We present quantitative measurements of a dilute micellar solution past a falling sphere. The dilute micellar solution that consists cetyltrimethylammonium bromide and 5-methyl salicylate (CTAB/5mS) in deionized water exhibits shear-thickening behavior beyond a critical shear rate of $\dot{{\gamma }_c}\approx 0.4$ (1/s). Previous experiments have demonstrated that this micellar solution forms unentangled rodlike micelles at equilibrium [Davies et al., J. Am. Chem. Soc. 128, 6669 (2006)]. At a vanishingly small Reynolds number $\mathrm{Re}=0.03$, the drag coefficient for the falling sphere is similar to that of a Newtonian fluid. However, surprisingly, for conditions that correspond to $0.09\le \text{Re}\le 9.86$, falling spheres experience a significant drag reduction. Moreover, an unusually extended wake which spans over a long distance $>80a$ downstream of the sphere is detected by particle image velocimetry. These unusual results could be rationalized by invoking the phenomenon of flow-induced structure formation. We hypothesize that strong shear and/or extensional flows around the falling sphere could trigger aggregation of rodlike micelles into giant wormlike structures. Such wormlike micelles may induce significant sphere drag reduction and extended elastic wakes in the rear of sphere. This interpretation is consistent with the steady shear and transient extensional experiments, whereby a strong shear and elongational thickening have been recovered.

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• Fluid-particle interactions
• Non-Newtonian fluids
• Rheological properties
• Rheology
• Rheology techniques
• Shear deformation