Rheology of Dispersions

Tharwat F. Tadros was appointed lecturer in Physical Chemsitry (1962-1966) at Alexandria University. Between 1966 and 1969, he spent a sabbatical at the Agricultural University of Wageningen and T.N.O. in Delft, The Netherlands. Thereafter he joined I.C.I. and ZENECA until 1994, where he researched various fields of surfactants, emulsions, suspensions, microemulsions, wetting spreading & adhesion and rheology. During that period he was also appointed visiting professor at Imperial College London, Bristol University and Reading University. In 1992, he was elected President of the International Association of Colloid and Interface Science. Since leaving ZENECA, Tharwat F. Tadros has worked as a consultant for various industries and also given several courses in his specialized field. He is the recipient of two medals from the Royal Society of Chemistry in the UK, and has more than 250 scientific papers to his name.

"In summary, the book Rheology of Dispersions by Tharwat Tadros can be highly recommended for practically oriented researchers working in the field of dispersions." (Materials Views, 16 January 2012)

• Preface xiii1 General Introduction 1References 62 Interparticle Interactions and Their Combination 72.1 Hard-Sphere Interaction 72.2 “Soft” or Electrostatic Interaction 72.3 Steric Interaction 102.4 van der Waals Attractions 142.5 Combination of Interaction Forces 162.6 Flocculation of Dispersions, and Its Prevention 182.6.1 Mechanism of Flocculation 192.6.1.1 Flocculation of Electrostatically Stabilized Suspensions 192.6.1.2 Flocculation of Sterically Stabilized Dispersions 222.6.1.3 Bridging or Charge Neutralization by Polymers 232.6.2 General Rules for Reducing (Eliminating) Flocculation 232.7 Distinction between “Dilute,” “Concentrated,” and “Solid” Dispersions 242.8 States of Suspension on Standing 272.9 States of the Emulsion on Standing 292.9.1 Creaming and Sedimentation 302.9.2 Flocculation 312.9.3 Ostwald Ripening (Disproportionation) 322.9.4 Emulsion Coalescence 342.9.5 Phase Inversion 35References 363 Principles of Steady-State Measurements 373.1 Strain Rate or Shear Rate 383.2 Types of Rheological Behavior in Simple Shear 383.2.1 Models for Flow Behavior 393.2.1.1 Law of Elasticity (Hooke’s Model) 393.2.1.2 Newton’s Law of Viscosity 393.2.1.3 The Kinematic Viscosity ν 403.2.1.4 Non-Newtonian Flow 403.2.2 Rheological Models for the Analysis of Flow Curves 413.2.2.1 Newtonian Systems 413.2.2.2 Bingham Plastic Systems 413.2.2.3 Pseudoplastic (Shear Thinning) System 423.2.2.4 Dilatant (Shear Thickening) System 433.2.2.5 The Herschel–Bulkley General Model 433.2.2.6 The Casson Model 443.2.2.7 The Cross Equation 443.3 Time Effects During Flow: Thixotropy and Negative (or Anti-) Thixotropy 463.4 Rheopexy 483.5 Turbulent Flow 503.6 Effect of Temperature 523.7 Measurement of Viscosity as a Function of Shear Rate: The Steady- State Regime 533.7.1 Capillary Viscometers 543.7.2 Measurement of Intrinsic Viscosity of Polymers 553.7.3 Capillary Rheometry for Non-Newtonians 563.7.4 Rotational Viscometers 573.7.4.1 Concentric Cylinder Viscometer 573.8 Non-Newtonians 583.8.1 Shear Thinning or Pseudoplastic 583.8.2 Bingham Plastic 593.9 Major Precautions with Concentric Cylinder Viscometers 593.9.1 Shear Rate Calculations 593.9.2 Wall Slip and Sample Evaporation During Measurement 603.9.2.1 The Vane Rheometer 603.9.2.2 Cone and Plate Rheometer 613.9.2.3 Parallel Plates (Discs) 623.9.2.4 The Brookfield Viscometer 62References 644 Principles of Viscoelastic Behavior 654.1 Introduction 654.2 The Deborah Number 654.3 Strain Relaxation after the Sudden Application of Stress (Creep) 664.4 Analysis of Creep Curves 674.4.1 Viscous Fluid 674.4.2 Elastic Solid 674.4.3 Viscoelastic Response 684.4.3.1 Viscoelastic Liquid 684.4.3.2 Viscoelastic Solid 694.5 The Berger Model (Maxwell + Kelvin) 704.6 Creep Procedure 714.7 Stress Relaxation after Sudden Application of Strain 724.8 Dynamic (Oscillatory) Techniques 744.8.1 Analysis of Oscillatory Response for a Viscoelastic System 744.8.1.1 Vector Analysis of the Complex Modulus 764.8.1.2 The Cohesive Energy Density E c 784.8.1.3 The Weissenberg Effect and Normal Forces 794.8.2 Viscoelastic Measurements 794.8.2.1 Constant Stress (Creep) Measurements 804.8.2.2 Dynamic (Oscillatory) Measurements 824.8.2.3 Shear Modulus (Rigidity) Measurement 83References 845 Rheology of Suspensions 855.1 Introduction 855.2 The Einstein Equation 865.3 The Bachelor Equation 865.4 Rheology of Concentrated Suspensions 865.5 Rheology of Hard-Sphere Suspensions 875.5.1 Analysis of the Viscosity–Volume Fraction Curve 895.6 Rheology of Systems with “Soft” or Electrostatic Interaction 895.6.1 Viscoelastic Behavior of Electrostatically Stabilized Suspensions 905.6.1.1 Elastic Modulus (G′)–Distance (h) Relation 925.6.1.2 Scaling Laws for Dependence of G′ on φ 935.6.2 Control of Rheology of Electrostatically Stabilized Suspensions 945.7 Rheology of Sterically Stabilized Dispersions 945.7.1 Viscoelastic Properties of Sterically Stabilized Suspensions 955.7.2 Correlation of the Viscoelastic Properties of Sterically Stabilized Suspensions with Their Interparticle Interactions 965.7.3 The High-Frequency Modulus–Volume Fraction Results 985.8 Rheology of Flocculated Suspensions 995.8.1 Weakly Flocculated Suspensions 1005.8.2 Strongly Flocculated (Coagulated) Suspensions 1065.8.2.1 Analysis of the Flow Curve 1075.8.2.2 Fractal Concept for Flocculation 1085.8.2.3 Examples of Strongly Flocculated (Coagulated) Suspensions 1095.8.2.4 Strongly Flocculated, Sterically Stabilized Systems 1115.9 Models for the Interpretation of Rheological Results 1165.9.1 Doublet Floc Structure Model 1165.9.2 Elastic Floc Model 117References 1186 Rheology of Emulsions 1216.1 Introduction 1216.2 Interfacial Rheology 1216.2.1 Interfacial Tension and Surface Pressure 1216.2.2 Interfacial Shear Viscosity 1226.2.2.1 Measurement of Interfacial Viscosity 1226.2.3 Interfacial Dilational Elasticity 1236.2.4 Interfacial Dilational Viscosity 1246.2.5 Non-Newtonian Effects 1246.2.6 Correlation of Emulsion Stability with Interfacial Rheology 1246.2.6.1 Mixed-Surfactant Films 1246.2.6.2 Protein Films 1246.3 Bulk Rheology of Emulsions 1266.3.1 Analysis of the Rheological Behavior of Concentrated Emulsions 1286.3.1.1 Experimental ηr – φ Curves 1316.3.1.2 Influence of Droplet Deformability 1316.3.2 Viscoelastic Properties of Concentrated Emulsions 1326.3.2.1 High-Internal Phase Emulsions (HIPES) 1336.3.2.2 Deformation and Break-Up of Droplets in Emulsions During Flow 138References 1467 Rheology Modifiers, Thickeners, and Gels 1497.1 Introduction 1497.2 Classification of Thickeners and Gels 1497.3 Definition of a “Gel” 1507.4 Rheological Behavior of a “Gel” 1507.4.1 Stress Relaxation (after Sudden Application of Strain) 1507.4.2 Constant Stress (Creep) Measurements 1517.4.3 Dynamic (Oscillatory) Measurements 1527.5 Classification of Gels 1537.5.1 Polymer Gels 1547.5.1.1 Physical Gels Obtained by Chain Overlap 1547.5.1.2 Gels Produced by Associative Thickeners 1557.5.1.3 Crosslinked Gels (Chemical Gels) 1597.5.2 Particulate Gels 1607.5.2.1 Aqueous Clay Gels 1607.5.2.2 Organo-Clays (Bentones) 1617.5.2.3 Oxide Gels 1627.5.2.4 Gels Produced using Particulate Solids and High-Molecular-Weight Polymers 1637.6 Rheology Modifiers Based on Surfactant Systems 164References 1678 Use of Rheological Measurements for Assessment and Prediction of the Long-Term Physical Stability of Formulations (Creaming and Sedimentation) 1698.1 Introduction 1698.2 Sedimentation of Suspensions 1698.2.1 Accelerated Tests and Their Limitations 1718.2.2 Application of a High-Gravity (g) Force 1728.2.3 Rheological Techniques for the Prediction of Sedimentation or Creaming 1738.2.4 Separation of Formulation: Syneresis 1748.2.5 Examples of Correlation of Sedimentation or Creaming with Residual (Zero-Shear) Viscosity 1758.2.5.1 Model Suspensions of Aqueous Polystyrene Latex 1758.2.5.2 Sedimentation in Non-Newtonian Liquids 1758.2.5.3 Role of Thickeners 1768.2.6 Prediction of Emulsion Creaming 1778.2.6.1 Creep Measurements for Prediction of Creaming 1798.2.6.2 Oscillatory Measurements for Prediction of Creaming 1798.3 Assessment and Prediction of Flocculation Using Rheological Techniques 1808.3.1 Introduction 1808.3.2 Wall Slip 1808.3.3 Steady-State Shear Stress–Shear Rate Measurements 1808.3.4 Influence of Ostwald Ripening and Coalescence 1818.3.5 Constant-Stress (Creep) Experiments 1818.3.6 Dynamic (Oscillatory) Measurements 1828.3.6.1 Strain Sweep Measurements 1828.3.6.2 Oscillatory Sweep Measurements 1838.3.7 Examples of Application of Rheology for Assessment and Prediction of Flocculation 1848.3.7.1 Flocculation and Restabilization of Clays Using Cationic Surfactants 1848.3.7.2 Flocculation of Sterically Stabilized Dispersions 1858.3.7.3 Flocculation of Sterically Stabilized Emulsions 1868.4 Assessment and Prediction of Emulsion Coalescence Using Rheological Techniques 1878.4.1 Introduction 1878.4.2 Rate of Coalescence 1878.4.3 Rheological Techniques 1888.4.3.1 Viscosity Measurements 1888.4.3.2 Measurement of Yield Value as a Function of Time 1898.4.3.3 Measurement of Storage Modulus G′ as a Function of Time 1898.4.4 Correlation between Elastic Modulus and Coalescence 1908.4.5 Cohesive Energy Ec 191References 191Index 193