Rheology part 2

Rheology

Maryam kazemiPh.D student of pharmaceutics

Shiraz university of medical sciences

Measurement of thixotropy The most apparent characteristic of a thixotropic

system area of hysteresisPlastic( bingham) bodies :

Thixotropic coefficient:

Thixotropic coefficient

B=thixotropic coefficientU=plastic viscosityF=Shear stress G=Shear ratef=Bingham yield value

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

Bulgs & Spures

Agueous bentonit gel (10-15% by weight) hysteresis loop with bulge in the up curve.

Procain penicillin gel IM

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.

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.

The equilibrium system :

1) readily pourable

2)the material returned to the Sol like properties.

Anti thixotropy compare with dilatancy or rheopexy

Anti thixotropy:1)Floculated2)Low solid content(1-10%)3)Equilibrium form is sol

Rheopexy or Dilatancy1)Defloculated2)Greater than 50% by volume of solid dispersed phase .3)Equilibrium form is gel

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

Thixotropy in Formulation Desirable properly in liquid pharmaceutical sys

Ideally should have a high consistency in the container

Spread easily

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.

Determination of the rheological behavior and viscosity of various materials and compounds

Choise of viscometer:1) Single point viscometer : Newtonian systemSingle shear stress & shear rate .

2) Multipoint viscometer/rheometer: non-Newtonian, Newtonian systemMultiple shear stress & shear rate

Instrumentation

Newtonian Fluids Viscometers1- Capillary Viscometer

Ostwald U-tube viscometer

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

Poiseuilles law• Application:Calculate the pressure difference in the arteries and capillaries.Poiseulles law :r : the radius of the inside of the capillaryt : the time of flowp : the pressure head in dyne/cm2 under which the liquid flows.I : the length of the capillaryV : the volume of liquid flowing

pressure head is depended on density ,height, gravity .

• Poiseuille law according to the r:

• Falling sphere viscometer

2- Falling Sphere ViscometerHoeppler falling ball viscometer 1.59 mm diameter ball (glass or steel cylinder

(320 mm height x 25 mm width)

• Bob and cup viscometer

Multipoint Viscometers1- 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 KF Shear stress (dyne cm-2) = Stress reported by viscometer x Shear

stress factor (dyne cm-2)

G = KG x n

Shear rate (s-1) = Shear rate factor x Speed of rotation (rpm)

omega : the angular velocity in radians/sec produced byT: 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.

• Newtonian liquid

• Plastic viscosityU : the plastic viscosity in poises wf :the yield value intercept in grams

• yield value of a plastic system

• Kf is equel to:

• The Brookfield viscometer is a rotational viscometer of the Searle type that is popular in the quality-control laboratories of pharmaceutical manufacturers

• Pulg flowFlow of paste Bob and cup