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Jose Manuel Valverde, M J Espin, M A S Quintanilla, A Castellanos
Jamming and rheology of fluidized beds of magnetized particles

Appl. Rheol. 21:3 (2011) 35179 (9 pages)

The interaction between magnetic particles in a bed fluidized by a gas is determined by the magnetizing action of an externally applied magnetic field. As the strength of the field is increased there comes a point at which the bed transits from a fluidlike to a solidlike stable state. Interparticle attractive forces induced by the applied field causes chainlike agglomeration of the particles, which confers the stabilized structure with a mechanical strength. In this paper we report experimental results on the yield stress of fluidized beds of fine magnetic particles stabilized by an externally applied magnetic field. Our results show that, in analogy with magnetorheological fluids (MRFs), particle structuring determines essentially the yield stress of magnetofluidized beds (MFBs). Moreover, our work shows that the dependence of the yield stress on particle size, which stands as a controversial issue in the study of MRFs, can be understood from the analysis of the jamming transition as affected by the size of the particles and the strength of the field.

Cite this publication as follows:
Valverde JM, Espin MJ, Quintanilla MAS, Castellanos A: Jamming and rheology of fluidized beds of magnetized particles, Appl. Rheol. 21 (2011) 35179.

J.M. Valverde, A.T. Perez, A. Castellanos, R.E. Viturro
Rheological Testing of Xerographic Liquid Inks: A Need for Printing Technology

Appl. Rheol. 14:4 (2004) 190-196

Rheological properties of xerographic liquid inks of different concentrations of solid particles have been tested. Generally we have found that viscosity decreases with increasing shear rate, i.e. the system is pseudoplastic as corresponds to the break down of aggregated particles by the applied shear. The viscosity of inks may vary in orders of magnitude depending on solids concentration, reaching up values of ~ 108 Pas for solids concentration of 40 wt%. The existence of a yield critical stress has been discussed and we estimate that it increases exponentially with solids concentration. We have looked for possible differences in the rheological behavior of ink samples obtained either diluting more concentrated inks or drying less concentrated ones. Concentration and dilution of xerographic inks do not change their rheological properties meaning that the microscopic structure of the concentrated dispersion is broken (when diluting) and recovered (when drying) reversibly. On the other hand processed ink (previously subjected to high mechanical and electrostatic stresses) behaves differently than non-processed ink. The properties of processed inks are not totally recovered when diluting, manifesting itself in a higher effective apparent viscosity. This result should be of main concern to liquid ink based printing technologies, for which elimination of waste by recycling processed ink is a major goal.

Cite this publication as follows:
Valverde JM, Perez AT, Castellanos A, Viturro RE: Rheological Testing of Xerographic Liquid Inks: A Need for Printing Technology, Appl. Rheol. 14 (2004) 190.

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