(454i) Flow Heterogeneity and Flow Reversal in High Elasticity Poloxamer Wormlike Micelles (WLMs) with a Yield Stress

A significant body of work has examined the nonlinear flow behaviour of viscoelastic wormlike micelles, but in some formulations WLMs behave like gels,¹ with extremely long relaxation times and apparent yield stresses. In these gel-like WLMs, the nonlinear flow behaviour to-date is not as well characterized. Here, shear flows of WLM gels formed from 15% wt poloxamer P234 in 2 M NaCl are examined and revealed to be heterogenous over a range of shear rates with a fluidized region adjacent to an immobile, solid-like region. Using a combination of nonlinear rheology and particle tracking velocimetry, the formation and dynamics of this heterogeneous flow are examined during shear start-up. The heterogeneous flow formation more closely resembles that in yield stress fluids, due to stress heterogeneity in the flow geometry rather than the constitutive-instability-driven shear banding like in viscoelastic WLMs. Interestingly, heterogeneous flow is preceded by a period of flow reversal, a phenomenon where a portion of the fluid moves opposite to the imposed flow direction. Flow reversal has been observed in yield stress fluids² and only recently in wormlike micelles (WLMs),³ where flow reversal has been correlated with WLM elasticity, wall slip, entanglement density, and flow curve stress plateau width.⁴ By analysing flow reversal experimentally in these poloxamer P234 WLMs and related WLM systems, and by performing complementary cessation of flow simulations, we then demonstrate that two key ingredients are necessary for flow reversal: elasticity (recoverable strain) and relaxation time heterogeneity along the flow gradient axis. In addition to validating these criteria, we demonstrate that cessation of flow protocols combined with particle tracking velocimetry can glean a wealth of information about the fluid elasticity, relaxation time, and heterogeneity during shear start up.

¹ S.R. Raghavan and Y. Feng, Wormlike micelles: solutions, gels, or both? Royal Society of Chemistry (2017).

² T. Divoux, et al., Soft Matter. 7, 9335 (2011).

³ H. Mohammadigoushki, Soft Matter. 15, 1061 (2019).

⁴ P. Rassolov and H. Mohammadigoushki, J. Rheol., 67 169 (2023).