9/25 Chapter 14 Flashcards
1. Define rheology 2. Differentiate between Newtonian and non-Newtonian flow 3. Describe plastic, pseudoplastic and dilatant flow 4. Contrast thixotropy and antithixotropy 5. Discuss how to determine rheological parameters of materials (14 cards)
Define rhenology
The study of viscosity of a substance. Divided into Newtonian and non-Newtonian Measured in poise(p)
Packaging into containers and removal prior to use can affect what 3 factors?
Patient acceptability Physical stability Biological stability
Newtonian flow vs non-Newtonian flow
Newtonian
- constant velocity
- defined by a single viscosity at a T.
- proportional to shearing stress and rate of shear. EX: water and mineral oil
non-Newtonian
- change in viscosity
- CANNOT be defined by a single viscosity at a T -time dependent
- **MAJORITY of pharm. products are made under this category**
EX: emulsions, suspensions, ointments
KNOW FLOW CURVES
pg. 3
Know equations for the viscosity of the Newtonian System.
G = dv/dt F = F’/A n = F/G
Name the 4 non-Newtonian materials
- Plastic 2. Pseudoplastic 3. Dilatant 4. Floc
Define Plastic
-Does not flow until yield is exceeded! -Below flow point = elastic -Van der Waals bonds plastic together -Exceeding yield brings out Newtonian properties AKA Bingham bodies
Define Pseudoplastic
-No yield value due to involvement in the plastic system. -Shear-thinning - “n” decreases as shear rate increases EX: Polymetric solutions
Define Dilatant
-Inverse of Pseudoplastic properties -Shear-thickening -“n” increases as shear rate increases -Characterized by having a high percentage of deflocculated particles in the formulation. EX: Titanium dioxide in water
Floc
-“Fl”oc = “Fl”ow -Flowing aggregation of molecules using WEAK bonds.
Define Thixotropy
Thixotropy – Refers to the **decrease** in viscosity with time when flow is applied to a material previously at rest and the recovery of viscosity in time when flow is discontinued. -Applies to SHEAR-THINNING -Desirable in pharmaceutical systems • High consistency in container, but pour and spread easily
Define Antithixotropy
Antithixotropy – When flow causes a reversible, time-dependent **increase** in viscosity.
Define Rheopexy
Rheopexy – Refers to the phenomenon that the gel formation of a system may be facilitated by low shear, compared to keeping it at rest; the reversibility is not defined.
What is a Viscometer?
– Instrument used to measure viscosity – Types: Capillary, falling-sphere, cup-and-bob and cone-and-plate – All can be used to determine viscosity of Newtonian systems – Only those with **variable-shear rate controls** can be used for non-Newtonian systems
