Rheology PDF

Rheology (The science of Flow) BP 351 Dosage Form Design I Dr. Ashok Kumar Janakiraman Learning Outcomes At the end of this topic, students are expected to know: : ◼ The classification of liquid into different types of flow. ◼ The characteristics of Newtonian and Non-Newtonian liqu...

Rheology (The science of Flow) BP 351 Dosage Form Design I Dr. Ashok Kumar Janakiraman Learning Outcomes At the end of this topic, students are expected to know: : ◼ The classification of liquid into different types of flow. ◼ The characteristics of Newtonian and Non-Newtonian liquids. ◼ The reasons a liquid shows certain type of flow. ◼ The advantages and disadvantages of a flow type compared to another. ◼ The methods to characterize types of flow & the calculations involved ◼ The interpretations of rheograms ◼ Thixotropy phenomena and their importance ◼ Factors affecting the rheological properties of dispersed systems. ◼ Ways to modify & improve a problematic dispersed formulation. 2  Classification of fluids based on rheological behavior Fluids Non- Newtonian Newtonian Time Time dependent independent Anti- Negative Plastic Pseudoplastics Dilatant Thixotropic Thixotropic Rheopexy Rheopexy 3  Rheology ▪ Rheology describes the structural behavior & physical properties of materials. ▪ Rheology is the science of flow & deformation of matter under stress. ▪ Viscosity: ▪ The index (magnitude) of resistance of a liquid to flow. ▪ Viscosity () is the resistance of a material to flow under stress. 4 Concept of Viscosity ✓ The higher the viscosity of a liquid, the greater is the resistance to flow. ✓ Eg; ✓ Groundnut oil, honey, syrup, all resist the flow more in comparison to water or alcohol. ✓ Syrup has larger internal friction; it pours very slowly. ✓ On molecular level: ✓ Motion is transferred between molecules of a syrup at lower rate than for molecules of water. 5  Consider a ‘block of liquid’ consisting of parallel layers of molecules, similar to a deck of cards: A A B B C Shear stress C Applied on ‘A’ D D E E _____ _____ _____ _____ _____ _____ _____ _____ N N 6 ▪ When pressure is applied horizontally on the top layer ‘A’, the liquid begins to flow. ▪ In above figure arrows indicate the magnitude of flow velocity. ▪ Since force is applied on the top layer (A), it moves at greater velocity. ▪ While moving ahead, the first layer (A) induces flow in the second layer (B). ▪ The velocity of second layer (B) is somewhat less than that of the first layer (A), because of the viscous drag offered by the third layer (C). ▪ Similarly, the velocity of 3rd layer (C) is less than that of 2nd layer (B), but higher than that of 4th layer (D). ▪ This phenomena continue & bottom layer (N) remains stationary. ▪ Thus, liquids resist flow when force is applied. ▪ This resistance is estimated & expressed as viscosity 7  Mathematical Treatment – Viscosity ▪ Shear Stress is defined as the force per unit area, F’/A, which is applied to bring about to flow. ▪ Shear Stress, F = F’/A ▪ Velocity Gradient or Rate of Shear: dv/dr ▪ As the change in the velocity, ‘dv’, with an infinitesimal change in distance, ‘dr’. ▪ Rate of shear, G = dv/dr 8 Consider a ‘block of liquid’ consisting of parallel layers of molecules, similar to a deck of cards: Velocity= v Sheared (F’) dv dr B Velocity = 0 9 ▪ The higher the viscosity of the liquid, the greater is the force per unit area required to produce a certain rate of shear. ▪ Hence, the relationship between shear stress and rate shear is as follows: ▪ Shear stress  Rate of shear F' dv  A dr ‘η’ is the coefficient of F' dv viscosity, and usually or = A dr referred as viscosity. or F = G F  = G 10 ▪ Coefficient of viscosity ▪ The force per unit area required to maintain unit difference in velocity between two parallel layers in the liquid, one centimeter apart. ▪ Units: Poise ▪ Poise is shearing force (stress) required to produce a velocity of 1 cm/sec between two parallel planes of a liquid each 1 cm2 in area and separated by a distance of 1 cm ▪ Centipoise (cp) = 0.01 poise ▪ 1 poise = dynes. sec/ cm2 = gm/cm/sec = 0.1 Nsm–2 (dynamic viscosity –SI) 11  Kinematic Viscosity As viscosity (η) divided by the density (ρ) of the liquid. Official USP, BP, NF,… Kinematic Viscosity (v) = η/ρ Units: stoke (s) & centistoke (cs) Fluidity,  = reciprocal of viscosity (coefficient) = 1/η Relative Viscosity, ηr (suspension, emulsion) The coefficient, ηr, is the ratio of viscosity of the dispersion (η) to that of the solvent, ηo, (vehicle). It is mathematically expressed as: Relative viscosity = ηr = η/ηo 12  Specific Viscosity, ηsp (solution, emulsion, suspension) The relative increase in the viscosity of the dispersion over that of solvent (vehicle ) alone. Specific Viscosity = ηsp = η – ηo/ηo Reduced Viscosity (suspension) The ratio of specific viscosity to the concentration (c). Reduced viscosity = ηsp/c 13 Intrinsic Viscosity (Staudinger Index) The reduced viscosity is determined various concentrations of a substance and the results are plotted. 0.7 The resulting line can be extrapolated 0.6 to c = 0 to obtain the intercept. 0.5 Reduced Viscoity The intercept value is known as 0.4 intrinsic viscosity 0.3 0.2 Use: 0.1 To determine the MW of polymers. 0 0 50 100 150 Concentration 14  To enhance viscosity of systems Polymers or hydrocolloids: Suspending agents – MC, bentonite, sodium CMC Bodying agents Glucose, fructose and sucrose Honey, Simple syrup (66.66% w/w) MC of 2% w/w in water shows apparent viscosity of 80 poise. 15  Mechanism for enhanced Viscosity The lower sugars, - glucose, sucrose contain many alcohol functional groups. React with water molecules through hydrogen bonding. The higher numbers such as starch & other cellulose polymers also react with water. Eg: MC has 3 to 4 ether groups & 1 to 2 hydroxyl groups When water is added to it, the groups get easily hydrated in solution. When polymer molecules moves, the hydrated solvent sheath also moves. As a result, the size of polymer unit increases & hence, increases the resistance to flow. Polystyrene in benzene -Vander Waal 16 Factor influencing the Viscosity 1. Intrinsic Factors Chemical nature, Molecular size, Shape, Intermolecular forces Molecule related The heavier the molecular weight - ↑ the viscosity Large & irregularly shaped molecules - ↑viscous ↑ intermolecular forces – ↑ viscosity Molecules with spherical shape are expected to slide past one another, and thus have low viscosity. 17 2. Extrinsic Factors Pressure, Temperature & Added substances Pressure: An increase in pressure enhances the cohesive of interaction, leading to an increase in the viscosity. Small quantities of nonelectrolytes like sucrose, glycerin when added to the water, the solution exhibits increased viscosity. Polymers & macromolecules also enhance the viscosity of solvents (water) Small quantity of electrolytes (e.g. NaCl) – ↓ viscosity Alkali metals, ammonium ions (results?) 18  Temperature As the temperature increases, the system acquires thermal energy which facilitates the breaking of the cohesive forces. The viscosity of liquids decreases. Gases – increase temperature increase the viscosity. Increased molecular collisions & interactions. (Arrhenius equation) Relationship η=A eE /RT v A, related to MW & molar volume Ev is an energy of activation, to initiate the flow between the molecules. Defined as the energy required to remove a molecule from the liquid. 19  Newtonian Flow Liquids that obey Newton’s law of flow are called as Newtonian fluids. F = G Shear stress – shear rate relationship is normally represented in the form of a curve namely rheogram or consistency curve. when data are plotted by taking F on X-axis & G on Y-axis, a flow curve (RHEOGRAM) is obtained. The rheogram passes through the origin & slope gives the reciprocal of coefficient of viscosity (1/). 20  Systems that follow linear relationship are called Newtonian fluids This class includes liquids such as: Water Solutions of syrups Glycerin Very dilute colloids G Chloroform Slope =1/ coefficient of viscosity F 21  Non – Newtonian fluids Rheological properties of heterogeneous dispersions such as: Emulsions, Suspensions and Semisolids are more complex and do not obey Newton’s equation of flow Based on the pattern of consistency curves, non – Newtonian fluids are categorized as: Plastic flow Pseudoplastic flow Dilatant flow 22  Plastic flow The consistency curve for plastic flow does not pass through the origin. The substance initially behaves like an elastic body and fails to flow when less amount of stress is applied. Further increase in shear stress leads to a nonlinear increase in the shear rate which progressively gets increased. The linear portion when extrapolated intersects the x – axis at a point called yield value. 23  Therefore, plastic flow resembles Newtonian flow above the yield value. Plastic flow can be adequately expressed in terms of yield value and plastic viscosity. Plastic flow is associated with the presence of: Flocculated particles in concentrated suspensions, Butter, Certain ointments, Pastes and Gels. 24  Mechanism: Floccules are the aggregation of particles with inter- particle contacts The structure is maintained when system is at rest. Yield value represents the stress required to break the inter–particle contacts so that particles behave individually. Therefore, YIELD VALUE is indicative of the forces of flocculation. Frictional forces between moving particles also contribute to the yield value. 25  Once the Yield Value exceeds, further increase in shearing stress (F – f) will bring about a proportional increase in the rate of shear. Materials that exhibit Plastic Flow are often called as Bingham Bodies. The quantitative behavior of is expressed in terms of Bingham Equation. The slope of the rheogram – mobility & Its reciprocal – Plastic Viscosity, U U = (F – f)/G F = shear stress f = yield value G = rate of shear Yield Value, f is the intercept on the shear stress axis units: dy/cm2 26 ◼ Conclusions: 1. In plastic flow, flow begins only when a certain shear stress, known as the yield value, is exceeded. 2. At shear stress less than the yield value, elastic deformation occurs. 3. At shear stress greater than the yield value, Newtonian behavior is exhibited. 4. Plastic flow is associated with flocculated particles in concentrated suspensions. 5. Higher yield values indicate a higher degree of flocculation. 6. Systems exhibiting plastic flow may be characterized by using the yield value and the plastic viscosity. 7. Materials exhibiting plastic flow are known as Bingham bodies. 27  Pseudoplastic Flow The consistency curve for a pseudoplastic flow begins at the origin Nearly zero at lower shear stress conditions. Rate of shear, G Shear stress, F Pseudoplastic Flow 28  As the shear stress increases progressively, shear rate also increases, but the trend is nonlinear. Therefore, the viscosity of pseudoplastic system cannot be expressed in by a single value. Eg: Tragacanth in water Sodium alginate in water MC in water Sodium CMC in water 29  Mechanism Under normal storage conditions, the long chain molecules of the polymers are randomly arranged in the dispersion. On applying a shear stress, these molecules begin to arrange their long axes in the direction of force applied. This stress induced orientation reduces the internal resistance of the material. In addition, the solvent molecules will also be released. Thus, the effective size of the molecules is lowered. Now, the material allows greater shear rate on progressive increase in the shearing stress. 30  Pseudo plastic flow behaviour: Structural reasons Liquid not sheared Liquid sheared Orientation Extension Deformation Destruction of Aggregates 31  FN = η' G N = number given to the exponent η' = viscosity coefficient For pseudoplastic fluids, N is the higher than 1 (one) & rises as the flow becomes increasingly non-Newtonian. When N =1, above equation becomes: η = F/G ----------- Newtonian flow The greater the value of ‘N’ above unity, the greater the pseudoplastic behavior of the material. Apply log: N log F = log η' + log G rearrange log G = N log F – log η' (Straight line – some fluids do not obey this approach, though they exhibit pseudoplastic flow) 32  Dilatant Flow The system exhibits enhanced to flow with increasing rate of shear Rate of shear, G Shear stress, F Dilatant Flow 33  ❖ When sheared, these systems increase their volume & hence are called as dilatant. ❖ Dilatant materials are also often termed as shear thickening systems ❖ Because of increased apparent viscosity at higher rates of shear. ❖ When the stress is removed, the system returns to its initial state of fluidity. Dilatant flow is exhibited by: Suspensions containing high concentrations of solids (>50%) small, deflocculated particles Suspension of starch in water Inorganic pigments in water Kaolin 12% in water Zinc oxide 30% in water 34  Mechanism The dilatant system is at rest, the molecules are closely packed. A minimum void volume (inter–particle spaces) is available & the amount of vehicle is sufficient to fill void volume This situation allows the particles to move relative to one another. Therefore, the system at rest exhibits relatively low consistency. Thus, one may pour a dilatant suspension from a bottle. 35 ◼ At rest, particles are closely packed with minimum void space filled with fluid. ◼ At increasing shear stress, the particles need to take an open packing form to move past each other, resulting in expansion or dilation of the system. ◼ The volume of fluid then becomes insufficient to fill the increased void space. ◼ Increased resistance to flow results because the particles are no longer completely wetted or lubricated by the fluid. 36  Finally, system will show a paste –like consistency. The sediment in the deflocculated suspension is dilatant and resists any attempt of stirring or shaking. This effect is known as caking or claying of suspensions This behavior should be avoided. The dilatant liquids follows FN = η' G N, is less than 1 and decreases as the degree of dilatancy increases. 37 Conclusions: ◼ In dilatant flow, increasing shear stress produces an increase in viscosity. ◼ Dilatant systems increase in volume under applied shear force. ◼ Dilatant systems are also known as “shear-thickening” systems. ◼ When the shear stress is removed, the system returns to its original state of fluidity. ◼ Dilatant systems generally contain a high solids concentration (>50 percent) of small-deflocculated particles. 38  THIXOTROPY Shear Thinning System: When agitated and kept aside, are expected to return to its original state of fluidity But it takes longer time to recover compared to the time taken for agitation. This behavior is called THIXOTROPY. Definition: It is an isothermal and comparatively slow recovery on standing of a material, and of a consistency lost through shearing. 39  The rate of shear is progressively increased, and the corresponding stress is measured using a stable instrument. When these readings are plotted as: b Pseudoplastic system B Plastic Rate of shear, G system c a Shear stress, F Thixotropic behavior exhibited by plastic & pseudoplastic systems 40  The phenomenon of thixotropy is explained in terms of particle – particle interactions. AT REST Multi-point contacts ___ Gel state (on (high consistency storage) Or High viscosity) Rapid process ON SHEAR Contacts breakdown __ Sol state (equilibrium ) (low consistency or low viscosity) Not instantaneous SET ASIDE Particle contacts are (Removal of established due to ___ stress) Brownian motion Gel state (high consistency or high viscous) 41  At rest particles in the dispersion impart rigidity on the system through multipoint contacts. The system behaves like a gel As shear is applied, the contacts begin to break down, the particles are aligned, and the flow starts. The material undergoes a gel–to–sol transformation inducing the system to exhibit shear thinning. Upon removal of stress, the system starts regaining its original state. This process is not instantaneous. Particles slowly come into contact with one another owing to random Brownian movement and progressively the original consistency will be restored. 42  If the system is viscous or consists of large, heavy particles, the Brownian motion is too slow to reestablish the broken links. More or less, extensive period of the rest is required to rebuild the original structure and reach the initial viscosity These events are termed as gel–sol–gel transformations. The rheograms of thixotropic material depends The rate at which shear is increased or decreased The length of time during which a sample is subjected to any one rate of shear. 43  Bulges Concentrated aqueous magma (gel) of bentonite (10 to 15% by weight) produces hysteresis loop with a characteristic bulge in the up-curve. Mechanism The crystalline plates of bentonite form a ‘house of cards like structure’ that causes the swelling of magmas. This 3-D structure results in a bulged loop. 44  Spurs Procaine penicillin gel produces a rheogram wherein the bulged curve may actually develop into a spur like position. The structural breakdown is indicated by high spur value, Y in up- curve. The spur value represents a sharp point of structural breakdown at a low shear rate in the up-curve. It is possible that one may miss the spur value, if care is not taken to record the shear stress at low shear rates and allow the sample to age undisturbed above shear rate. Significance: Procaine penicillin gels exhibiting a definite spur value, Y, produce prolonged blood levels of drug, because they form intramuscular depots on injection. 45  Spurs Rate of shear, G Spur value, Y Shear stress, F Rheogram of thixotropic material showing spur value in the hysteresis loop (procaine penicillin G) 46  Negative Thixotropy At rest magnesia magma shows sol –like properties. On shaking, the system behaves like a gel and imparts greater suspendability. However, at equilibrium, it is readily pourable (SOL). Anti-thixotropy or Negative thixotropy represents an increase in consistency on the down curve. Eg: Magnesia magma (milk of magnesia) exhibits an enhanced resistance to flow with increased time of shear compared to resting state. 47  Rheogram, the down-curve shifts to the right of the up-curve. A B C D Rate of shear, G Shear stress, F Rheogram of antithixotropic behavior (Magnesia magma) 48  When magnesia magma is sheared alternatively with increasing & then decreasing rates of shear, the magma thickens. As these cycles continue, the extent of increase in the thickening reduces gradually and finally reaches equilibrium state. There will be no change in the consistency curves on further cycles of shear rate. 49  Mechanism In the resting state, the system consists of a large number of individual particles & small floccules. When product is sheared, the polymer (large) molecular collisions are increased at a greater frequency. As a result, interparticle bonding increases. At equilibrium, large floccules are available in small numbers. However, the system exhibits sol form at equilibrium. When the product is allowed to rest, the large floccules breakup & gradually return to the original state of small floccules & individual particles. 50 Differentiation between NEGATIVE THIXOTROPY & DILATANCY. Dilatant systems are deflocculated, and the volume of solids is high (more than 50%); Whereas negative thixotropy is seen in a flocculated system containing low solid content (1 to 10%) 51  Particle – particle interactions in an anti-thixotropic material AT REST Individual particles are in large in number (on storage) Small flocs (low viscosity) ON SHEAR Particle collisions particle Sol state contacts are more; (equilibrium (Suspendability state ) Large flocs in small number (high consistency) & pourability) SET ASIDE Flocs contacts break (Removal of individual particles stress) (low consistency) 52  Rheopexy Rheopexy is a phenomenon in which a solid transforms to a gel state more readily when gently shaken. Activity 1 Negative Thixotropy and Rheopexy are similar phenomena. True or False? Justify 53  Measurement of Thixotropy 1 In thixotropic system, the hysteresis loop is formed by the up and down-curves of the rheogram The area of hysteresis loop has been proposed as a measure of thixotropic breakdown It can be obtained readily with the help of a planimeter. 54 2. In a thixotropic system, the nature of rheogram largely depends on the rate at which shear is increased or decreased. t2 t1 d b c Rate of shear, G 1/U2 1/U1 e B = (U1 – U2)/ln(t2/t1) a Shear stress, F Plastic system exhibiting thixotropic behavior, when the material is subjected to a constant rate of shear for t1 & t2 seconds 55  Consider a material that follows plastic flow. Suppose shear rate is increased at a constant rate on the system up to a point ‘b’ and then decreased. When the results are plotted, ‘abe’, rheogram is obtained. If the shear rate is maintained at ‘b’ for time t1 seconds and then decreased, ‘abce’ rheogram is obtained. Similarly at point ‘b’ if the shear rate is maintained for time t2 seconds & then decreased, ‘abde’ curve is obtained. The structural breakdown with respect to time at constant rate of shear gives the rheogram. 56  Based on such rheograms, the thixotropic coefficient, B, is calculated as: B = (U1 – U2)/ln(t2/t1) U1 & U2 are plastic viscosities of the two down curves. Thixotropic coefficient, B represents the rate of breakdown with time at constant shear rate. For an accurate measurement, rheograms should be obtained at several rate of shear. 57 3. The system is subjected to different rates of shear (say, v1 and v2) and the rheogram is obtained, which shows two hysteresis loops. The thixotropic coefficient, ‘M’ (dynes.sec/cm3)is calculated with: M = 2(U1 – U2)/ln(2/1) U1 and U2 are plastic viscosities of the down-curves having shearing rates of 1 and 2, respectively. Thixotropic coefficient, ‘M’ represents the loss in shearing stress per unit increase in shear rate. 58  The limitations of this approach are that the value of M shows considerable variation & it depends on the proper selection of the rates of shear.  2 1/U2 Rate of shear, G  1 1/U1 Shear stress, F Plastic system exhibiting thixotropic behavior, when the material is subjected to increasing shear rates 59  Applications of Thixotropy: Thixotropy is a desirable property particularly in emulsions, suspensions and creams. The greater the thixotropy, the higher is the physical stability of suspension. During the storage, a suspension should have high consistency (gel) in the container, so that the suspended particles do not settle rapidly. On moderate shaking, the suspension should become fluid (sol), so that the contents can be poured easily from the container Thus, the principles of thixotropy are useful in dispensing and administration of a dose. At rest, the suspension regains its original consistency. This gel–sol–gel transformations improve the physical stability of dosage forms. 60  The degree of thixotropy is related to the specific surface of penicillin used. Parenteral suspension containing 40 to 70% of procaine penicillin G in water has inherent thixotropy. While injecting the preparation, the structure of the suspended particles break down so that the product can pass through the hypodermic needle After injection, the original structure of gel will be rebuilt. This leads to depot of the procaine penicillin G at the site of injection in the muscle, from which it is slowly released so as to provide sustained levels of drug in the body. 61  Apparatus for Studying Rheological Properties Determination of flow properties Selection of Viscometer: Based on type of flow Newtonian: Rate of shear is proportional to shearing stress Non-Newtonian: variable shearing stress Other Rheological Terms: Tackiness, Stickiness, Body, Slip and Spreadability (difficult) Viscosity, Yield value, Thixotropy (can be determined) For Newtonian flow, the equipment works at a single rate of shear, is sufficient. Single point Viscometer – used Capillary Viscometer Ostwald Viscometer or U – shaped Viscometer 62 Falling Sphere Viscometer  Measurement of Viscosity: Capillary Viscometer (a) Ostwald, (b) Cannon-Fenske (c) Ubbelohde 63 η1 = t11/t2ρ2. η2 64  Poiseulle’s Equation hg r4 t = 8 LV h is the average height of the hydrostatic column g is the gravitatio nal constant  is the solution density r is the capillary radius t is the time it takes the sample to flow between A and B L is the length of the capillary V is the sample volume  t  r= =  ; If ρ  ρo , o to  o t r 65 to  The viscosity of liquid may be expressed as: η = K’t1ρ1 K is constant of the viscometer Viscosities of the unknown (η1), and standard (η2) liquids can be expressed as: η1 = K’t1ρ1 η2 = K’t2ρ2 η1 = t11/t2ρ2. η2 66 Hoeppler Falling Sphere Viscometer  = r (Sb-Sf )B 67 Cone and Plate Viscometer Constant shear rate throughout Useful for low and high viscosity materials Shear rate ranging from 10-2-10 s-1 3M = 2 R 3  =  M = torque,  = rotational speed,  = angle Cup-Bob Viscometer 68  Identify the name of the instrument 69  Viscoelasticity Viscoelastic measurements are based on the mechanical properties of materials that exhibit both viscous properties of liquids and elastic properties of solids Eg: creams, lotions, ointments, suppositories, suspensions & colloidal dispersing, emulsifying & suspending agents. Biological materials: Blood, sputum & cervical fluid also show viscoelastic properties Rheology of Dispersed Systems: Many factors Explain why a product formulated shows certain type of flow. Use the knowledge to improve a formulation which possesses poor rheological properties. 70 Factors affecting the Rheology of Emulsions & Creams: Type of flow, fluidity, viscosity & yield value, etc. % or fraction of volume of the dispersed phase. Viscosity of the external or continuous phase. Apparent fraction of volume of internal or dispersed phase. Viscosity & solidity of internal or dispersed phase. Size of internal or disperse phase droplets. Effect of surfactants (emulsifiers). Effect of other additives. 71  Percentage (%) of fraction of volume (Ø) of the dispersed phase affects Distance among globules: the higher the Ø the closer the distance Density of globules: the higher the Ø the denser the globules Interaction among globules: the higher the Ø the more the interaction. Touching among globules: the higher the Ø the more the contacts by touching among the globules. 72  If Ø Pseudoplastic,  Strong interaction> Plastic,  If Ø>0.5 Globules are very close together, they are dense, there are touching & interaction among globules, they may flocculate. If globules are quite loose> Plastic,  with low yield value (F1) 74  If globules are dense > Plastic,  with high yield value (F1) Relationship between  &  for o/w emulsion While  than the  of external phase. Flow usually become Casson type of flow. 78  (B) If internal phase is liquid with high viscosity Globules are not easily flatter or deformed For pseudoplastic,  can be reduced while sheared For plastic flow,  can be reduced while sheared but the reduced  is still >> than the  of external phase. Casson or Bingham type of flow. (C) If internal phase is solid Globules do not flatter or deform Resistance to flow is greater then (A) or (B)  can be plastic or pseudoplastic Bingham type of flow 79 Size of internal phase or dispersed phase For the % of internal phase, homogenization (milling) can result in different size of internal phase The smaller size of globules will: Increase the number of globules Lower the distance among the globules Increase interaction among globules Increase viscosity Change type of flow from Newtonian flow → Non-Newtonian flow, Pseudoplastic flow → Plastic Plastic with low F1 → Plastic with high F1 80 Effect of Surfactants (Emulsifiers) Effect of surfactant concentration There is adsorption between surfactant & globule The surfactant is soluble in water phase. Increasing the concentration of surfactant will: Increase adsorption →  apparent fraction of volume () →   of emulsion  number of micelles  interaction →  flocculation Change type of flow from Newtonian flow → Pseudoplastic, Pseudoplastic → Plastic, Plastic with low F1 → Plastic with high F1 81  Effect of types of surfactants used Two types: ionic & nonionic Nonionic: There is no charge on the surface of globule.  of emulsion will increase because of the  of  only (cause by the adsorption of surfactants to the globules) Ionic: There are charges on the surface of globule,  of emulsion will increase because of the  of  (cause by the adsorption of surfactants to the globules) Repulsion of globules having the same charges → electro-viscous effect. 82  If the emulsion having the same fraction of volume, will have different . Emulsion having ionic surfactant will have greater  compared to emulsion having nonionic surfactant (Only apply to O/W emulsion type) Effect of HLB value Solubility of surfactant is related to its HLB value HLB < 8 → surfactant is more dissolved in oil (w/o) HLB > 8 → surfactant is more dissolved in water (o/w) 83  For w/o emulsion: If the surfactant has HLB 8 → most of the surfactant will be in water phase (dispersion medium) → the affect on the type of flow & viscosity is not significant or very little Effect of Physical states of surfactants (fluid or solid)  of liquid using solid surfactant >  of liquid using liquid surfactant 84   of emulsion using solid surfactant >  of emulsion using liquid surfactant Solidity of solid globules > solidity of liquid globules having solid surfactant is not easy to be deformed (or flattened) compared to globules having liquid surfactant For liquid surfactant → usually Casson flow For solid surfactant → usually Bingham flow 85  Effect of the structure of the part of surfactant molecules protruding into the external phase Effect of the structure of the part of surfactant molecules protruding into the external phase can have interaction with external phase & may affect  & type of flow For o/w emulsion, the effect of surfactant having long or branched chain on , interaction & yield value be greater compared to short chained surfactant The longer or more branched chain →interaction → resistance to flow →  & F1 The shorter or less branched the chain →  interaction →  resistance to flow →  & F1 or no yield value For w/o emulsion, the hydrophobic chain or ring will play the role. 86 Effect of other additives The effect of additives depend on the properties of the additives. Eg: flavouring agent: Liquid flavouring agent →   Viscous flavouring agent →  Thickener →  Emulsifying agent → ; →  interaction; → affect flow type Liquid with low viscosity →  Flocculating agent →  interaction, affect flow type Newtonian → Pseudoplastic Pseudoplastic → Plastic Low yield value → High yield value Deflocculating agent, etc Pseudoplastic → Newtonian Plastic → Pseudoplastic 87  Factors affecting the rheological properties of Suspension % or  of suspended particles < =2% of suspended solids → Newtonian flow → Particles are far apart  → particles are closer, there will be change of flow type either changing to pseudoplastic or plastic flow Diameter of particles For the same % of dispersed phase, homogenization (milling) can result in different diameter of dispersed phase. Smaller diameter of particles will: Increase the number of particles Lower the distance among particles Increase interaction among particles Increase viscosity. 88  Uniformity of particle size The size distribution of particles diameter can affect viscosity Comparing two formulations of suspension: The average diameter sizes of both formula may be the same, but their respective size distribution of diameter size may be different The suspension having more uniform particle size (or small size distribution) has bigger , hence more viscous. Shape of the particles Shapes of the particles can be spherical or non-spherical. If the weight of particles is the same, the fraction of volume () of spherical particles is less than the fraction of volume () of non-spherical particles (especially while particles flow) The apparent viscosity () of suspension having spherical particles is less than the apparent viscosity of non-spherical particles 89  Surface charge of particles If there are charges on the surface → repulsion, especially if  is high & particles are close together →  (effect of electroviscous) & particles will be deflocculated The high  & repulsion among the particle will slow sedimentation & flocculation (=Stable for short- or medium-term storage) After long storage time: particles can overcome the resistance created by the repulsion of same charges Particles are close & caking formed. Charges on the surface of agglomerated/closely packed particles → difficulty in resuspending. If there are no changes → it may flocculate before sedimentation → ease in resuspending (loose- flocculate) 90  Effect of flocculating agent Flocculating agent →  interaction, affect flow type Newtonian → Pseudoplastic Pseudoplastic → Plastic Low yield value → High yield value Effect of deflocculating agent Deflocculating agent →  interaction, affect flow type: Pseudoplastic → Newtonian Plastic → Pseudoplastic High yield value → Low yield value 91  Effect other additives  Suspending agent, viscous syrup, viscous suspending liquid →   Nonviscous colouring agent, flavouring agent & etc, →   Material that can neutralize particles charges →   Polymeric suspending agent usually from network & will change flow type from Newtonian → Pseudoplastic → Plastic  Thixotropic agent can change flow from non-thixotropy to thixotropy. 92  Factors Affecting the Rheological Properties of GEL Gel is semisolid, dispersed system which does follow plastic flow It is formed when: Concentration of polymer > Critical gel concentration. If there is enough polymer, network will be formed by interaction among chains of polymer resisting flow. Temperature