Mass-distribution scaling and pattern formation in magneto-rheological suspensions

Abstract

Pattern formation in magneto-rheological fluids composed of micrometric particles of Fe 3 O 4 dispersed in silicone oil in the presence of an external static magnetic field is studied. The aggregation processes that originate the complex fibrous structure acquired by the suspension when the external field is turned on can be characterized by means of well-defined mass-scaling relations. The radial mass distribution is calculated. This function also clearly exhibits the different mass-scaling stages related to the formation of three different generations of clusters, and allows the hierarchical structure of the system to be described.Magneto-rheological suspensions (MRS) are dispersions of magnetic particles in a liquid phase, usually a mineral oil. Upon the application of an external magnetic field, the MRS suffer a quasi-reversible transition from a liquid system to almost a solid body, in times of the order of a few dozens of milliseconds. The many-body interactions among the particles originate the complex structure of the suspension. The link between this structure and the macroscopic physical properties of the system still present several unanswered fundamental questions [1]. We have studied pattern formation in MRS formed by micrometric iron oxide particles dispersed in silicone oil, in the presence of an external field. We have found that the resulting structure exhibits three aggregation stages, which can be well characterized by the behaviour of their measured mass fractal dimension [2]. In this work, we calculate and discuss the mass radial distribution, this function shows another aspect of the different regimes of mass scaling, and could provide additional elements to describe the hierarchical structure of the MRS.For the preparation of the MRS, we synthesize iron oxide microcrystals (Fe 3 O 4 , magnetite phase) with average size 10 µm and prismatic morphology by means of a coprecipitation procedure. These ceramic powders are obtained as precipitates in a small volume reaction of aqueous solutions of ammonium oxalate, iron (III) nitrate, and iron (II) chloride, followed by thermal decomposition [3]. The identification of the phases was done by X-ray diffraction. We prepare MRS by dispersing the microcrystals in silicone oil. To be able to observe the pattern formation by optical microscopy, we use particle concentrations less than 0.1 in volume fraction. Using a cover glass we prepare square cells, 15 mm wide and 1 mm thick, to contain the dispersion. To observe the pattern formation, one of these cells is set on the stage of the optical microscope and the magnetic field is applied using electromagnets. The observations were done in transmission mode.The whole process of pattern formation is filmed with a digital camera. In the absence of the field the particles are homogeneously dispersed in the silicone oil and we can observe the individual particles moving through the liquid. The aggregation processes that occur when the field is turned on can be described as follows: the magnetization induced in t...