Pieter B. Visscher, Dept. of Physcs and Astronomy and MINT Center, University of Alabama
Mourad Benakli, Dept. of Physcs and Astronomy and MINT Center, University of Alabama

We present three-dimensional animated visualizations of Brownian dynamics simulations of a magnetic colloid. The system is chosen to model a material of great technological interest, the magnetic ink (a colloidal suspension of cigar-shaped iron or iron-oxide particles) that is painted on a substrate in the manufacture of magnetic tapes and disks. We have modeled the response of the colloid to an AC field in a direction perpendicular to an applied DC field, i.e. the transverse susceptibility.

The simulation shows a system of cigar-shaped particles, initially randomly oriented in a gel-like structure in the absence of a magnetic field. When the DC magnetic field is turned on, the particles align along the field. The AC susceptibility is measured by superposing a small transverse AC field and recording the resulting AC magnetization, and is plotted as a function of time. This plot has also been obtained experimentally, and we see good semiquantitative agreement. This probe (transverse susceptibility) is quite sensitive to how easily the structure is broken up by the orienting DC field, hence to the detailed structure and has been used industrially as a gauge of dispersion quality (lack of clumping). The user can freeze the system in a particular configuration and rotate or zoom it with the mouse to examine the orientation and local structure.

Supported by NSF-MRSEC, grant # DMR-9809423.