Enhancement of electrorheological effect by particle-fluid interaction

The influence of interactions between particle surface and host fluids in electrorheological suspensions is explored. It is observed that dispersions of nanosized particles of titania in octanoid acid exhibit an anomalously large electrorheologic effect when compared with a similar dispersion of micrometric particles or with a more conventional colloidal suspension of silica in silicone oil. The effect is interpreted as originated by the formation of a thin layer of octanoid acid molecules with the surface of the titania solid particle. The experimental data are fitted with the outcomes of a modified version of conductive models existing in the literature. It is suggested that anomalous large electrorheological effect is mainly originated by the increasing of the effective radius of the nanometric particles, which results in an increasing of the effective volume fraction of the dispersed phase. It is also shown that the deformation of the soft shell around the solid particles, induced by Coulombic force, plays a not negligible role. Some hints for tailoring electrorheologic fluids suitable for different applications are proposed.

Figure 1

Shear stress of a dispersion ($\varphi =0.05$) of SiO${}_2$ particles ($d=80$ nm) in silicone oil at different values of the applied field (dc biasing). (Inset) Increment in the shear stress obtained at different values of the applied field (see text for details).

Figure 2

Shear stress of a dispersion ($\varphi =0.05$) of TiO${}_2$ particles ($d=32$ nm) in octanoid acid at different values of the effective electric field (ac biasing, 20 Hz). (Inset) Comparison between the increment in the shear stress, obtained at a selected value of the effective field, for dispersions of microsized and nanosized particles at fixed volume fraction, $\varphi =0.05$.

Figure 3

Shear stress of a dispersion ($\varphi =0.05$) of TiO${}_2$ particles ($d=44$ $\mu$m) in octanoid acid at different values of the effective electric field (ac biasing, 20 Hz).

Figure 4

Yield stress of a dispersion ($\varphi =0.05$) of SiO${}_2$ particles ($d=80$ nm) in silicone oil at different values of the effective electric field (dc biasing). Continuous line, fitting outcomes. (Inset) Field dependence of the $\Gamma$($E$) function, as obtained from the fitting procedure (see text). The vertical dashed line marks the limit of validity of the model.

Figure 5

Yield stresses from the investigated dispersions ($\varphi =0.05$) of TiO${}_2$ particles in octanoid acid at different values of the effective electric field (ac biasing, 20 Hz). Continuous and dashed lines, fitting outcomes. (Inset) Field dependence of the $\Gamma$($E$) function, as obtained from the fitting procedure (see text). Continuous line, $d=44$ $\mu$m; dashed line, $d=32$ nm.

Figure 6

Geometry of the adopted model and symbol definitions. (a) Ellipsoid parameters and coordinates definition; (b) yield angle definition.

Figure 7

Maximum reduced field in the fluid as a function of the reduced interparticle distance, as calculated from the model for the investigated dispersion of SiO${}_2$ particles in silicone oil. Dashed line, model calculation under the assumption of conducting particles [$\Gamma (E_f=0)$].

Figure 8

Model result: yield stress for SiO${}_2$ ($d=80$ nm) in silicone oil dispersions ($\varphi =0.05$) as a function of the shear strain at different values of the applied field.