The yield stress of magnetorheological ͑MR͒ fluids depends on the induced solid structure. Since thick columns have a yield stress much higher than a single-chain structure, we improve the yield stress of MR fluids by changing the fluid microstructure. Immediately after a magnetic field is applied, we compress the MR fluid along the field direction. Scanning electron microscopy images show that particle chains are pushed together to form thick columns. The shear force measured after the compression shows that the structure-enhanced static yield stress can reach as high as 800 kPa under a moderate magnetic field, while the same MR fluid has a yield stress of 80 kPa without compression. This improved yield stress increases with the magnetic field and compression pressure and has an upper limit well above 800 kPa. The method may also be useful for electrorheological fluids.

High-T c superconducting particles of mm size in a strong electric field bind themselves together to form macroscopic balls in milliseconds. Each ball holds over 10 6 particles and bounces between the electrodes without losing any. The ball formation is a result of superconductivity. As the c-axis coherence length is shorter than the Thomas-Fermi screening length, the electric field produced by the charged surface layer turns off the coupling between the interlayers. This loss of Josephson energy becomes a positive surface energy induced by the charged surface layer, the minimization of which leads to the balls. PACS numbers: 74.80.Bj, 68.10.Cr, 83.80.Gv In nature, there are very few cases where granular particles could aggregate together by themselves to form a round ball. Application of an electric field further destroys space's isotropy. Therefore, it is against common sense that an electric field could drive high-T c superconducting (HTSC) particles together to form a round ball. However, this is our finding, which reveals a new property of high temperature superconductivity [1,2]. Moreover, this effect may be used to produce superconducting films on a solid substrate, leading to applications.The particles used in our experiment, YBa 2 Cu 3 O 72x (99.99% purity), NdBa 2 Cu 3 O x (99.9% purity), Bi 2 Sr 2 CaCu 2 O 81x (99.9% purity), and YbBa 2 Cu 3 O x (99.9% purity) were provided by Superconductive Components, Inc. (SCI), Columbus, OH. As shown in Fig. 1, the particle size is 1 2 mm. They were prepared either by a solid state method [3] or the Sandia Chemical Preparation [4]. One batch of YBa 2 Cu 3 O 7 granulated by us has a particle size of 4 5 mm. The superconductivity properties of individual particles were carefully studied by

To enhance the yield shear stress of magnetorheological (MR) fluids is an important task. Since thick columns have a yield stress much higher than a single-chain structure, we enhance the yield stress of an MR fluids by changing the microstructure of MR fluids. Immediately after a magnetic field is applied, we compress the MR fluid along the field direction. SEM images show that the particle chains are pushed together to form thick columns. The shear force measured after the compression indicates that the yield stress can reach as high as 800 kPa under a moderate magnetic field, while the same MR fluid has a yield stress of 80 kPa without compression. This enhanced yield stress increases with the magnetic field and compression pressure and has an upper limit well above 800 kPa. The method is also applicable to electrorheological fluids.

The behavior of an electrorheological (ER) chain under a shear force is investigated theoretically and experimentally. Contrary to the conventional assumption that the ER chain under a shear force becomes slanted and breaks at the middle, we have found that there is symmetry breaking. When the shear strain is small, the chain becomes slanted with a space gap between the first and second particles (or between the last and next last particles). As the shear strain increases, the gap becomes wider and wider. When the shear strain exceeds a critical value, the chain breaks at the gap. The experiment also confirms that an ER chain under the shear breaks at either end, not at the middle. This symmetry breaking reflects the space's anisotropy, which is the result of the applied electric field.

We have observed a strong electrorheological (ER) effect of a suspension of fine aluminum particles in liquid nitrogen. The particles have diameter ~ 10//m and an insulate surface. In an ac field, aluminum particles quickly form chains across the two electrodes. The chains vibrate vigorously as the liquid nitrogen has a constant random flow motion caused by bubbles. In addition to low temperature, liquid nitrogen has extremely low viscosity, and low conductivity. These special properties enable us to observe several interesting phenomena which are absent in ER fluids at room temperature. We have determined the Theological properties of our cryogenic ER fluid.