Dynamics and lateral interactions of dipolar chains

The dynamics and lateral interactions of dipolar chains in magnetorheological suspensions determine the long-time microscopic structure and resulting rheological response. In this paper we characterize proposed lateral interaction mechanisms and their implications for long-time coarsening of structure and compare them to direct measurements of the lateral interaction of dipolar chains using optical trap micromanipulation. We observe a long-range far-field attraction between flexible chains, while the near-field interaction can be repulsive or attractive. At high field strengths, we observe the short-range attraction of rigid chains. Chain dynamics measured with videomicroscopy and diffusing wave spectroscopy are described by a local-mode model and are consistent with fluctuation-mediated interaction theories. The subdiffusive behavior at intermediate and long times scales as t(0.75), identical to semiflexible molecules. Finally, we show examples of how defects in chains can create lateral attractions or repulsions.