Simultaneous mapping of droplet deformation and banding in surfactant-free emulsions by multidirectional neutron dark-field imaging

Soft matter systems, such as colloids, emulsions, and polymer solutions, are common in nature and industry. A key feature is that their structures respond to external mechanical forces, such as shear flow, leading to complex non-linear behavior and flow-induced phase transitions. In this study, we explore the interplay between flow and microstructure in a Couette cell geometry. As a sample, we select a surfactant-free glycerol-in-silicone oil mixture, which was recently reported to form a stable emulsion, not coalescing over time. To probe the structural changes of this emulsion due to shear flow, we pioneer the use of in situ neutron dark-field imaging (DFI) using a single hexagonal grating, introducing a novel imaging technique capable of capturing spatially resolved anisotropic small-angle neutron scattering (SANS) signals. We demonstrate droplet elongation due to shear forces and the appearance of reversible bands with alternating droplet density. By simultaneously creating a full field map and measuring droplet deformation and orientation during shear, DFI enables in situ structural characterization of flowing soft matter. This structural mapping is complemented by the simultaneous monitoring of the macroscopic rheological response of the fluid, resulting in a multimodal probe for soft matter structure. We demonstrate DFI to be a powerful methodology to uncover in situ structural changes under flow, which can exploit common neutron contrast matching techniques. DFI can be applied to concentrated and opaque fluids, uncovering structures from the nanometer to the micrometer range, thereby complementing the typical sample and size limitations of SANS.