Formation of magnetic particle clusters in shear thinning fluids: A first approach

Theoretical expressions used to study the assembly of magnetic particles under the action of external magnetic fields are generalized to incorporate carrier fluids with varying viscosity (shear thinning fluids). The theory is essentially local, and the drag force assigned to each particle considers local viscosity and flow changes due to pseudoplastic behavior. A benchmark is performed with finite element simulations for the case of pairwise interactions and static external fields. General agreement is observed, although simulations reveal the complexity that arises in these systems due to nonlinear hydrodynamic interactions. In the case of external rotating fields, the generalized theory predicts the transition from chains to cluster-like assemblies as the angular frequency of the field increases, similar to what happens in Newtonian media. The difference lies in that particles forming clusters in shear thinning fluids exhibit a higher degree of orientational ordering and are larger than those formed in Newtonian media. An experimental test is performed using magnetite dispersed in a power-law fluid made with xanthan gum. Image analysis reveals that the clusters formed in the power-law fluid are larger than those formed in a reference Newtonian fluid. Further analysis indicates that aggregation in the non-Newtonian fluid is primarily a dynamical process and is little influenced by direct interactions between the magnetite and the polymer.