Pseudo-plastic behaviour

Gels and emulsions show plastic and pseudo-plastic behaviour. At rest, these systems are more viscous than when they flow. Creams and ointments should be easily spreadable, but they should not drip from the skin. Emulsions and suspensions should be as stable as possible and pouring should be easy. 

Similar trends of the effect of microfibre at 0.5% vol. reinforcement were reported by Yao era/. [125] for carbon microfibre, showing a pseudo-plastic behaviour for system with 0.2% carbon microfibre (7 /nm diameter, 5 mm long) and 0.3% vol. steel (0.5 mm diameter, 30 mm long), with a relatively smooth transition in the load-deflection curve around the matrix cracking stress. Qian and Stroeven [126] demonstrated the enhanced efficiency of hybrid reinforcement of 0.4 w/c ratio concretes, showing that a system of steel macrofibre and polypropylene microfibre could be used to obtain a flexural reinforcing effect at a fibre content of 0.75% vol., similar to that obtained with a higher content (0.9% vol.) of mono-steel macrofibres. 

The non-aqueous HIPEs showed similar properties to their water-containing counterparts. Examination by optical microscopy revealed a polyhedral, poly-disperse microstructure. Rheological experiments indicated typical shear rate vs. shear stress behaviour for a pseudo-plastic material, with a yield stress in evidence. The yield value was seen to increase sharply with increasing dispersed phase volume fraction, above about 96%. Finally, addition of water to the continuous phase was studied. This caused a decrease in the rate of decay of the emulsion yield stress over a period of time, and an increase in stability. The added water increased the strength of the interfacial film, providing a more efficient barrier to coalescence. 

The effect of pseudo-plasticity was reportedly known for different plastisols, in particular, for plastisols II described in Sect. 3. The rheological behaviour of these systems with an accuracy sufficient for the purposes of engineering is given by the Bingham-Shvedov law within the extended range of shear rates (10-2 < y < 102) ... 

Boger, D.V. Demonstration of upper and lower Newtonian fluid behaviour in a pseudo-plastic fluid. Nature 1977,265, 126-128. 

If flocculation occurs slowly, the viscosity, measured at low rates of shear, increases with time during the rest period after an efficient shearing. When this happens the paint is said to be thixotropic. If there is no dependence on time or on the previous treatment of the paint and if the viscosity decreases as the rate of shear increases, then the paint is said to be pseudo-plastic. If there is a minimum stress required before any flow can occur at all, the viscosity behaviour is said to be plastic. All these types of behaviour are, of course, contrary to Newton s equation and are grouped together under the heading of non-Newtonian viscosity. 

The given presentation of the mechanism of the interaction of polymer molecules with turbulent flow admits a peculiar theoretical examination. The presence of polymer addition besides the increase of longitudinal viscosity is resulted in the appearance of such rheological solution properties as elastic plasticity, pseudo-plasticity, anisotropy. In [3] the influence of different rheological fluid characteristics on the wall turbulence is theoretically analyzed within the limits of monoharmonic approximation, which affords to take into account turbulent blows-out. Different variants of rheological behaviour were considered. For all that we succeded to show, that the decrease of turbulent friction arose only in mediums, possessing... 

Two types of flow behaviour can be distinguished, namely Newtonian and non-Newtonian flow. Non-Newtonian flow is further divided into plastic, pseudo-plastic and dilatant flow. 

Viscosity enhancing substances not only increase the viscosity but also introduce rheological qualities such as (pseudo) plastic, dUatant and thixotropic behaviour (see Sect. 18.2.1). 

The basic principles of rheology and the various experimental methods that can be applied to investigate these complex systems of food colloids have been discussed in detail in Chapter 7. Only a brief summary is given here. Two main types of measurements are required (1) Steady-state measurements of the shear stress versus shear rate relationship, to distinguish between the various responses Newtonian, plastic, pseudo-plastic and dilatant. Particular attention should be given to time effects during flow (thixotropy and negative thixotropy). (2) Viscoelastic behaviour, stress relaxation, constant stress (creep) and oscillatory measurements. 

Betsed on laboratory test loop measurements the kaolin and fluidic ash slurries can be determined as time independent non-Newtonian yield pseudo-plastic suspensions. On the contrary a considerable effect of shearing during initial period of flow in pipe was found out for fly ash-gypsum mixture, see Fig. 2. However, after a relative short time about one hour, the decrease of pressure losses finished and slurry became stabilised. This behaviour is probably due to physico-chemical activity of ash and gypsum after mixing with water. The Fig. 3 shows plot of wall shear stress t, versus pseudo-shear rate (8 V/D) for the kaolin slurry flow in three different pipes and for representative ash-water mixtures in pipe of diameter D = 17.5 mm. The transition between laminar and turbulent flow is shown by the steep change in slope of the flow curves. 

A theoretical idealisation of pseudo-plastic with yield value behaviour is the Bingham plastic (see curve 3 of Fig. 6.3), which deforms elastically until the yield stress Ty is reached and then flows like a Newtonian liquid with 7 linearly related to t. When r is greater than Ty ... 

As a consequence of the non-spherical symmetry of the copper(II) ion, d9 configuration, and of the influence of the Jahn-Teller, and pseudo Jahn-Teller effect on six-coordinate geometries, the stereochemistries of the copper(II) ion are characterized by non-rigid geometries (fluxional behaviour), and ranges of distorted geometries (Plasticity Effect). The latter may be connected by a series of Structural Pathways, which may be characterised by an Electronic Criterion of Stereochemistry for a related series of complexes, e.g. the [Cu(bipy)2X] [Y] complexes. 

It is possible to think of yield and plastic deformation in polymers as a type of viscous flow, especially since glassy polymers.are basically frozen liquids that have failed to crystallize. Eyring developed a theory to describe viscous flow in liquids and it can be readily adapted to describe the behaviour of glassy polymers. The segments of the polymer chain can be thought of as being in a pseudo-lattice and for flow to occur a segment must move to an adjacent site. There will be a potential barrier to overcome,... 

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