Exam 1 Spring 17 Flashcards
Consistency can affect what?
flow of materials, mixing, choice of processing equipment, packaging into container, patient acceptability, physical stability, bioavailability, removal from container for use
Define viscosity
- resistance to flow
- internal friction of a system
How do you determine viscosity (via layers)?
strength that a layer exerts on adjacent layers determines the system viscosity
What is the rate of shear and what is the variable that denotes it?
Rate of shear = G = dv/dr
dv = displacement of each layer
dr = thickness of each layer
What is the shearing stress and what is the variable that denotes it?
Shearing stress = F = Force/A
Force applied / surface area of each layer
What is the relationship between rate of shear and shearing stress?
F ∝ G
In variables, what is Newton’s law of flow?
F = ηG
the more shearing stress = the more rate of shear
How does viscosity affect F and G?
The higher the viscosity, the greater is the shearing stress required to produce a certain rate of shear
What are the types of non-Newtonian flow?
- Plastic flow
- Pseudoplastic flow
- Dilatant flow
What is η?
Coefficient of Viscosity (poise)
In a rheogram, when you plot F vs G, you get a straight line/slop (direct relationship). What is the slope (in variable) and what does it represent?
1/η
- Represents fluidity (Φ)
In variables, define kinematic viscosity.
η/ρ
Viscosity/Density
What are the units for kinematic viscosity?
stokes = centipoise / [g/cm^3]
What are the characteristics of a rheogram of a Newtonian system?
Rate of shear (y) and shearing stress (x) are directly related
What are some examples of non-Newtonian systems?
ointments, suspensions, emulsions
What is another name for plastic flow?
Bingham bodies
What is a yield value?
f = the amount of force you have to apply to a non-newtonian fluid in order for it to move
What happens when shear stress doesn’t exceed yield value?
the material will act as an elastic material
How can you determine the yield value?
when shearing stress exceeds yield value, the relationship between F and G are proportion; extend the line of their slop and where it touches the x-axis is the yield value
What is the variable for plastic viscosity and how do you solve for it?
U = (F - f) / G
What are the contributions to yield value?
Vanderwaals forces and interparticle friction
What is an example of plastic fluid?
flocculated suspensions
What effect does flocculated suspensions have on yield values?
more flocculated = larger the yield value
What is another name for pseudoplastic flow?
Shear thinning systems
Define pseudoplastic flow
neither plastic or totally newton; curve starts at origin with linear approach but never reaches a straight line because slope is constantly changing; viscosity is different at every point so F value must be reported when determining viscosity
What is an example of pseudoplastic material?
natural and synthetic gums
What are the reasons for the curved rheogram with respect to pseudoplastic flow?
- shear force aligns polymers along their long axis, reducing the internal resistance to flow
- Uncoiling releases the entrapped solvent resulting in lowering the concentration and size of the dispersed molecules
What are examples of shearing stress and how does that affect viscosity of pseudoplastic materials?
- Examples: shaking suspension, pumping blood, blinking eye
- Applying stress makes the flat/linear which creates a larger surface area so that flow increases and viscosity decreases
What is another name for dilatant flow?
Shear thickening systems
In dilatant flow, describe the relationship between shearing stress and viscosity. What happens when you remove this stress?
as you increase shearing stress, the viscosity increases
when the stress is removed, the system returns to the original state of fluidity
What are characteristics of dilatant materials?
- higher percentage of dispersed solids (more than 50%)
- small, deflocculated particles
What causes a solution to dilate?
increased stress -> increased volume -> dilate
In a Newtonian system, what happens when the shearing stress is removed from the system?
viscosity is restored at the same rate at which it was lost
In a non-Newtonian system, what happens when the shearing stress is removed from the system?
viscosity is not restored at the same rate at which it was lost
Define thixotropy
An isothermal and comparatively slow recovery, on standing of a material, of a consistency lost through shearing
The curves of the rheograms of thixotropic systems are highly dependent on?
- the rate at which stress is applied or removed
- the length of time the material is subjected to stress
How does length of time the material is subjected to stress affect recovery time?
it’s much slower
Under which systems is thixotropy applicable?
shear-thinning systems only; plastic and pseudoplastic flow
What are benefits of thixotropy ?
upon shaking container, formulation is easily spread
What is negative thixotropy?
when a solution increases in consistency on the downcurve