Thixotropic behaviour

Pectins in aqueous solutions show pseudoplastic non-thixotropic behaviour, independent of their degree of methoxylation. Figure 1 shows the viscosity curve of a 2,5 % pectin solution, sheared the preselected shear rate-time function. The viscosity curves for the increasing and decreasing shear rate are superimposed. The pseudoplasticity of pectin solutions decreases with decreasing concentration. 

Thixotropic behaviour of pectins can be observed with the drop of their degree of esterification, and with the onset of a distinct reactivity towards divalent cations. The common measuring principal is pictured in figure 3. 

The most surprising result is that such simple non-linear relaxation behaviour can give rise to such complex behaviour of the stress with time. In Figure 6.3(b) there is a peak termed a stress overshoot . This illustrates that materials following very simple rules can show very complex behaviour. The sample modelled here, it could be argued, can show both thixotropic and anti-thixotropic behaviour. One of the most frequently made non-linear viscoelastic measurements is the thixotropic loop. This involves increasing the shear rate linearly with time to a given... 

The silanol groups on the polymer particle surface play a very important role upon the particle aggregation such as the interparticle hydrogenbonding and the hydrothermal reaction thereafter. The hydrogen bond between these particles is about 5 times stronger than that of electrostatic and van der Waals forces, but weak enough to be broken by a mechanical force in such cases as the thixotropic behaviour of aerosil powder mixed in... 

Shear thinning (and shear thickening) are instantaneous, in contrast to thixotropic behaviour, which occurs when a response occurs well after a perturbation. ... 

Gels are.essentially dispersions in which the attractive interactions between the elements of the disperse phase are so strong that the whole system develops a rigid network structure and, under small stresses, behaves elastically. In some instances the gel may flow plastically above a limiting yield stress [Chapter 8, Figure 8.3(e)] the gel then often exhibits thixotropic behaviour, the rigid gel state being re-formed when the stress is removed. 

Dolz M., Hernandez M. J., Delegido J., Alfaro M. C., Munoz J. 2007. Influence of xanthan gum and locust bean gum upon flow and thixotropic behaviour of food emulsions containing modified starch. r. Food Ena.. 81,179-186. 

The term false body has been introduced to describe the thixotropic behaviour of viscoplastic materials. Although the thixotropy is associated with the build-up of structure at rest and breakdown of structure under shear, viscoplastic materials do not lose their solid-like properties completely and can still exhibit a yield stress, though this is usually less than the original value of the virgin sample which is regained (if at all) only after a long recovery period. 

Other examples of materials exhibiting thixotropic behaviour include concentrated suspensions, emulsions, protein solutions and food stuffs, etc. [Barnes, 1997). 

If a stress above the yield stress is applied, the gelled structure is broken into smaller units (floes), which can then move past each other. If floe attrition is affected by the strength of the hydrodynamic and attractive forces, pseudoplastic behaviour is observed and the viscosity decreases with shear rate. The strong shear forces at high shear cause flow units to be smaller and flow is facilitated. The destruction of floes releases constrained solvent, which results in a decrease in the effective volume-fraction of the floes. This phenomenon may create thixotropic behaviour in the system if the breakup and formation of floes is reversible. 

First it should be mentioned that thixotropy, like gel formation, is favoured by an anisodiametric shape of the colloidal particles or of the particles of coarser suspensions showing thixotropy. The systems must be capable of forming a continuous cohering structure Where examples of spherical particles, showing a thixotropic behaviour have been met with, this faculty is always confined to relatively concentrated systems. 

This aim is even better achieved if, after a temporary high shear stress, the recovery of the higher viscosity is delayed. This phenomenon is called thixotropy. This phenomenon can often be seen in daily practice. For example, highly viscous emulsions like tomato ketchup and other cooking sauces temporarily have a lower viscosity after vigorous shaking. Also cutaneous emulsions often exhibit thixotropic behaviour. 

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). 

Figure 7 Shear stress vs. shear rate for a) ink 1 (glycerol + 2 wt% PVP vehicle), and b) ink 2-4 (glycerolAvater 80/20 -t- 5 wt% PVP vehicle, propylene glycol/water + S wt% PVP vehkks). Inks 2-4 show hysteresis, which is indicative of thixotropy. Ink 1 does not exhibit thixotropic behaviour...

Molten polymers are viscoelastic materials, and so study of their behaviour can be complex. Polymers are also non-ideal in behaviour, i.e. they do not follow the Newtonian liquid relationship of simple liquids like water, where shear-stress is proportional to shear strain rate. Unlike Newtonian liquids, polymers show viscosity changes with shear rate, mainly in a pseudoplastic manner. As shear rate increases there is a reduction in melt viscosity. This is true of both heat-softened plastics and rubbers. Other time-dependent effects will also arise with polymer compounds to complicate the rheological process behaviour. These may be viscosity reductions due to molecular-mass breakdown or physical effects due to thixotropic behaviour, or viscosity increases due to crosslinking/branching reactions or degradation. Generally these effects will be studied in rotational-type rheometers and the extrusion-type capillary rheometer. 

The term thixotropy has often been confused with the pseudoplastic state which is a reduction in viscosity as the rate of shear increases. Thixotropy is a reduction in viscosity with time. Almost all thixotropic hquids are also pseudoplastic, but not all pseudoplastic liquids are thixotropic. Both pseudoplastic and thixotropic behaviour are caused by structure which involves the interactions between particles in the liquid. The strength of these interactions, and the rate at which they form, controls the rheology of the system. Viscosity measurement is not an instantaneous process and the rate of formation of interactions relative to the time of measurement determines the rheology. If interactions are formed relatively fast an equilibrium will be set up between formation (constant) and destruction (rate dependent on shear) of interactions, and the system will appear to be pseudoplastic. If interactions are formed slowly the apparent viscosity will decrease, this is because the viscometer will record a value before equilibrium is reached. In a brushing paint, the removal of brush marks requires a slow rate of forming structure. If the paint is applied to a vertical surface a fast build-up of structure is required to prevent flow of the paint down the surface, a phenomenon termed sagging . 

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