Rheology part 2

Rheology concepts

1. Rheology Maryam kazemi Ph.D student of pharmaceutics Shiraz university of medical sciences

2. Measurement of thixotropy The most apparent characteristic of a thixotropic system area of hysteresis Plastic( bingham ) bodies :

3. Thixotropic coefficient: Thixotropic coefficient B=thixotropic coefficient U=plastic viscosity F=Shear stress G=Shear rate f=Bingham yield value

4. Determine the structural break down M=thixotropic coefficient V=maximum shearing rate

5. Bulgs & Spures Agueous bentonit gel (10-15% by weight ) hysteresis loop with bulge in the up curve. Procain penicillin gel IM

6. Bulgs & spures cont … A sharp point of structural break down spurevalue or yield value at the low shear rate. The spur value is obtained by using an instrument in which the rate of shear can be slowly and uniformly increased preferably automatically Penicillin gels having definite I values were very thixotropic, forming intramuscular depots upon injection that afforded prolonged blood levels of the drug.

7. Negative thixotropy or Anti thixotropy Represents an increase rather than a decrease in consistency on the downcurve . This increase in thickness or resistance to flow with increased time of shear. It was detected at shear rates of greater than 30 1/sec Below 30 1/sec the magma showed normal thixotropy It was observed that when magnesia magma was alternately sheared at increasing and then decreasing rates of shear , the magma continuously thickened (an increase in shearing stress per unit shear rate) but at a decreasing rate, and it finally reached an equilibrium state in which further cycles of increasing-decreasing shear rates no longer increased the consistency of the material.

8. The equilibrium system : 1) readily pourable 2)the material returned to the Sol like properties.

9. Anti thixotropy compare with dilatancy or rheopexy Anti thixotropy : 1) Floculated 2)Low solid content(1-10%) 3)Equilibrium form is sol Rheopexy or Dilatancy 1) Defloculated 2)Greater than 50% by volume of solid dispersed phase . 3)Equilibrium form is gel

10. Increased collision frequency of dispersed particles or polymer molecules in suspension Anti thixotropy results from Inter particle bonding with time Eventual equlibrium Small number of relatively large floccules (at rest ) Large floccules break up and gratually return to the original state of small floccules and indivitual particles

11. Thixotropy in Formulation Desirable properly in liquid pharmaceutical sys Ideally should have a high consistency in the container Spread easily

12. Degree of thixotropy & rate of sedimentation Concentrated parenteral suspensions containing 40-70% w/v procain pen g in water high inherent thixotropy . Break down of the structure occurred (pass through the needle) Consistency was then recovered as rheologic structure reformed. Formulation of a depot of drug at the site of IM injection drug was slowly removed & made available to the body Degree of thixotropy was related to the specific surface of the Penicillin used.

13. Determination of the rheological behavior and viscosity of various materials and compounds Choise of viscometer: Single point viscometer : Newtonian system Single shear stress & shear rate . 2) Multipoint viscometer/ rheometer : non-Newtonian, Newtonian system Multiple shear stress & shear rate

14. I nstrumentation

15. Newtonian Fluids Viscometers 1- Capillary Viscometer Ostwald U-tube viscometer

16. Example: Acetone (45 s, 0.786 g cm -3 ) Water (100 s, 0.997 g cm -3 , 0.8904 cp ) viscosity acetone?

17. Poiseuilles law Application: Calculate the pressure difference in the arteries and capillaries. Poiseulles law : r : the radius of the inside of the capillary t : the time of flow p : the pressure head in dyne/cm2 under which the liquid flows. I : the length of the capillary V : the volume of liquid flowing

18. pressure head is depended on density ,height, gravity .

19. Poiseuille law according to the r:

20. Falling sphere viscometer

21. 2- Falling Sphere Viscometer Hoeppler falling ball viscometer 1.59 mm diameter ball (glass or steel cylinder (320 mm height x 25 mm width )

22. Bob and cup viscometer

23. Multipoint Viscometers 1- Bob and Cup Viscometer (concentric cylinder viscometer ) Couette (Cup rotates, and Bob stationary) Searle (Cup stationary and Bob rotates) Rotovisco , Stormer rotovisco : F = S x K F Shear stress (dyne cm -2 ) = Stress reported by viscometer x Shear stress factor (dyne cm -2 ) G = K G x n Shear rate (s -1 ) = Shear rate factor x Speed of rotation (rpm)

24. omega : the angular velocity in radians/sec produced by T : the torque in dynes cm. H : The depth to which the bob is immersed in the liquids , Rb and Rc :the radii of the bob and cup.

25. Newtonian liquid Plastic viscosity U : the plastic viscosity in poises wf : the yield value intercept in grams yield value of a plastic system Kf is equel to:

26. The Brookfield viscometer is a rotational viscometer of the Searle type that is popular in the quality-control laboratories of pharmaceutical manufacturers Pulg flow Flow of paste Bob and cup