1. Non-Newtonian Fluids Flashcards
Definition of Newtonian Fluid
Newtonian fluid example water, molten metals, low molecular weight polymers. Viscosity can depend on temperature, pressure, and chemical composition of the fluid.
Definition of Non-Newtonian Fluids
Viscosity depends on flow conditions such as flow geometry, shear rate (or stress) developed within the fluid, time of shearing, kinematic history of the sample. Divided into categories:
- Substances for which the rate of shear (or viscosity) is dependent only on the current value of the shear stress, or vice versa. Includes purely viscous, time independent or generalised Newtonian fluids.
- More complex materials for which the relation between the shear stress and the shear rate also depends upon the duration of shearing, the previous kinematic history, etc. Known as time dependent fluids.
- Materials exhibiting combined characteristics of both an elastic solid and a viscous fluid and showing partial elastic and recoil recovery after deformation, the so-called viscoelastic fluids. Mu = fn(elastic + viscous components).
Pseudoplastic fluids
The viscosity of shear thinning or pseaudoplastic fluids decrease as the shear rate is increases. They are the most commonly encountered type of time-independent fluid behaviour.
For some fluids, it is thought that at low shear, particles or molecules within the fluid are more randomly dispersed or entangled, but at high shear they align in the direction of the shear, thus reducing the viscosity. Pseudoplastic fluids often contain polymers or large molecules that could behave in this manner. Examples include polymeric solutions, paints, mayonnaise, paper pulp.
Can be modelled by the power law. For pseudoplastic, n<1, thus if the shear rate doubles, the shear stress will increase by a lower ratio.
Dilatant fluids
The viscosity increases as the shear rate is increased. The power law model can also be used but n>1. Examples include high solids content solutions of starch and sand. At low shear rates, water acts as a lubricant, but at higher rates, water moves out of the solid structure and there is less lubrication so the viscosity increased -> denser particle packing.
Displacing liquids -> solids touching/interacting -> harder to flow
Bingham (plastic) fluids
Characterised by the existence of a yield stress that must be exceeded before the fluid will deform or flow. In other words, Bingham plastic fluids behave as a solid until a critical shear rate is achieved. Often have 3D structures sufficient to resist flow until the yield stress is achieved. Examples include drilling mud, toothpaste and grain.
Thixotropic fluids
Fluid thins with time. For example, when the aqueous suspensions of red mud and of bentonite, crude oils, and certain foodstuffs etc are sheared at a constant rate of shear following a period of rest, their apparent viscosity gradually decreases as their internal structure is progressively broken down. In these fluids, the structure is broken down or molecules or particles are aligned as the fluid is subjected to shear.
Rheopectic fluids
Under certain conditions, fluid movement can lead to reformation of structure which results in an increased resistance to relative motion. Thus, apparent viscosity increased with length of shearing. This behaviour is known as rheopexy. These fluids are rare. Examples are printer ink and gypsum pastes.
Viscoelastic fluids
In addition to viscous behaviour, these fluids exhibit some elastic behaviour. They generally contain elastic particles which are stretched during flow but which recoil when the stress is reduced. Viscoelastic behaviour is important in the flow of hot plastics into extrusion moulds. Other examples include jelly, liquid soap and some polymers. Usually modelled by the Kelvin Model and the Maxwell Model.
