||Extensional flow is important in many processes, causing structure formation as well as break up in complex fluids. In biology, there us extensional flow during silk formation by spiders, while in industrial applications it is important during extrusion, for example in 3D printing done by our industry partner. The challenge is to understand the mechanical response of complex fluids in extensional flow and the underlying microstructural response, especially for (bio-) polymers as main constituents of complex fluids. Capillary breakup extensional rheometry (CaBER) became the main technique to mechanical characterize extensional flow. Here, a liquid droplet is placed between two plates and elongated by pulling the plates apart with a defined speed, resulting in liquid filament thinning with an ideal uniaxial extensional flow. CaBER has been successfully employed to measure the extensional viscosities of polymer solutions. However, the ordering dynamics of polymers in extensional flows are still controversial as in situ optical techniques like birefringence are difficult to perform and interpret due to the small waist of the standing capillary that is formed and used in this experiment. Thus, the microscopic structural evolution giving rise to the macroscopic extensional flow still needs to be unravelled. We propose to combine CaBER with SANS to resolve structural changes in-situ, making use of the high contrast that SANS offers. Our novel CaBER instrument facilitates SANS experiments, directing the neutrons along the axis perpendicular to the required optical axis. As in microfluidics, the scattering volume of the liquid thread is a limiting factor. However, by operating SANS in the time-stamping mode, we will collect data from a multi-fold of stretches. Here, the large beam waste of the SANS line will be of practical use, as the liquid thread will always be inside of the beam. To conclude, we will gain unprecedented information on polymer configuration during stretching.