Activation energy for the viscoelastic flow: Analysis of the microstructure-at-rest of (water- and milk-based) fruit beverages.

Fresh fruit beverages are a tasty way to follow the recommendations of World Health Organization. A study on the rheological behaviour of water-based and milk-based fruit beverages with emphasis on the temperature effect was made. The power law model adjusted data of apparent viscosity. The flow index of each beverage was practically constant with temperature variation. Then, one could discuss the variation with temperature of the consistency index. This parameter decreased around 40% in both cases in the temperature interval 5-30 °C. The stability of beverages against the sedimentation of the fruit pulp was clearly affected by the increase of temperature. Viscoelastic moduli were obtained in the linear region. Temperature dependency of both moduli was described in the interval 5-30 °C. The activation energy for linear viscoelastic flow was slightly higher than the activation energy for viscous flow. This result was interpreted as an indication of the strength of the microstructure-at-rest.

The Hamaker constant of anatase aqueous suspensions.

The linear relationship of the yield stress with the square zeta potential may be used to determine the Hamaker constants in suspensions. In this work we have obtained the Hamaker constant for the attractive forces between anatase particles in aqueous suspensions using this method and compared them with those obtained by contact angle measurement. The results show excellent agreement.

Intrinsic viscosity of SiO2, Al2O3 and TiO2 aqueous suspensions.

The viscosity of dilute suspensions of several metal oxides (SiO2, Al2O3 and TiO2) was measured at different pH values. The intrinsic viscosity, [eta], was derived from the concentration dependence of the viscosity. This magnitude was pH-dependent. A correlation with the shape of the kinetic unity has been proposed.

The primary electroviscous effect in colloidal suspensions.

Although a well-defined electrokinetic phenomenon, the primary electroviscous effect in dilute colloidal suspensions is still an unsolved problem. Most of the experimental tests of the different theories that we have studied have shown a lack of agreement. We have developed, during the last years, new theoretical approaches obtaining, finally, a much better agreement with the experimental results. The corrections are defined in two lines: first, it is accepted that ions present into the Stern layer, in which the fluid is stagnant, can tangentially move; second, it is accepted that the hydrodynamic interaction between colloidal particles exists although the suspensions are extremely diluted. The remarkable conclusion of our work is that the combination of both corrections should give correct theoretical results.

Stern-layer parameters of alumina suspensions.

The specific adsorption of Mg(+2) and S(2)O(-2)(8) ions onto the gamma-alumina particle surface has been demonstrated. An estimation algorithm based upon the correlation between electrophoretic mobility and conductivity increment experimental data, employing a dynamic Stern-layer theory, has been developed and used to get the adsorption parameters into the Stern layer for different gamma-alumina/Mg(+2) and S(2)O(-2)(8) aqueous solutions interface.

An experimental test of Booth's primary electroviscous effect theory.

The primary electroviscous effect has been investigated in dilute suspensions of titanium oxide (anatase), the viscosities of which were measured by means of a capillary viscometer with automatic timing. The linear relation between viscosity and solids volume fraction was first determined at the isoelectric point of the particles when the particles are uncharged, and the electroviscous contribution to the intrinsic viscosity was then determined at other values of pH. Booth's theory (Proc. R. Soc. London Ser. A203, 533 (1950)) agrees well with the experimental results when the particle zeta potential is small and the double layer is thin (kappa alpha approximately 7.3), but agreement is poor when the double layer is thick (kappa alpha approximately 0.6).

Primary Electroviscous Effect with a Dynamic Stern Layer: Low kappaa Results.

The classical treatments of the primary electroviscous effect show important discrepancies with respect to the experimental data. A possible better agreement may be found if the contribution of the ions adsorbed on the Stern layer, which can move tangentially near the particle surface, is taken into account. This contribution has been incorporated into the Watterson-White theory. A study of the influence of the Stern-layer parameters on the primary electroviscous coefficient has been made. Copyright 2000 Academic Press.