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When particles are dispersed in a rheologically complex medium, a striking alignment of particles into strings can be observed during shear flow. This occurs even under conditions where the suspensions would be considered to be dilute. However, a conclusive explanation for the specific role of the rheological properties of the suspending medium has not been given since the phenomenon was first discovered, now around 40 years ago. The present work aims to elucidate the role of the fluid properties as well as the effect of geometrical conditions by performing experiments, which separate the different factors that could play a role in string formation. First, the presence of significant normal stress differences is shown to help, but it is not a necessary condition for string formation. Second, moderate confinement is shown to be enhancing alignment as well. Third, shear thinning and its effect on lubrication forces are identified to be the crucial factor for alignment. In particular, shear thinning allows to maintain the individual rotation of each particle in a string. This is a requirement to meet the zero torque condition; otherwise, the string would tumble as a whole. In confirmation of this it is observed that when small particles are added as depleting agent to push larger particles closer together, at a critical concentration the flow induced alignment is suppressed. Particle doublets and triplets are then observed to rotate as a whole and tumble on drifting Jeffery orbits ending up in a log-rolling state, oriented along the vorticity direction. The suppression of the string formation upon increasing attraction confirms that enabling the rotation of the separate particles within the string is crucial for maintaining the structure aligned in the flow direction. This suggests that the pertinent rheological parameters of the suspending fluids are those related to the lubrication flows occurring in the gap, i.e., the overall level of the viscosity and the amount of shear thinning.status: publishe

String formation in sheared suspensions in rheologically complex media: The essential role of shear thinning