Influence of liquid phase on nanoparticle-based giant electrorheological fluid

Abstract

We show that the chemical structures of silicone oils can have an important role in the giant electrorheological (GER) effect. The interaction between silicone oils and solid nanoparticles is found to significantly influence the ER effect. By increasing the kinematic viscosity of silicone oils, which is a function of siloxane chain length, sol-like, gel-like and clay-like appearances of the constituted ER fluids were observed. Different functional-group-terminated silicone oils were also employed as the dispersing media. Significant differences of yield stress were found. We systematically study the effect of siloxane chain lengths on the permeability of oils traveling through the porous spaces between the particles (using the Washburn method), oils adsorbed on the particles' surface (using FT-IR spectra), as well as their particle size distribution (using dynamic light scattering). Our results indicate the hydrogen bonds are instrumental in linking the silicone oil to GER solid particles, and long chain lengths can enhance the agglomeration of the GER nanoparticles to form large clusters. An optimal oil structure, with hydroxyl-terminated silicone oil and a suitable viscosity, was chosen which can create the highest yield stress of ∼300 kPa under a 5 kV mm(-1) DC electric field.

Giant Electrorheological Effect: A Microscopic Mechanism

“…Silicone oil is a typical dispersant for the GER nanoparticles, with previous experimental studies showing the interaction between the nanoparticles to be dispersant dependent [11]. This indicates that the silicone oil chains have a crucial effect on the GER phenomenon.…”

“…Moreover, the formation of aligned dipolar filaments is an interfacial phenomenon, which directly implies the surface area scaling characteristic of the GER mechanism [8] that favors nanoparticle dispersions. A nonwetting oil would give rise to phase separation and the formation of large aggregates, thus suppressing the GER effect [10,11]. We give a simple argument, involving the dimensionality dependence of the polarizability for molecular dipoles, to explain all the observed phenomena in the simulations.…”

“…The resulting electrical energy density yields an excellent account of the observed GER yield stress variation as a function of the electric field. Electrorheological (ER) fluids [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15] are a type of colloidal dispersions which can vary their rheological characteristics through the application of an external electric field. The traditional ER mechanism is based on induced polarizations arising from the dielectric constant contrast between the solid particles and the fluid [6,12].…”

“…The traditional ER mechanism is based on induced polarizations arising from the dielectric constant contrast between the solid particles and the fluid [6,12]. The recent discovery of the giant electrorheological (GER) effect [7][8][9][10][11][12], in urea-coated barium titanyl-oxalate nanoparticles ½NH 2 CONH 2 @BaTiOðC 2 O 4 Þ 2 , or BTRU for short, dispersed in silicone oil, has shown that the theoretical upper bound of the ER effect is no longer applicable to this new type of materials. Instead, a phenomenological model of the GER mechanism, based on aligned urea molecular dipoles in the small contact regions of the nanoparticles, yielded an adequate account of the observed effect [7,9,12].…”

“…However, a microscopic picture of how this can occur has so far eluded persistent efforts. Moreover, as the GER effect is highly sensitive to whether the dispersing oil can wet the solid particles [10,11], in contrast to the traditional ER fluids, a natural question is how this observation can be integrated into a coherent GER mechanism. In view of the fact that the GER effect has now been reproduced in many different material systems and therefore is becoming a much more general effect [14,15], answers to the above questions would not only be timely, but may also shed light on how to devise general strategies for harnessing and controlling the large electric energy stored in molecular dipoles.…”