| What is rheology? | Rheology is the study of deformation and flow of matter, i.e. response to stress and strain. |
| What are the motivations for rheological study? | Direct impact on design of processing equipment.
Allows insight into material structure.
Used in raw material and process control.
Relevance to consumer acceptability. |
| What are the difficulties associated with rheology? | Very wide range of materials to study.
Any material will have different rheology under different conditions (temp, pressure). |
| What are the two types of non-Newtonian fluids? | Time independent.
Time dependent. |
| Give examples of time independent fluids | Shear-thinning
Shear-thickening
Viscoelastic materials - Bingham plastic & Herschel-Bulkley model |
| Give an example of a Bingham plastic fluid | Toothpaste |
| Give an example of a fluid that follows the Herschel-Bulkley Model | Yoghurt |
| What are the types of time dependent fluids? | Thixotropic
Rheopectic |
| What is a thixotropic fluid? | Shear thinning fluids where structural interactions (apparent viscosity) decrease with time at a constant shear rate. |
| Give an example of a thixotropic fluid. | Paraffin oil, gelatine, creams. |
| What is a rheopectic fluid? | Shear-thickening fluids that increase in apparent viscosity with time, at a constant shear rate. |
| Give an example of a rheopectic fluid | Coal-slurries, |
| When is the Cauchy versus Hencky form of rate of strain used? | Cauchy - for solids which remember its original shape, Lo.
Hencky - for materials which do not remember Lo. |
| What is the definition of stress? | Stress = force per unit area.
Units of N/m2, or Pa.
It may be tensile, compressive or shear.
Small element may be considered in terms of Cartesian coordinates x, y, z. |
| When are normal stresses considered positive or negative. | Positive - tensile stresses (acting outward)
Negative - compressive stresses (acting inward) |
| On any given surface of an element, how many stresses are acting? | 3 stresses acting:
2 shear stresses
1 normal stress
Therefore only 9 separate quantities are required to completely describe the state of stress in a material. |
| What is a elastic (Hookean) solid? | An elastic solid has no viscous properties and does not flow.
Follows Hooke's law of elasticity where stress-strain curve is a straight line through the origin.
Perfectly elastic: stress = constant * strain
E.g. rubber. |
| What is a Newtonian liquid? | Has no elastic properties.
Newtonian liquids obey Newton's law of viscosity where shear stress-shear rate curve is a straight line through origin.
i.e. shear stress = constant * shear rate.
E.g. water, most aqueous solutions, silicones. |
| What is the cause of viscosity in a liquid? | Cohesion is the predominant cause of viscosity in a liquid.
As cohesion decreases with temperature, therefore so does viscosity. |
| What are the advantages of rotational viscometers? | Given sample can be sheared for as long as is desired, so changes in time can be observed.
Uniform shear rate can be applied throughout sample. |
| What is the main disadvantage of rotational viscometers? | Excessive rise in temperature incurred at high shear rates. |
| What is a concentric cylinder viscometer? | In a concentric cylinder viscometer, the material is confined between vertical coaxial cylinders, one of which can be rotated at various speeds while the torque on the other is measured. |
| What is a cone and plate viscometer? | A cone and plate viscometer consists of a flat plate and a cone of radius R having a very obtuse angle.
Cone's axis is normal to plate and its apex just touches the plate surface. |
| What is a parallel plate viscometer? | A parallel plate viscometer is an instrument which consists of two parallel circular plates of radius R separated by a narrow gap of height h.
One
plate rotates at a steady angular velocity Ω, and the torque T on the other plate is measured. |
| What does the Rabinowitsch-Mooney equation show? | Gives the shear rate at the pipe wall for the laminar flow of a time-independent fluid, in which the derivative is evaluated at a particular value of τw. |
| What is the generalised Reynolds number for? | The generalised Reynolds number is used to get unique friction factor-Reynold's number curve for laminar flow. |
| What are viscoelastic materials? | Materials which possess varying degrees of viscous as well as elastic properties. |
| What parameter is λ for Maxwell liquids? | λ is known as the relaxation time or characteristic time of a Maxwell liquid.
Dimensions of time. |
| What is the Voigt model/ Kelvin model? | Viscoelastic material but more solid-like behaviour compared to Maxwell model.
Obtained by assembling a spring and dashpot in parallel.
Strain is same for both elements and stresses are additive. |
| What is the Maxwell model? | A Maxwell material is the most simple model viscoelastic material showing properties of a typical liquid.
Stress same for both elements and strains are additive. |
| What parameter is λ for Voigt model liquids (delayed elasticity)? | λ is referred to as the retardation time. |
| What is the generalised Voigt model analogous to? | The generalised Voigt model is analogous to a viscoelastic solid |
| What does it mean if one of the elements in the generalised Voigt model has a zero modulus? | System is left with a simple dashpot which does allow unlimited viscous flow.
System therefore could be considered as a viscoelastic liquid. |
| What is the creep function φ (t)of a viscoelastic material? | The creep function, φ (t) , of a viscoelastic material is defined as the strain per unit stress expressed as a function
of time when the relaxed material is suddenly subjected to a constant stress. |
| What is the creep function useful for? | The creep function is perhaps the most convenient method of specifying the properties of a viscoelastic material
which has predominantly solid properties, i.e. a material which is capable of supporting its own weight without
appreciable distortion as compared with a liquid which flows and must be confined |
| What are the typical operating modes of a dynamic testing rheometer? | Strain sweep
Frequency sweep
Isothermal time sweep |
| Describe strain sweep in a dynamic rheometer | In strain sweep, the strain amplitude is varied over a wide range. This mode is used to determine the limits
of linear viscoelastic behaviour; in the linear region, the rheological properties are strain independent. |
| Describe Frequency sweep in a dynamic rheometer | Frequency sweep is a common mode of dynamic testing because it shows how the viscous and elastic
behaviours of the material change with the rate of application of strain, i.e. the frequency of the harmonic
strain input ω. |
| Describe isothermal time sweep in a dynamic rheometer | Isothermal time sweep: in this mode the frequency and amplitude of the input strain are kept constant.
This test can reveal structural changes in materials with thixotropic characteristics. A time sweep can be
conducted in conjunction with a controlled increase in temperature. Such a test would reveal if there are
any drastic changes in rheological behaviour that are caused by heating such as in protein substances or
starch pastes. |
| What are the three dimensionless groups used in viscoelasticity problems? | Reynolds number, Re
Deborah number, De
Weissenberg or elasticity number, We |
| What does Re measure? | Re is a measure of the relative importance of inertial and viscous forces |
| What does De measure? | De is a measure of the degree of viscoelasticity of a material, i.e. whether it will act as a solid or liquid.
De<<1 corresponds to a slow deformation of the fluid.
De>>1 corresponds to fast deformation, behaviour like an elastic solid. |
| What does We measure? | Alternative to De.
Measures relaxation time, determined from steady flow. |
