Showing papers by "James E. Martin published in 2004"

Using triaxial magnetic fields to create high susceptibility particle composites.

Abstract: We report on the use of triaxial magnetic fields to create a variety of isotropic and anisotropic magnetic particle/polymer composites with significantly enhanced magnetic susceptibilities. A triaxial field is a superposition of three orthogonal ac magnetic fields, each generated by a Helmholtz coil in series resonance with a tunable capacitor bank. Field frequencies are in the range of 150-400 Hz. Because both the field amplitudes and frequencies can be varied, a rich variety of structures can be created. Perhaps the most unusual effects occur when either two or three of the field components are heterodyned to give beat frequencies on the order of 1 Hz. This leads to a striking particle dynamics that evolves into surprising structures during resin gelation. These structures are found to have perhaps the highest susceptibility that a particle composite can have. The susceptibility anisotropy of these composites can be controlled over a wide range by judicious adjustment of the relative field amplitudes. These experimental data are supported by large-scale Brownian dynamics simulations of the complex many-body interactions that occur in triaxial magnetic fields. These simulations show that athermal three-dimensional field heterodyning leads to structures with a susceptibility that is as high as that achieved with thermal annealing. Thus with coherent particle motions we can achieve magnetostatic energies that are quite close to the ground state.

Effects of electric fields and volume fraction on the rheology of hematite/silicone oil suspensions

Abstract: Electrorheological (ER) fluids composed of iron(III) oxide (hematite) particles suspended in silicone oil are studied in this work. The rheological response has been characterized as a function of field strength, shear rate and volume fraction. The dielectric properties of the suspensions were first studied in order to get information about the conductivity of the solid. Rheological tests under a.c. electric fields elucidated the influence of the electric field strength and volume fraction on the field-dependent yield stress. It was found that this quantity scales as a square power law in both cases. The viscosities of electrified suspensions were found to increase by several orders of magnitude as compared to the unelectrified suspension at low shear rates, although at high shear rates hydrodynamic effects become dominant and no effects of the electric field on the viscosity are observed. The ER behaviour of the suspensions was analysed by considering the fundamental forces (of hydrodynamic and electrostatic origin) acting on the particles and it is found that, at a given volume fraction, all the dependencies of relative viscosity on shear rate and field strength can be described by a single function of the Mason number, Mn. Finally, two different chain models were used to explain the shear-thinning behaviour observed: rheological measurements showed a power-law dependence of relative viscosity decrease on the Mason number, η F ~Mn Δ , with Δ≈−0.95.

Evaluation of an ultrasonic displacement sensor

Abstract: An ultrasonic displacement sensor is currently being investigated. A possible application for this sensor is acoustic/seismic landmine detection in which a noncontact vibrometer is employed to measure the normal surface velocity of soil. The ultrasonic sensor is an alternative displacement sensor to a laser Doppler vibrometer (LDV). The ultrasonic wavelength suggests that the ultrasonic sensor is capable of resolving displacements in the presence of rough surfaces. This functionality is of particular interest because scattering of optical signals by rough surfaces can prohibit accurate LDV measurements. The ultrasonic sensor will be evaluated in an array of surface roughness conditions. The performance of the ultrasonic sensor will be compared to the LDV for each of these cases. The relative advantages and limitations of the ultrasound system will be presented. The applicability of the ultrasonic sensor to acoustic mine detection will also be addressed.