Pharmaceutics I
Rheology of Suspensions
Dr. Mohammad Bayan Faculty of Pharmacy, Philadelphia University, Amman, Jordan Introduction:
Rheology
Viscosity Poise and centipoise Dyne.sec.cm-2
Newtonian and Non-Newtonian Fluids
Dr. Mohammad Bayan 2 Newton’s law of flow
Dr. Mohammad Bayan 3 Shear Stress (F), F'/A
Strain or Deformation
Velocity Gradient (G), dv/dr
Dr. Mohammad Bayan 4 The Shearing Stress (F) is the force per unit area (F'/A) required to cause flow.
Rate of Shear or velocity gradient (dv/dr:G) is the velocity difference dv between two planes of liquid separated by an infinite distance dr.
Dr. Mohammad Bayan 5 F'/A = η dv/dr...... (1) Newton recognized that: •Where η is a constant known as the higher the Viscosity viscosity of a liquid η = F / G...... (2) F = η G...... (3)
•Fluidity is the reciprocal of viscosity the greater shearing stress
F'/A α dv/dr
Dr. Mohammad Bayan 6 Example: What is the shear rate when an oil is rubbed into the skin with a relative rate of motion between the fingers and the skin of about 10 cm per seconds and the film thickness is about 0.02 cm?
Rate of Shear dv/dr = G G= 10 cm per seconds / 0.02 cm = 500 sec -1
Dr. Mohammad Bayan 7 Newtonian flow
Dr. Mohammad Bayan 8 Newtonian flow
Temperature
Pressure
Chemical composition
Water, simple organic liquids, true solutions, dilute suspensions and dilute emulsions are examples of Newtonian systems.
Dr. Mohammad Bayan 9 Non-Newtonian flow
Fail to follow Newton’s equation of flow
Characterised by a change in the viscosity with increasing shear rate
Examples: colloidal solutions, emulsions, liquid suspensions, and ointments.
Dr. Mohammad Bayan 10 Non-Newtonian flow
Plastic Pseudoplastic Dilatant
Dr. Mohammad Bayan 11 Plastic flow
Bingham bodies
Starts after exceeding a certain yield value of shearing stress
Does not pass through the origin
Shear-thinning systems
Elastic below the yield value
Dr. Mohammad Bayan 12 Plastic flow
Tomatoes Sauce, Honey and Flocculated particles in a concentrated suspension usually show a plastic flow
Dr. Mohammad Bayan 13 Pseudoplastic flow
No yield value
The consistency curve begins at origin or at least approach it at low rate of shear
Shear-thinning systems
Examples: natural and synthetic gums such as liquid dispersions of tragacanth, sodium alginate, methyl cellulose, and sodium carboxymethyl cellulose
Dr. Mohammad Bayan 14 Pseudoplastic flow
Dr. Mohammad Bayan 15 Dilatant flow
Exhibit an increase in volume when sheared, and the viscosity increases with shear rate.
Shear-thickening systems
High percentage of solids in the formulation (50 % or greater). When shear is removed the system returns to original state of fluidity.
Example: deflocculated suspension
Dr. Mohammad Bayan 16 Explanation of dilatant flow behaviour
Dr. Mohammad Bayan 17 Dilatant flow
• The inverse of pseudo plastic diagram
Dr. Mohammad Bayan 18 Thixotropy “An isothermal and comparatively slow recovery, on standing of a material whose consistency is lost through shearing”.
Only applied for plastic and pseudoplastic systems
Reversible gel–sol transformation
Dr. Mohammad Bayan 19 Upon setting, a network gel forms and provides a rigid matrix that will stabilise suspensions and gels.
When stressed (by shaking), the matrix relaxes and forms a sol with the characteristics of a liquid dosage form for ease of use.
A thixotropic agent such as microcrystalline cellulose is incorporated into the suspensions or emulsions to give a high viscosity.
Dr. Mohammad Bayan 20 Rheology of suspensions
High apparent viscosity at low rates of shear
At higher rates of shear, the apparent viscosity should fall sufficiently for the product to be poured easily
External use spread easily without excessive dragging
Injection pass easily through a hypodermic needle with only moderate pressure
Dr. Mohammad Bayan 21 A flocculated system partly fulfils the previous criteria
A pseudoplastic or plastic behaviour associated with thixotropy is exhibited as the structure progressively breaks down under shear and then shows time-dependent recovery
It may not be sufficient to prevent rapid settling, unless a high concentration of dispersed phase is present
Suspending agents
A deflocculated system Newtonian behaviour
Dr. Mohammad Bayan 22 Dr. Mohammad Bayan 23