Electrorheology (ER) denotes the control of a material's flow properties (rheology) through an electric field. We have fabricated electrorheological suspensions of coated nanoparticles that show electrically controllable liquid-solid transitions. The solid state can reach a yield strength of 130 kPa, breaking the theoretical upper bound on conventional ER static yield stress that is derived on the general assumption that the dielectric and conductive responses of the component materials are linear. In this giant electrorheological (GER) effect, the static yield stress displays near-linear dependence on the electric field, in contrast to the quadratic variation usually observed. Our GER suspensions show low current density over a wide temperature range of 10-120 degrees C, with a reversible response time of <10 ms. Finite-element simulations, based on the model of saturation surface polarization in the contact regions of neighbouring particles, yield predictions in excellent agreement with experiment.

No abstract

Electrorheological fluids constitute a type of colloids that can vary their rheological characteristics upon the application of an electric field. The recently discovered giant electrorheological (GER) effect breaks the upper bound of the traditional ER effect, but a microscopic explanation is still lacking. By using molecular dynamics to simulate the urea-silicone oil mixture trapped in a nanocontact between two polarizable particles, we demonstrate that the electric field can induce the formation of aligned (urea) dipolar filaments that bridge the two boundaries of the nanoscale confinement. This phenomenon is explainable on the basis of a 3D to 1D crossover in urea molecules' microgeometry, realized through the confinement effect provided by the oil chains. 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 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.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. More...

This letter shows that by decreasing the size of the barium titanyl oxalate nanoparticles coated with urea, achieved through Rb doping, the giant electrorheological (GER) effect can attain a yield stress of Ͼ250 kPa. The shear thinning effect observed in parallel-plate sandwich configuration is also reported, and attributed to the centrifugal sedimentation effect of the inhomogeneous structures induced by applied electric field. In addition, it is found from experiments that the ER effect is very sensitive to the solid particles volume fraction.

No abstract

Characterization of metal clusters (Pd and Au) supported on various metal oxide surfaces (MgO and TiO 2 )

In the present paper it is pointed out that the frictional forces and the energy transfer rate appearing in the balance equations for the hot-electron transport problem all depend explicitly on the electric field. Their expressions are rederived with the electric field dependent corrections to O(E') included. These expressions are identical with those of Lei-Ting except that in the latter the electron energy appearing in the delta function and the distribution function are replaced by come sponding new-defined field dependent ones. The significance of such substitutions for obtaining the correct result is discussed in detail.

In view of the recent disputes of Huang and Wu and Lei about Lei's balance equation theory for an arbitrary energy band, the balance equations for hot-electron transport in an arbitrary energy band are reformulated. In the new formulation the adoption of the effective Hamiltonian method is avoided and all calculations are carried out in laboratory coordinates. In the mean time the crystal momentum k is carefully distinguished from the physical momentum. By introducing a correct equilibrium density matrix of the drifting electron gas in place of that used by Lei, the balance equations together with the distribution function can be derived directly with no need to distinguish the degree of freedom of the center of mass from the relative degree of freedom of the electrons, which is in our opinion neither possible nor necessary at least for a nonparabolic energy band. Some problems associated with the foundation of the balance equation theory are also examined. The result shows that the comment to Lei's theory given by Huang and Wu is correct and to the point. Nevertheless, contrary to their assertion that this theory is inapplicable to the nonparabolic band transport, Lei's balance equations, after a few, however, important corrections, are still applicable to transport for nonparabolic energy bands. I ) 96 Jingzhai Road, Anhui, Hefei 230026, People's Republic of China. 9 nhvcica (h\ 197/1