Scattering Insights into Flow-Induced Scission and Alignment of Rod-Like Micelles

Guan-Rong Huang^1,2*, Ryan P. Murphy³, Lionel Porcar⁴, Chi-Huan Tung⁵, Changwoo Do⁵, Wei-Ren Chen⁵

¹Engineering and System Science, National Tsing Hua University, Hsinchu, Taiwan
²Physics Division, National Center for Theoretical Sciences, Taipei, Taiwan
³NIST Center for Neutron Research, National Institute of Standards and Technology, Gaithersburg, 20878, MD, USA
⁴Neutrons for Society, Institut Laue-Langevin, Grenoble, France
⁵Neutron Scattering Division, Oak Ridge National Laboratory, Oak Ridge, 37831, TN, USA

* Presenter:Guan-Rong Huang, email:huangrn@ess.nthu.edu.tw

Understanding how rod-like micelles undergo scission under mechanical stress is essential for controlling their stability and rheological behavior in industrial and biological systems. In this study, we investigate the interplay between micellar length, flexibility, and external shear forces using small-angle neutron scattering (SANS) combined with rheological measurements on aqueous cetyltrimethylammonium bromide (CTAB)/sodium nitrate solutions. Steady-shear tests reveal shear thinning without shear banding, confirming a uniform flow field suitable for quantitative scattering analysis. With increasing shear rate, the SANS patterns exhibit pronounced angular anisotropy, which is analyzed through spherical harmonic decomposition to resolve micellar orientation and length evolution.

A two-step analysis is employed: (1) a model-independent spectral eigendecomposition identifies a systematic reduction in micellar length, and (2) regression-based reconstruction quantifies changes in the length distribution and mean contour length as functions of shear rate. Concurrently, the orientational distribution function demonstrates increasing micellar alignment with flow. These results provide direct experimental evidence of flow-induced alignment and scission in rod-like micellar systems, establishing a quantitative framework for interpreting shear-dependent microstructural transformations. (3) the scaling law of mean contour length and shear rate is consistent with the results predicted by first-principles analytical theory and dissipative particle dynamics.

Keywords: Mechanically driven rod-like micellar solution, Small angle neutron scattering, Real spherical harmonic expansion, Soft Condensed Matter