## 2 Instrumental Concept and Performance  
### General Construction  
The main parts of the sample environment are three foam cells, in which the foam will form. Figure 1a,b show an engineering drawing and a photograph of a single foam cell. Each cell consists of a 250 mm long quartz glass cylinder with an inner diameter of 30 mm and a wall thickness of 2 mm. Quartz is almost transparent to neutrons and the circular design avoids the rupture of lamellae at edges. A porous quartz glass plate (pore size 10-16 \(\upmu\)m, porosity P16 (ISO 4793)) is fused to the bottom of the cylinder. The whole cylinder is mounted to a gas-inlet socket via an O-ring in a crimp connection. From below, a gas (e.g., air or nitrogen) is pressed through the porous plate, which breaks the continuous gas flow into small bubbles. This results in the formation of foam, when an appropriate foaming solution is poured into the cylinder. Finally, the gas exits the cylinder at the open top, which ensures pressure equilibrium. For temperature control, two thermostating jackets are fitted to each foam cylinder, using titanium screws from the bottom of the gas-inlet socket. One of the main features and advantages of this foam cell is the possibility to measure at any position along the height of the foam cylinder. Therefore, the thermojackets are designed in a way to leave a slit-shaped gap between them. The width of this gap governs the maximum scattering angle accessible. Figure 1c shows a sketch in top view for the estimation of the maximum scattering angle at the geometrically least favorable position. For a scattering event occurring at the edge of a 10 mm wide primary neutron beam (highlighted in orange) at the side facing the primary beam, the scattering angle limit \(2\theta_{lim}\) is 13.8\({}^{\circ}\). Assuming a neutron wavelength of \(\lambda=5\) A, the maximum scattering vector \(q_{max}\) is calculated by  
\[q_{max}=\frac{4\pi}{\lambda}\sin(\theta_{lim})\approx 0.3\,\text{\AA}^{-1} \tag{1}\]  
This corresponds to a minimum measurable size of 2 nm in real space, which is sufficient to resolve structures such as foam films or incorporated objects such as micelles or nanoparticles. It is worth noting that the SANS instruments LoKI and SKADI at the ESS will be operated with a predicted neutron wavelength band of 2 A to 22 A and 3 A to 21 A, respectively [2]. We expect an accessible \(q\) range of 0.005-0.6 A\({}^{-1}\) for our foam cell at these ESS instruments. The lower limit is governed by the instrument's design and the upper limit by the cell geometry. This will lead to a range of measurable sizes of ca. 1-125 nm. The gas-inlet socket and thermostating cylinder are made of AlMg\({}_{4.5}\)Mn\({}_{0.7}\), a special aluminum alloy that is easy to process and not activated by neutrons. The hose connectors are made of AlMg4.5 and, therefore, are also not activated by neutrons. All three foam cells are placed in a folded AlMg4.5 socket. The side facing the primary neutron beam is protected by an aluminum plate covered with 1 mm B4C including rectangular slits at the respective positions of the foam cells. Since the cells are open to the top, an aluminum cover was placed above the cells to avoid dust incorporation. A backview of the mounted cells is shown in Figure 2a. The setup is placed on a translation stage (travel range 508 mm), which allows remote controlled sample changes between the different foam cells as shown in Figure 2b,c. The entire setup is mounted on a breadboard (900 \(\times\) 1200 mm\({}^{2}\), Newport Spectra-Physics GmbH, Darmstadt, Germany). Later on at the ESS, the entire setup will be mounted on a lifting table, which is also part of the unified carrier system used by all FlexiProb SEs. This will ensure fast setup changes by removing the entire setup including the board. The lifting table installed at LoKI and SKADI will have a vertical displacement range of \(>\)300 mm with a 0.1 mm positional accuracy, which is sufficient for the foam cell SE. Figure 1: Individual foam cell.