Silva, Adriana S. and Bond, Robert and Plouraboué, Franck and Wirtz, Denis *Fluctuation dynamics of a single magnetic chain.* (1996) Physical Review E, vol. 54 (n° 5). pp. 5502-5510. ISSN 1063-651X

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Official URL: http://dx.doi.org/10.1103/PhysRevE.54.5502

## Abstract

"Tunable" ﬂuids such as magnetorheological "MR" and electrorheological "ER" ﬂuids are comprised of paramagnetic or dielectric particles suspended in a low-viscosity liquid. Upon the application of a magnetic or electric ﬁeld, these ﬂuids display a dramatic, reversible, and rapid increase of the viscosity. This change in viscosity can, in fact, be tuned by varying the applied ﬁeld, hence the name "tunable ﬂuids". This effect is due to longitudinal aggregation of the particles into chains in the direction of the applied ﬁeld and the subsequent lateral aggregation into larger semisolid domains. A recent theoretical model by Halsey and Toor "HT" explains chain aggregation in dipolar ﬂuids by a fluctuation-mediated long-range interaction between chains and predicts that this interaction will be equally efficient at all applied fields. This paper describes video-microscopy observations of long, isolated magnetic chains that test HT theory. The measurements show that, in contrast to the HT theory, chain aggregation occurs more efficiently at higher magnetic field strength (H0) and that this efﬁciency scales as H0½. Our experiments also yield the steady-state and time-dependent ﬂuctuation spectra C(x,x')≡ [h(x)-h(x')]²>½ and C(x,x',t,t')≡ <[h(x,t)-h(x',t')]² >½ for the instantaneous deviation h(x,t) from an axis parallel to the ﬁeld direction to a point x on the chain. Results show that the steady-state ﬂuctuation growth is similar to a biased random walk with respect to the interspacing ͉ |x-x'| along the chain, C(x,x')≈|x-x'| α, with a roughness exponent α =0.53±0.02. This result is partially confirmed by Monte Carlo simulations. Time-dependent results also show that chain relaxation is slowed down with respect to classical Brownian diffusion due to the magnetic chain connectivity, C(x,x',t,t')≈|t-t'|β, with a growth exponent β=0.35±0.05<½. All data can be collapsed onto a single curve according to C(x,x',t,t')≈|x-x'| α ψ (|t-t'| / |x-x'| z ), with a dynamic exponent z= α /β≅ 1.42.

Item Type: | Article |
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Additional Information: | Thanks to APS editor. The original PDF is available at http://pre.aps.org |

Audience (journal): | International peer-reviewed journal |

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Institution: | French research institutions > Centre National de la Recherche Scientifique - CNRS Other partners > Ecole Supérieure de Physique et de Chimie Industrielles - ESPCI (FRANCE) Other partners > Université de Paris Diderot - Paris 7 (FRANCE) Other partners > Université Pierre et Marie Curie, Paris 6 - UPMC (FRANCE) Other partners > Johns Hopkins University - JHU (USA) |

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Deposited By: | Franck PLOURABOUE |

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