1

Rheology: rheo (to flow) and logos (science)

Described the flow of liquids and the defor-

mation of solids.

Viscosity :

Expression of the resistance of a liquids to flow; the

higher the viscosity, the greater the resistace.

Pharmacy: Application in formulation and and analysis of pharmaceutical product .e.g. emulsions, pastes, suppositories, and tablet coatings.

2

Rheology:Involved in:

The mixing and flows of materials,

Packaging into containers, and their removal

prior to use.

Affects its patient acceptability,

Physical stability

Biological availability.

Influence the selection of processing equip-

ment used in its manufacture

3

NEWTONIAN SYSTEMS

'

dr

dv

A

F

dv/dr: rate of shear = G

F’/A : shearing stress = F

= poise =dyne s/cm2 = g/cm s.

Fluidity : η

1Kinematic Viscosity = / stoke

1 centipoise (cP) = 1 millipascal second (mPa s)1 poise (P) = 0.1 pascal second (Pa s)

G

F

4

Calculate the rate of shear (dv/dr) applied by a patient in rubbing a 200 m thick film of ointment on the skin surface at a rate of 10 cm/s.

1-2

s 500102

10

dr

dv

S=G : rate of shear: Slop = fluidity = 1/

5

The following data show the rate of shear as function of shear stress for a Newtonian fluid at 25o . Plot the data and determine the fluidity and viscosity of the fluid.

(rate of shear, s-1) 200 400 600 800 1000

(shear stress,dynes/cm2) 50 100 150 200 250

12

12

xx

yy

cp 25poise 25.04

1

1

sdyne/cm 4 50250

2001000 2

6

Absolute Viscosity Of Some Newtonian Liquids At 20o C

liquid viscosity (cp)

Castor Oil)ChloroformEthyl alcoholGlycerin, 93%Olive OilWater

1000 0,563 1,19 400 100 1,0019

Temperature Dependance

RTEvAe /TR

EA v 1

ln ln

A = constantEv = Activation energyR = Gas constantT = Absolut temperature

7

The viscosity of human plasma at 370C is 1.2 cp. Assuming

that plasma behave as a Newtonian fluid, determine the

viscosity of plasma required for infusion that is kept at

room temerature (250C). The activation energy of plasma

is 4.25 X 103 cal/mole. R = 1.987 kal. 0K-1 mol-1

cp 1021,1718,6ln

310

1

987,1

1025,4ln (1,2)ln

K) 310(T 37At

3

3

0

AA

A

C

cp 58,1460,0ln

298

1

987,1

1025,41021,1lnln

2525

33

25

First determine the A value, using the equation TR

EA v 1

ln ln

8

NONNEWTONIAN SYSTEM

S=G: rate of shear:

Plastic Flow

Plastic viscosity, U:

G

fFU

f : yield value

UG=F-f

F=UG+fF

G

Yield value: the minimum shear stress required by the system before it deforms and begin to flow

9

Extrapolating the straight line to the x axis, the yield value,

f, will be approximately 50 dynes/cm2

plastic viscosity U= 1/slop or (x2-x1)/(y2-y1)

U =(725-162.5)/(1500-250) = 0.45 p = 45 cps

The following data show the rate of shear as function of shear stress for topical formulation that exhibts plastic rheologic profile. Plot the data and determine the yield value and plastic viscosity of the formulation.

Rate of shear (s-1) 250 500 750 1000 1500

Shear stress (dyne/cm2) 162.5 275 387.5 500 725

10

S=G: rate of shear:

Pseudoplastic Flow

GF N 'η Log G= N log F – log ’

11

Pseudoplastic system reffered as shear-thinning

system because at increasing shear stress, the

viscosity decreases..

Clasical example is ketchup (tomato sauce). To

get ketchup out of the bottle, one must shake

the bottle several times. Shaking the bottle is

incresing shear stress.

12

S=G: rate of shear:

Dilatan Flow

shear-thickening system

Suspensions containing high concentration (>40-50%) of small, deflocculated particles.

13

THIXOTROPY

An isothermal and comparatively slow recovery of the material on standing that was lost by shear thinning.Shear thinning is a desirable property of pharmaceutical suspensions and emulsions that should have high viscosity upon standing to prevent settling and aggre-gation but should pour easily from the container once it is shaken.

14

Tixotropy Measurement

1

2

21

lnt

tUU

B

1

2

21

lnv

vUU

M

15

Structure of hysteresis loop

bulge spur

Bentonit gel, 10-15% by weight

Procaine penicillin gel for i.m. injection

16

Negative Thixotropy (Antithixotropy)

Represents an increase rather than decrease in consistency on the down curve.

The difference between dilatancy and rheopexy. Dilatant systems are deflocculated and ordinarily contain greater than 50% by volume of solid dispersed phase, whereas antithixotropic systems have low solids content (1% — 10%) and are focculated.

Rheopexy is a phenomenon in which a solid forms a gel more readily when gently shaken or otherwise sheared than when allowed to form the gel while the material is kept at rest. In a rheopectic system, the gel is the equilibrium form, whereas in antithixotropy, the equilibrium state is the sol.

17

18

D E T E R M I N A T I O N O F V I S C O S I T Y

Capilary Viscometer

(a)Ostwald viscometer , (b) Ubbelohde,

(c) Ostwald-Cannon-Fenske

22

11

2

1

t

t

19

Hoeppler Falling Ball Viscometer

BSSt fb

t is the time interval in seconds for the ball to fall between two points, S b

and S f are specific gravities of the ball and fluid. B is constant for particular ball and is supplied by the manufacture.

Falling Ball Viscometer

20

Cup And Bob Viscometer (Rotational Viscometer):

Viscometer Stormer

vw

Kv

where Kv is a constant for the instrument. V, the rpm generated by the weight, w, in grams,

21

Ferranti-Shirley

Cone-and-Plate Viscometer

v

TC

where C is an instrumental constant, T is the torque reading, and v is the speed of the cone in revolutions per minute.

22

Rotational Viscometer :

Viscometer Brookfield

23

24

VISCOEI.ASTICITY

Oscillatory and creep measurements are also of considerable importance for investigating the properties of semisolid drug products, foods, and cosmetics that are classified as viscoelastic materials.

Viscoelastic measurements are based on the mechanical properties of materials that exhibit both viscous properties of liquids and elastic properties of solids.

Many of the systems studied in pharmacy belong to this class, examples being creams, lotions, ointments, suppositories, suspensions, and the colloidal dispersing, emulsifying, and suspending agents. Biologic materials such as blood, sputum, and cervical fluid also show viscoelastic properties

2626

Semi solid: Liquid

Solid

G

F

Hooke’s Law

Newton Law

F

E

a) Liquid b) Solid

c) ViscoelasticParallel combination,

Voight or Kelvin

element

27

/1 teJJ

The strain applied to the mechanical model is expressed as a deformation or compliance, J, of the test material, in which J is strain per unit stress. The compliance of a viscoelastic material following the Voigt model is given as a function of time, t, by the expression:

where J: is the compliance or strain per unit stress at

infinite time and is viscosity per unit modulus, /E (dyne sec cm2/dyne cm2), which is called retardation time and has the unit of seconds.

28

A-B : elastic movement

B-C : viscoelastic region

C-D : viscous flow

DE equivalent AB

EF equivalent BC

The compliance equation corresponding to the observed behavior of wool fat as simulated by theMaxwell—Voigt model :

F

eJeJF

J nm tn

tm

γ11

γ τ/τ/0

0 : instantaneous strain, F : constant applied shear stress; J m and Jn : mean compliance of bond in the material; m and n mean retardation time for the two Voight units..

29

30

PSIKORHEOLOGY Topical preparations must meet criteria of: feel,

spreadability, color, odor, and other psychologic and sensory characteristics.

Sensations in the mouth, between the fingers, and on the skin are important considerations for manufacturers of foods, cosmetics, and dennatologic products.

Boylan showed that the thixotropy, consistency, and yield value of bacitracin ointment, USP, decreased markedly as the temperature was raised from 20°C to 35°C.

The consistency of a material could be accurately as- sessed by a panel of untrained subjects by the use of only three attributes: smoothness, thinness, and warmth. Smooth-ness was related to a coefficient of friction and thinness to non-Newtonian viscous parameters that could be measured with appropriate instruments. The characteristic of warmth was found to be sufficiently complex to require further study..

32

32

APLICATION OF RHEOLOGY IN PHARMACY

Polymer solutions :in ophthalmic preparations, as wetting solutions for contact lenses, and as tear replacement solutions for the condition known as dry eye syndrome. Both natural (e.g., dextran) and synthetic (e.g., polyvinyl alcohol) polymers are used with the addition of various preservatives.

The rheologic properties of suppositories at rectal temper- atures can influence the release and bioabsorption of drugs from suppositories, particularly those having a fatty base.

The cosmetic sticks contained propylene glycol, polyethylene glycol 400, and polyethylene glycol 600 as humectants and the topically active drugs panthenol, chiorphenesin, and lignocaine. Thixotropic breakdown was much lower in these medicated sticks than in comparable bases.

33