Dispersed systems, viscosity measurements

The rheological properties of the dispersion systems were assessed using relative viscosity measurements, where the relative viscosity is defined as the viscosity of the suspension divided by the viscosity of the solvent-dispersant system. The measurements of shear stress vs shear rate were performed on dispersions with 1 volX solids, which were mechanically agitated for 72 hr at -20°C. Dispersions referred to as water added followed the procedure outlined above, except that 0.15 volX water was added to unexposed slurries after 72 hours of agitation in a glove box. 

At least, in absolute majority of cases, where the concentration dependence of viscosity is discussed, the case at hand is a shear flow. At the same time, it is by no means obvious (to be more exact the reverse is valid) that the values of the viscosity of dispersions determined during shear, will correlate with the values of the viscosity measured at other types of stressed state, for example at extension. Then a concept on the viscosity of suspensions (except ultimately diluted) loses its unambiguousness, and correspondingly the coefficients cn cease to be characteristics of the system, because they become dependent on the type of flow. 

Stopped flow mixing of organic and aqueous phases is an excellent way to produce dispersion within a few milliseconds. The specific interfacial area of the dispersion can become as high as 700 cm and the interfacial reaction in the dispersed system can be measured by a photodiode array spectrophotometer. A drawback of this method is the limitation of a measurable time, although it depends on the viscosity. After 200 ms, the dispersion system starts to separate, even in a rather viscous solvent like a dodecane. Therefore, rather fast interfacial reactions such as diffusion-rate-limiting reactions are preferable systems to be measured. 

Table 1.6 also lists the radius of gyration. This is an average dimension often used in colloid science to characterize the spatial extension of a particle. We shall see that this quantity can be measured for polydisperse systems by viscosity (Chapter 4) and light scattering (Chapter 5). It is therefore an experimental quantity that quantifies the dimensions of a disperse system and deserves to be included in Table 1.6. Since the typical student of chemistry has probably not heard much about the radius of gyration since general physics, a short review seems in order.

Until now, we have been primarily concerned with the definition and measurement of viscosity without regard to the nature of the system under consideration. Next we turn our attention to systems containing dispersed particles with dimensions in the colloidal size range. Viscosity measurements can be used to characterize both lyophobic and lyophilic systems we discuss both in the order cited. 

It is well known that filler-filled systems have very complex rheological properties and structures which vary in different flow fields and show special phenomena accompanied by structural changes. The processability of these dispersed systems is determined not only by their viscosities but also by their elasticities. In order to clarify the nature and the reformation process of their internal structure, dynamic viscoelasticity measurements were carried out extensively [6-8,47-51,53]. For systems filled with solid particles, the viscoelastic properties especially the dynamic ones as well as the steady flow properties have been studied extensively, and it has been found that the rheological properties of the dispersed systems differ from those of polymer solutions and melts in several ways. 

The flow properties of a colloidal system are very much dependent on its microstructure, as determined by the molecular arrangement and interaction of its components. ME systems show flow typical of a Newtonian liquids, for which the shear stress is directly proportional to the shear rate. Since viscosity measurements are dynamic experiments, they will give information on dynamic properties of the ME. These will depend on the miCTOStructure, type of aggregates, or interactions within the ME, which in turn are determined by the concentration of the various components and the temperature. The dispersion of one component in another, e.g., water in oil, will generally increase the bulk viscosity in comparison to the individual components (oil and water) [58]. For at true colloidal dispersion, viscosity will increase with increasing volume fraction of dispersed phase according to the formula generated by Einstein ... 

In practice, complete rheological curves for concentrated emulsions often cannot be obtained. This is due to the instability of disperse systems with a high content of dispersed phase at high shear rates. Difficulties with the instrumental techniques employed for such measurements may also arise because the values of the viscosity and the shear stress at the transition from the imperturbed to a completely destroyed structure vary over a wide range. Under certain conditions, incomplete rheological curves must be used for analysis and prediction of the viscosity of emulsions. 

Thus, measuring rheological properties of dispersions of low-molecular-weight (with viscosity about 60 Pa s) and high-molecular-weight poly(isobutilene) (with viscosity about 10s Pa s) with the same content of filler we see that the values of creep viscosity r c of these systems are practically equal, in spite of the difference of the viscosity of the dispersion medium more than 1000 times [3],... 

This formula may be useful as a rheological method for determining the thickness of adsorption layer, which is formed as a result of interaction between a dispersion medium and filler, by the results of measuring the t] versus q> dependence. Especially curious phenomena, connected with surface effects, arise when a mixture of fillers of different nature is used according to concentration of an active filler the introduction of the second (inert) filler can either increase or decrease the viscosity of a multicomponent system [35],... 

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