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It is a well-known fact that, under the application of a magnetic field and a progressively growing shear flow, the inner structure of magnetorheological fluids evolves from a collection of upright chains to a striped pattern consisting of particle layers contained in the flow-gradient plane. This picture totally disagrees with the scenario conceived by the traditional chain model [1] that predicts the progressive destruction of the chains till a point where only individual particles are homogeneously distributed in the sample.
Previous works in the literature have concluded that the onset for layer formation is controlled only by the Mason number (Mn, ratio between shear and magnetic forces). Namely, it happens at the (critical) Mn responsible for the breakage of the shortest (two-particle) chains in the sample [2]. However, the majority of those works studied the layer pattern once the flow had ceased or did not use samples of interest in current applications (e.g. carbonyl iron particles).
In this work, the chain-layer transition and pattern formation are reported as Mn is increased during rheograms thanks to the use of a video camera fully synchronized with a rheometer in a plate-plate configuration. Both the sample rheological signature and inner structure (pattern morphological properties) are analysed and correlated with each other. It is found that the layered pattern is strongly affected not only by Mn but also by the gap between plates. In particular, the layer onset happens at smaller Mn as the gap increases while the layer period and width increase linearly with the gap.
Finally, a numerical model is proposed, based on chain interactions able to explain the previous experimental findings and reconcile them with the widely-accepted chain model.

Acknowledgements:
This project has received funding from the European Union’s Horizon 2020 research and innovation programme under the Marie Skłodowska Curie grant agreement No 101030666.

References:
[1] Martin, J. E., Anderson, R. A. (1996). The Journal of Chemical Physics, 104, 4814.
[2] Volkova, O., Cutillas, S., Bossis, G. (1999). Physical Review Letters, 82, 233.

flow-field superposition, layered pattern, stripes, Mason number