Unit 5-rheology

physical pharmacy

1. Unit 5 RHEOLOGY KPJ HEALTHCARE UNIVERSITY COLLEGE

2. RHEOLOGY Science describing the flow and deformation of matter under stress. Rheo = the flow Viscosity (η) is the resistance of a fluid material to flow under stress. The higher the viscosity, the greater the resistance.

3. Importance of rheology – Formulation of medicinal and cosmetic creams, pastes and lotion. – Formulation of emulsion, suspension, suppositories and tablet coatings. – Fluidity of solutions for injection. – In mixing and flow of materials, their packaging into containers, their removal prior to use, whether by pouring from a bottle, extrusion from a tube, or passage through a syringe needle. – Can affect patient acceptability, physical stability, and even biological availability.

4. Viscosity Viscosity is an index of resistance of a liquid to flow. The higher the viscosity the greater the resistance Water and alcohol has the least resistance to flow (lowest viscosity)

5. Types of viscosity Kinematic viscosity: It is defined as the viscosity( neta ) divided (row) of the liquid. Viscosity is expressed as kinematic viscosity in official pharmacopoeias. It is expressed as kinematic viscosity= neta /row The unit of kinematic viscosity is stokes (s) and centistokes( cs )

6. Types of viscosity Relative viscosity: the coefficient, abbreviated, neta r is defined as the ratio of viscosity of the dispersion ( neta ) to that of the solvent neta0 (vehicle) it is mathematically expressed as Relatively viscosity, nr =neta-neta0/neta0

7. Types of viscosity Specific viscosity: this term is defined as the relative increase in the viscosity of the dispersion over that of the solvent (vehicle) alone. It is mathematically expressed as Specific viscosity, neta sp =n-n0/n0 Reduced viscosity: this term is defined as the ratio of specific viscosity to the concentration (c). It is mathematically expressed as Reduced viscosity= netasp /c

8. Types of viscosity Specific viscosity: this term is defined as the relative increase in the viscosity of the dispersion over that of the solvent (vehicle) alone. It is mathematically expressed as Specific viscosity, neta sp =n-n0/n0 Reduced viscosity: this term is defined as the ratio of specific viscosity to the concentration (c). It is mathematically expressed as Reduced viscosity= netasp /c

9. Types of viscosity Intrinsic viscosity: the reduced viscosity is determined at various concentrations of a substance and the results are plotted. The resulting line can be extrapolated to c=0 to obtain the intercept. The intercept value is known as intrinsic viscosity. This parameter is useful to determine the molecular weight of polymers.

10. Other important definitions Shear stress is defined as the force per unit area F’/A, which is applied to bring about the flow Shear stress, F=F’/A Velocity gradient or rate of shear, dv/ dr is defined as the change in the velocity, dv, with an infinitesmal change in distance, dr. Rate of shear, G=dv/ dr The higher the viscosity of the liquid the greater is the force per unit area required to produce a certain rate of shear.

11. Other important definitions Hence the relationship between shear stress and rate of shear is given as follows F’/ A∞dv / dr Or F’/A= neta dv/ dr Or F= netaG In which neta is the coefficient of viscosity and usually referred to as viscosity. Viscosity is calculated by neta =F/G This equation is called as Newtonian equation

12. Rheogram G V neta =1 F Rheogram: Rheogram is a plot of rate of shear Vs shearing stress. For Newtonian liquid if G is plotted Vs F the flow curve gives straight line passing through the origin and the slope is the coefficient of viscosity and is equivalent to 1

13. Other important definitions Coefficient of viscosity is defined as the force per unit area required to maintain unit difference in velocity between two parallel layers in the liquid, one centimeter apart Units of Viscosity: In ISO system the unit of viscosity is the poise, named after Poiseulle . It is practically expressed as centipoise, cp (=0.01 poise) In CGS system the units for viscosity is dynes/cm 2

14. Other important definitions Poise: it is defined as the shearing stress required to produce a velocity of 1cm/sec between two parallel planes of a liquid each 1cm 2 in area and separated by a distance of 1cm. Fluidity : this term fluidity ø is used to denote the reciprocal of viscosity i.e. Fluidity ø=1/ neta

15. Thixotropy and Antithixotropy “ Change by touch” Thixotropy is defined as an isothermal and comparatively slow recovery process on standing of a material of a consistency lost through shearing. This is applied for plastic and pseudoplastic system. This is also called as gel to sol to gel transformation. Antithixotropy : This is also called as negative thixotropy , this represents an increase in consistency on the down curve. The down curve shifts to the right of the up curve. This is also called as sol to gel to sol transformation

16. Classification of Rheological Systems Newtonian System Non- -Newtonian System

17. NEWTONIAN SYSTEMS NEWTONIAN ‘S LAW OF FLOW

18. Shear stress NEWTONIAN FLOW Newton assumed that all materials have, at a given temperature, a viscosity that is independent of the shear rate. In other words, twice the force would move the fluid twice as fast. Simple Newtonian Rheogram Shear rate viscosity

19. Shear: is the movement of material relative to parallel layer. Shear stress (F ’ ): is the force per unit area required to bring about flow (F/A) Shear rate (S) difference in velocity dv, between two planes of liquids separated by distance dr (i.e. dv/dr) F/A α dv/dr

20. Absolute (dynamic) viscosity Viscosity= η = F’ = shear stress = dyn m -2 = dyn m -2 s S shear rate sec -1 The fundamental unit of viscosity measurement is the poise . A material requiring a shear stress of one dyne per square centimeter to produce a shear rate of one reciprocal second has a viscosity of one poise, or 100 centipoise.

21. Definitions Fluidity; it is t he reciprocal of viscosity Ø = 1/ η Kinematic Viscosity: It is the absolute viscosity divided by the density of liquid at a specified temperature Kinematic viscosity = η/ p Where p is the density of the liquid The unite is Stock (s) or centistock (cs)

22. Definitions Relative viscosity: Is the relation of the solution viscosity η to the viscosity of the solvent “standard” η ο η rel = η/ η ο _ Specific Viscosity η sp = η rel -1

23. Example A) The viscosity of acetone at 25°C is 0.313 cp, its density at 25°C is 0.788 g/cm 3 . What is its kinematic viscosity at 25°C? B) Water is usually used as a standard of liquids. Its viscosity at 25°C is 0.89 cp. What is the viscosity of acetone relative to that of water (relative viscosity, η rel ) at 25°C? Solutions: a) Kinematic viscosity = 0.313 cp ÷ 0.788 g/cm 3 = 0.397 cs b) Relative viscosity η rel = 0.313 cp ÷ 0.8904 cp = 0.352 (dimensionless)

24. Liquid Viscosity(cp) Castoroil Chloroform Ethylalcohol Glycerol Oliveoil Water 1000 0.563 1.19 400 100 1.0019 ABSOLUTE VISCOSITY OF SOME OF THE COMMONLY USED LIQUIDS

25. NON- -NEWTONIAN SYSTEMS

26. NON- -NEWTONIAN SYSTEMS A non- -Newtonian fluid is defined as one for which the relationship between F’ and S is not a constant. In other words, when the shear rate is varied, the shear stress doesn't vary in the same proportion. The viscosity of such fluids will therefore change as the shear rate is varied. It can be seen in liquid and solid heterogeneous dispersions such as colloids, emulsions, liquid suspensions and ointments.

27. NON- -NEWTONIAN SYSTEMS THREE CLASSES: – Plastic flow – Pseudoplastic flow – Dilatant flow

28. 1.PLASTIC FLOW Plastic flow is associated with the preparation of flocculation or aggregations of particles in concentrated suspension , also known as Bingham bodies . A Bingham body does not begin to flow until a shearing stress corresponding to the yield value is exceeded. Yield value (f); is an indication of the force that must be applied to a system to convert it to a Newtonian System. Examples; suspension of Zno in mineral oil, certain paints, ointments

29. 1.PLASTIC FLOW

30. Plastic flow Rheogram Plastic flow rate of shear yield value (G) Shearing Stress (F)

31. PLASTIC FLOW (CONT.) The slope of the rheogram is termed Mobility , analogous to fluidity in Newtonian system, and its reciprocal is known as Plastic viscosity, U U= F – f S Where f is the yield value

32. PLASTIC FLOW (CONT.) Problem A plastic material was found to have a yield value of 5200 dyne.cm -2 . At a shearing stress above yield value, F was found to increase linearly with S. If the rate of shear was 150 sec -1 when F was 8000 dyne.cm -2 . Calculate the plastic viscosity of the sample. Solution: U = F – f = 8000 – 5200 = 18. .67 poise S 150

33. 2. Pseudoplastic Flow (shear- -thinning) The curve begins at the origin (or approach it), there is no yield value. Occurs in dispersion of polymers (e.g. syenthetic or natural gum, cellulose derivatives) As the shearing stress is increased, disarranged molecules orient themselves to the direction of flow. This orientation reduces internal friction and resistance of the molecules and allows a greater rate of shear at each shear stress. Some of the solvent associated with molecules will be released resulting in decreasing the viscosity. This type of flow behavior is sometimes called shear- -thinning .

34. Pseudo- -plastic flow behavior; Structural reasons

35. Pseudo- -plastic flow behavior; Structural reasons

36. Pseudoplastic Flow -Random arrangement polymer chains -Solvent interact with polymer - Chain entalgment Increased stress -Chains line up in the direction of the Applied stress -Layers move over each other more easily -More vehicle available

37. Pseudo-Plastic flow Rheogram Pseudo-Plastic flow rate of shear (G) Shearing Stress (F) The curve begins at (or near) the origin, there is no yield value - Apparent viscosity at any shear rate is determined from the slop of a tangent to the curve at that point

38. 3. Dilatant Flow (shear- -thickening) Certain suspensions with a high percentage (up to 50%) of deflocculated solids exhibit an increase in resistance to flow with increasing rate of shear. Such systems actually increase in volume when sheared and hence termed dilatant. This type of flow behavior is sometimes called shear--thickening . When stress is removed, a dilatent system returns to its original state of fluidity. E.g. corn starch in water.

39. Dilatant flow Rheogram Dilatant flow rate of shear (G) Shearing Stress (F)

40. Reasons for Dilatency 1. At rest particles are closely packed with minimal inter- -particle volume (void), so the amount of vehicle is enough to fill in voids and permits particles to move at low rate of shear. 2. Increase shear stress, the bulk of the system expand (dilate), and the particles take an open form of packing. 3. The vehicle becomes insufficient to fill the voids between particles. Accordingly, particles are no longer completely wetted (lubricated) by the vehicle. 4. Finally, the suspension will set up as a firm paste. 5. This process is reversible.

41. Resting Sheared - Closedpackparticles - Minimumvoidvolume - Sufficientvehicle - Relativelylowconsistency Openpackedparticles Increasedvoidvolume Insufficientvehicle Relativelyhighconsistency Characters of dilatent system

42. SIGNIFICANCE OF DILATENCY Such behaviour suggests that appropriate precautions should be used during processing of dilatent materials. Mixing (powder+granulating liquid) is usually conducted in high speed mixers, dilatent materials may solidify under these conditions thereby damage the equipments.

43. Sum up of all types of material flow

44. THIXOTROPY Non-Newtonian, Time Dependent behaviour. Definition of Thixotropy: It is the decrease in viscosity as a function of time upon shearing, then recovery of original viscosity as a function of time without shearing.

45. THIXOTROPY – A decrease in apparent viscosity with time under constant shear rate or shear stress, followed by a gradual recovery, when the stress or shear rate is removed. – Such system contain asymmetric particles forming a loose network through sample. At rest, this structure impart rigidity to system resembling gel. As shear applied, the structure begin to break and the material undergo Gel-to-Sol transformation. Finally, at rest the structure is restored again (Sol-to Gel). – It happens with pseudoplastic materials (shear- thinner)

46. THIXOTROPY

47. Shear stress THIXOTROPY Shear rate

48. THIXOTROPY Examples of thixotropic samples are ketchup, consumer paints, yoghurts, mayonnaise. Thixotropic samples break their structure under shear rate and rebuilds the structure at rest. The rebuild is occurring under a material specific time scale. The rebuild speed is highest at rest and slow at low shear rates.

49. Thixotropy in Formulation In suspension, particles will not settle down in the container (gel form), will become fluid (sol) on shaking for a dose to dispense. At rest, it will retain its consistency to maintain the particles suspended. This is also applied to emulsions, lotions and creams. 40- Parenteral suspensions used for intramuscular depot therapy, e.g. procaine penicilline G (40- 70% w/v in water)

50. Rheopexy – An increase in apparent viscosity with time under constant shear rate or shear stress, followed by a gradual recovery when the stress or shear rate is removed. Also called Anti-thixotropy or negative thixotropy.

51. Operation of Capillary and Cone & Plate Viscometers

52. Capillary Viscometer Select appropriate capillary size to give reasonable times Keep constant temperature Time fluid falling between two fiducial marks (a) and (b) Avoid parallax Also known as Ostwald ( U-tube)

53. Brookfield Cone & Plate Viscometer Shallow angled cone in very close proximity with a flat plate Important features Circulating bath to keep constant temperature Different cone sizes Level on the instrument Adjusting ring Motor speed in RPM Operation Adjust cup so pins barely not making contact Measure torque needed to overcome viscous resistance

54. Calibrations

55. Calibration Capillary Viscometer Second term neglected for sufficiently long times (>60 sec) Fluid of known viscosity used to determine parameter B

56. Calibration Brookfield Standard Density (g/mL) Viscosity (cP @ 25 °C) Average time (sec) Parameter B (cP*mL/g*sec) Fluid 100 0.974± 0.005 (2) 96.6 89.84 ± 0.22 (1) 1.104 ± 0.006 (2) Fluid 50 0.971± 0.005 (2) 47.9 44.74 ± 0.14 (1) 1.102 ± 0.006 (2) Cannon- Fenske Routine Capillary Viscometer: Size 400 with T=25 °C (1) Standard deviation (2) Propagated error

57. Calibration Brookfield cone and plate viscometer with cone size CP-41 and T=28.5 °C

58. Viscosity of an Unknown Fluid

59. Unknown Fluid Capillary Viscometer Accuracy: 0.7% vs ±0.2% reported Reproducibility: 0.19% vs ±0.1% Average Time (sec) Density (g/mL) Viscosity (cP) 89.89 ±0.17 (1) 0.974 ±0.005 (2) 96.7 ±0.7 (2) (1) Standard deviation (2) Propagated error

60. Unknown Fluid Motor speed (RPM) Viscosity Reading (cP) Viscosity (cP) 6 85.8 94.5 12 86.0 96.6 Brookfield Cone and Plate Viscometer Average viscosity=95.5 ±1.5 cP ( st dev) Accuracy: 1.6% vs ±1%

61. Results Unknown fluid determined to be Brookfield Fluid 100 ( μ =96.6 cP ) Capillary Viscometer (25 °C) 96.7 ±0.7 cP Error of 0.10% Cone and Plate Viscometer (28.5 °C) 95.5 ±1.5 cP Error of 1.1% Student’s T Test 84.4% Probability they are the same

62. Determination of Viscosity U-tube viscometers Falling sphere viscometers Rotation viscometers

63. .1Cappilary viscometers Also known as Ostwald ( U-tube( viscometer of glass cappilary viscometer. Used for Newtenian flow Classified as direct flow and reverse flow viscomenters

64. .2Falling sphere viscometers A glass or steel ball of known size and density is allowed to descend through the liquid hold stationary in a vertical glass tube. The rate at which the ball falls inversely proportion to the viscosity. - Electrical sensor is used for opaque liquids

65. 3. rotational Rheometers 1. Concentric cylinder rheometer A viscometer in which the liquid whose viscosity is to be measured fills the space between two vertical coaxial cylinders, the inner one suspended by a torsion wire; There is two types: The outer cylinder is rotated at a constant rate, and the resulting torque on the inner cylinder is measured. The inner cylinder is rotated at a constant rate, and the resulting torque on the outer cylinder is measures

66. 2. Cone and plate type

67. Important notes Flocculated suspensions exhibit plastic flow Newtonian fluids are expressed in terms of viscosity whereas non Newtonian fluids are expressed apparent (viscosity) Fluidity is a term associated with Newtonian fluids, whereas mobility is the term used for fluids exhibiting plastic flow Dilatant flow is characterized as a reverse phenomenon of pseudoplastic fluids

68. Important notes Deflocculated suspensions containing high concentration of the dispersed solids exhibits dilatant flow In antithixotropy the down curve is frequently positioned to (with respect to up curve) right At equillibrium , the thixotropic behaviour of a pseudoplastic system exhibit the state of a Sol Viscosity, body and slip, and spreadability are the rheological properties whereas surface tension is not a rheological property

69. Important notes Pseudoplastic flow behaviour can be explained by apparent viscosity High viscosity indicates stronger intermolecular attractions An o/w emulsion has the viscosity greater than that of the internal phase A system which undergoes gel to sol transformation is known as shear thinning Ostwalds viscometer measures kinematic viscosity When the IM injection of procaine penicillin G is given, a process of ‘depot’ formation occurs due to rapid thixotropic recovery