Definition
Liquid bandages, pastes (very concentrated suspensions)
Term
Using the formula FN=η'•G , what does it mean when N<1?
Definition
Dilatant flow
Term
What are some characteristics of suspensions with dilatant flow?
Definition
-High percentage of dispersed solids (>50%)

-Small, deflocculated particles

-Will break a blender
Term
Why do highly concentrated suspensions dilate?
Definition
-Under stress, the material takes on open form of packing with increased interparticle void volume

-Vehicle remains constant, therefore wetting decreases
Term
What viscometers are for Newtonian flow only?
Definition
Capillary and falling sphere

(One-point systems, single shearing stress)
Term
What viscometers can be used for non-Newtonian flow?
Definition
Cup & Bob, Cone & Plate

(Can operate at multiple shearing stresses)
Term
How does a Capillary viscometer (Ostwald) work?
Definition
It measures the time required for the liquid to pass between two marks as a flows through a vertical capillary tube. The measurement is compared with water to obtain relative viscosity.
Term
How does a falling ball viscometer (Hoeppler) measure viscosity?
Definition
It measures the rate at which a ball falls through a sample liquid

A water jacket outside the tube makes the temperature constant
Term
What formula calculates viscosity, as obtained by a falling ball viscometer?
Definition
η= t(Sb-Sf)B

t=time
Sb-Sf= specific gravity difference
B=constant
Term
How does a Cup and Bob viscometer work?
Definition
By putting the sample in between the cup and bob, and rotating one. The torque from the drag is measured.
Term
What is a disadvantage to a Rotovisco viscometer?
Definition
"Plug flow", that shearing stress decreases with the distance from the rotating bob to the side of the stationary cup.

Therefore, use as big a bob as possible.
Term
What happens when shearing stress is removed from a plastic system?
Definition

Hysteresis loop- it remains slightly less viscous even after the stress is withdrawn

 

[image]

Term

On a rheogram:

 

What are the axis?What does the slope indicate?

Definition
[image]
Term
What happens when shearing stress is removed from a pseudoplastic system?
Definition
Some "memory" of the force is retained; thixotropy
Term
Define thixotropy
Definition
An isothermal and comparatively slow recovery, on standing of a material, of a consistency lost through shearing

Like a "memory"
Term
What two things are rheograms of thixotropic systems dependent on?
Definition
1. Rate at which stress is applied or removed
2. Length of time the material is subjected to stress
Term
What kind of system is thixotropy applicable to?
Definition
Shear-thinning systems only
Term
In thixotropy, what is the technical term for the loose network breaks resulting in shear thinning?
Definition
Gel-to-sol transformation
Term
In thixotropy, why is it than when the stress is removed the structure does not instantly harden?
Definition
The asymmetric particles counter the forces of Brownian motion
Term
What is negative thixotropy?
Definition
Increase in consistency as the rate of shear decreases

Gel-like, great suspendability, and is easily pourable
Term
Define rheopexy
Definition
Rheoplexy is to dilatant flow what thixotropy is to plastic and uniplastic flow

A phenomenon in which a solid forms a gel more readily when greatly shaken; gel is the equilibrium state
Term
Give an example of a substance that demonstrates negative thixotropy
Definition
Magnesia magma
Term

How do you measure thixotropy?

 

(Just memorize; don't need to know math.)

Definition

By building hysteresis loops and measuring either:

 

1. Structural breakdown with a constant rate of shear

2. Structural breakdown with increasing shear

Term
What factor determines the level of thixotropy?
Definition
Thixotropic coefficient (mathematically derived from a hysteresis loop)

(We don't need to know how to do this math)
Term
List some pharmaceutical applications to thixotropy
Definition
1. Poloxamers (dermatological gels)

2. Opthalmic polymers (shear thinning during blinking)
Term
Define poloxamers
Definition
Brand name: pluronics

Clear, nontoxic, water-based gels as a dermatological base. Become non-Newtonian at high temp/conc
Term
What is the formula for activation energy to start flow?
Definition

η=A•e^(Ev/RT)

or

ln(η)= ln(A) + (Ev/R)•(1/T)

 

(Modified Arrhenius equation)

 

A=constant

Ev=activation energy required

T=temp in Kelvin

R=gas constant

Term
When plotting ln(η) versus (1/T), what is the slope of the line?
Definition
Ev/R
Term
Define viscoelasticity
Definition
Having both elastic (solid) and viscous properties

A quality of pharmaceuticals
Term
Define elasticity, mathematically
Definition
Elasticity=stress/strain

E=F/γ
(Hooke's law)
Term
Give some examples of viscoelastic materials
Definition
Creams, lotions, ointments, suspensions, suppositories, blood
Term
What quantifiable quality provides the pschorheologic quality "body"?
Definition
Viscosity
Term
What quantifiable quality provides the pschorheologic quality "slip"?
Definition
Yield value
Term
What quantifiable quality provides the pschorheologic quality "spreadability"?
Definition
Thixotropy
Term
What quantifiable quality provides the pschorheologic quality "feel"?
Definition
Kinematic viscosity
Term
What quantifiable quality provides the pschorheologic quality "sensory satisfaction"?
Definition
Viscoelasticity
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