Special characteristics of colloid dispersions rheology

When colloidal particles arc dispersed in a liquid, the flow of the liquid is disturbed and the viscosity (rf) is higher than that of the pure liquid (dispersion medium), designated as t]o- The problem of relating the viscosities of colloidal dispersions with the namre of the dispersed particles has been the subject of many experimental and theoretical investigations. In this respect, viscosity increments, especially the relative, specific and intrinsic viscosities are of greater significance than absolute viscosities. These functions are shown in Equations 8.12a and 8.12b. The intrinsic viscosity has units of reciprocal concentration but as the concentration is often expressed as the volume fraction of the particles (=volume particles/total volume), then the intrinsic viscosity is dimensionless. 

However, for real dispersions and emulsions, even dilute ones, the viscosity is expected to be lower than that given by Einstein s equation. Droplets in emulsions may with shearing change shapes to ellipsoids and even burst at high shear rates to form smaller droplets. Moreover, sinee small droplets are less easily deformed than larger droplets, the viscosity of an emulsion depends on particles size. 

Finally, notice that the constant 2.5 in Einstein s equation which is valid for spherical particles is the smallest possible value, i.e. values for elongated particles and other shapes are higher. Thus, having dispersions with spherical particles is a way to have low viscosities. 

The Mark-Houwink-Staudinger (MHS) equation provides a relationship between the intrinsic viscosity (Equation 8.12b) and the (average) molecular weight (M) of (synthetic) polymers  

the molecular weight of polymers can be estimated from viscosity measurements (of a series of dilute solutions). However, at low concentrations the relative viscosity is very close to unity and we 

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