The yielding behaviour of magnetorheological fluids has been investigated by videomicroscopy, ultrasonic and rheometry techniques simultaneously. Particularly, the effect of different factors such as, the magnetic field strength, particle size, surface chemistry of the particles, particle concentration and carrier fluid viscosity has been studied. Special attention has been paid to correlate the yielding information obtained by acoustical, optical and mechanical techniques. As a general trend, independently of the particular field strength and suspension formulation, the steady shear flow curve exhibits three well differentiated regions. In the first region, at small stresses, field-induced structures remain quasistatic and all magnitudes remain constant. For larger stresses the number of aggregates decreases but their size increases. This is identified with the onset of flow, and corresponds to the classical static yield stress and a decrease in time-of-flight and normal stresses. For even larger stress values, the suspensions fully flow. This stress value corresponds to the classical dynamic yield stress and is associated to a minimum in the time-of-flight and normal stresses.
Acknowledgments
This work was supported by MAT 2013-44429-R, DPI 2013-46915-C2-1-R and PLIN-2015-051 projects (Spain) and by Junta de Andalucía P10-RNM-6630 and P11-FQM-7074 projects (Spain).