Polar‐Molecule‐Dominated Electrorheological Fluids Featuring High Yield Stresses

Abstract

Recent works on the development of various electrorheological (ER) fluids composed of TiO 2 , SrÀTiÀO, and CaÀTiÀO particles coated with CÀO/ HÀO polar groups are summarized, in which an extremely large yield stress up to 200 kPa is measured and the dynamical yield stress reaches 117 kPa at a shear rate of 775 s À1 . Moreover, unlike that of traditional dielectric ER fluids, the yield stress displays a linear dependence on electric field strength. Experimental results reveal that it is the polar molecules adsorbed onto the dielectric particles that play the decisive role: the polar-molecule-dominated ER effect arises from the alignment of polar molecules by the enhanced local electric field in the gap between neighboring particles. The pretreatment of electrodes and the contrivance of new measuring procedures, which are desirable for the characterization and practical implementation of this material, are also discussed. The successful synthesis of these fluids has made many of the long since conceived applications of the ER effect available.

Giant Electrorheological Effect: A Microscopic Mechanism

“…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].…”

“…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.In this work we use molecular dynamics (MD) simulations to show that in a mixture of urea molecules with silicone oil chains confined between two bounding surfaces (denoted as substrates below) of a nanoscale contact, aligned urea molecular dipoles can form filaments snaking through the pores of the oil film to bridge the substrates. The required electric field for aligning the urea dipoles is found to be lowered by a factor of 2 to 3 in the presence of the oil chains, compared to that without the oil chains.…”

“…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.…”

“…The electric field at the contact region of two nanoparticles is enhanced by a factor of $ 100-300 (compared to the experimentally applied electric field) [31], owing to the large dielectric constant of the nanoparticles and hence the field concentration effect at the contact region [9,14,15]. Thus 0:5 V=nm field in the gap region would correspond roughly to $5000 V=mm (or less) of experimentally applied field.…”

Giant electrorheological fluids with ultrahigh electrorheological efficiency based on a micro/nano hybrid calcium titanyl oxalate composite

“…The polarization of molecular dipoles in the contact region between two neighboring nanoparticles is thought to be responsible for the GER effect. [15][16][17] Various inorganic and organic/inorganic GER materials have been synthesized; [18][19][20][21][22][23][24][25][26][27] for example, Wen et al 18 reported a new type of GER fluid consisting of polar group-modified nano-sized barium titanyl oxalate particles suspended in silicone oil.…”