8th Liquid Matter Conference September 6-10, 2011 Wien, Austria ...

P5.116Wed 711:10-14:00Fibrous structure formation in magneto-rheologicalfluidsMichael P. N. Juniper, 1 William W. Sampson, 2 and Roel P. A. Dullens 11 University of Oxford, Physical and Theoretical Chemistry Laboratory, South Parks RoadOX1 3QZ, Oxford, United Kingdom2 University of Manchester, Manchester, United KingdomMagneto-rheological fluids consist of paramagnetic colloidal particles dispersed in a non-magneticsolvent. Applying an external magnetic field induces a dipole moment in the particles, leading to ahost of field-induced structures [1-3]. The consequent change in rheological properties is not onlyof current scientific interest, but also has important technological applications such as use in semiactiveshock absorbers. It is well known that at low densities the assembled structure involvesmainly dipolar chains [1-3]. At intermediate densities, the formation of fibrous structures hasbeen reported [3]. Here we use optical video-microscopy to study the structure of these networksand the dynamics of their assembly from super-paramagnetic particles. Concepts from randomnetworks are employed to comprehensively characterize the statistical geometry of the system.Interestingly, the fiber length exhibits an exponential probability density function, indicating arandom nature. The effect of the competition between confinement and the characteristic fiberlength is also explored. Finally, we analyze the dynamics of the network formation by followingthe time evolution of the particles’ local environments. Simple rate equations describe the fieldinducedassembly surprisingly well.[1] Skjeltorp, A. T. , J. Appl. Phys. , 57, 3285-3288, (1985).[2]. Furst, E. M.; Gast, A. P. , Phys. Rev. E. , 62, 6916-6925, (2000).[3] Fermigier, M.; Gast, A. P. , J. Colloid Interface Sci. , 154, 522-539, (1992).116