Assessing Shear-induced Scission of Wormlike Micelles Using Flow-Small Angle Neutron Scattering*

Wormlike micelles (WLMs), long thread-like surfactant aggregates, remain an important class of surfactant-based materials in consumer products and oilfield applications. Although it is widely accepted that equilibrium micelle scission greatly influences the rheology of WLMs, due to the lack of direct structural measurements, there is still considerable theoretical debate regarding whether scission is affected by flow. We report new flow-small angle neutron scattering experiments on a carefully designed series of linear, weakly entangled WLMs. We find that the low-q intensity monotonically decreases with increasing shear rate, consistent with the net breakage of the WLMs. We use model calculations of scattering from deformable objects in flow to deconvolute the contributions to the scattering from changes in micelle orientation, stretching, and interactions from effects due to changes in micelle length in flow. We find that without including micelle scission, none of these other factors is able to explain the qualitative trends in experiments. This suggests that the length distribution of wormlike micelles is significantly affected by shear and opens up new possibilities for measuring and modeling the effects of flow on the reversible scission of WLMs.