R. Tao

Enhance the Yield Shear Stress of Magnetorheological Fluids

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.

Flexible Fixture Device with Magneto-Rheological Fluids

In manufacturing, fixtures are required to locate parts in a correct position to ensure machining accuracy. Especially for machining on irregular-shaped work-pieces and fine parts, flexible fixtures are in demand to reduce non-machining time. The quick phase change and strong yield strength of MR fluids lead us to investigate the possibility to produce MR-fluid-flexible-fixture. The yield strength and pre-yield elastic modulus are the key issues for a flexible fixture. However, a typical MR fluid currently has yield stress about 100 kPa that is not sufficient for flexible fixtures. To overcome the difficulties, we apply pressure to force single chains into thick columns. This produces a structure-enhanced yield stress. MR fluids in this process produce a yield stress of 800 kPa or above under a moderate magnetic field. The three-dimensional load capacity of MR fluids in this process is strong enough to ensure precision manufacturing. In addition, MR-fluid-flexible-fixtures are environmentally friendly and work at room temperature.

ELECTRORHEOLOGICAL FLUIDS UNDER SHEAR

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 spaces anisotropy, which is the result of the applied electric field.

Electrorheology for Efficient Energy Production and Conservation

Recently, a theory and new technology has been developed, which utilizes a pulsed electric field to change the rheology of complex fluid to reduce its viscosity, while keeping the temperature unchanged. The method is energy efficient, universal, and applicable to all complex fluids with suspended particles in nanometers, sub-micrometers, or micrometers. This technology has been applied to crude oil and refinery fuels. While the applications are still developing, the results are quite impressive, indicating that electrorheology can play an important role in energy production, transportation, and conservation.

Testing and Modeling a Cone-Shaped Squeeze-Film Mode Electrorheological Damper

Experiments on a cone-shaped squeeze-film mode ER damper are reported. An analytical model is developed to calculate the damping force as a function of the vibration amplitude, frequency, the yield stress of the ER fluid. Our calculations agree with the experiments very well at small amplitudes. The cone-shaped electrodes make the damper benefit from both shear mode damper and squeeze-film mode damper. The azimuth angle 0 of the cone electrodes plays an important role in magnifying the damping coefficient of this new type of ER damper.

Reducing the Viscosity of Diesel Fuel with Electrorheological Effect

Improving engine efficiency and reducing pollutant emissions are extremely important. Here the authors report their finding, using electrorheology to reduce the viscosity of diesel fuel. Diesel is made of many different molecules, 75% small molecules and 25% large molecules. In addition, it contains other nanoscale particles, such as sulfur. Therefore, diesel can be regarded as a liquid suspension. Under a strong electric field, the large molecules aggregate into small clusters, yielding a lower viscosity. For high-sulfur diesel, the applied electric field is around 1 kV/mm. However, for ultra-low-sulfur diesel, the required electric field must be around 2 kV/mm. This viscosity reduction leads to finer mist in fuel atomization, improving the combustion, and engine efficiency.

Electrorheology Improves E85 Engine Efficiency and Performance

E85 is an alternative fuel with 85% ethanol and 15% gasoline. However, it is widely reported that E85 vehicles have difficulties to start in winter and have poor performance. Here, the authors report that with proper application of electrorheology, the authors can solve these issues. E85 vehicles all have port-injected engines. The fuel is injected into cylinders as droplets. Before the ignition, the fuel evaporates. Because E85 is more viscous than gasoline, the injected E85 droplet size is not small. Especially, in winter the cold weather makes the viscosity even higher, leading to the E85 droplets being even bigger. Since evaporation starts from the droplet surfaces, large droplets are difficult to be evaporated before the ignition comes. When there are no enough fuel vapors, the engine cannot start. To solve this problem, the authors introduce a small device just before the fuel injection, which produces a strong electric field to reduce the fuel viscosity, leading to much smaller fuel droplets in atomization. The evaporation is much faster and the engine is easier to start. As the small fuel droplets produced by our device make the combustion fast and timely, engine efficiency and performance are also expected to be improved.

High temperature superconducting granular balls

Abstract When a strong electric field is applied to a suspension of micron-sized high Tc superconducting particles in liquid nitrogen, the particles quickly aggregate together to form balls. The millimeter-size balls hold over 106 particles each. They are sturdy, surviving constant heavy collisions with the electrodes. The ball formation is a results 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. Its minimization leads to the balls.

FLUID FLOW AND FALLING BALL EXPERIMENTS IN ER FLUIDS

The ability of the induced dipole–dipole model to predict various properties of an electrorheological fluid is tested in a series of experiments: the deformation of a single particle chain under fluid flow, the velocity of a falling ball at various electric fields, particle sizes, and concentrations, and flow valves operating at either constant pressure differential or constant flow rate. Analyses of these rather different experimental situations show that with proper application, the induced dipole model can give a fairly accurate description of observed characteristics.

Theory of the voltage biased Josephson model

The voltage biased weak link between two bulk superconductors is discussed using the conventional Josephson pendulum model. A parameter lambda is introduced to characterize the ratio between the electron pair tunnelling energy and the electrostatic energy required to store one electron pair in the link. If lambda is small, the Q-V curve, the relationship between the applied voltage and the charge stored in the link, is in a staircase form. When there are strong pair tunnelling processes, lambda >>1, the Q-V curve becomes a straight line. These results are consistent with experimental findings.