## 2 Sample Environment Components  
### Humidity Chamber  
We designed a spherical 3D-printed humidity metal chamber that provides precise control over temperature and, in conjunction with the gas flow assembly, humidity while offering a relatively large volume that can accommodate large samples and additional, interchangeable equipment. It is an upgraded version of a previous design of ours, with an improved control over environmental parameters and an easier general handling of the chamber [43]. The spherical design depicted in Figure 1 enables a homogeneous heat distribution for isothermal and temperature switching experiments while simultaneously preventing condensation due to the lack of sharp edges and corners, which are usually critical points for condensation [44]. The chamber size was reduced to a diameter of 146 mm, with a plane-to-plane distance of the two windows of 138 mm. It is still able to accommodate samples up to a size of 70 \(\times\) 70 mm\({}^{2}\), while the smaller overall chamber volume of 1.0 L compared to the previous volume of 1.4 L allows for faster temperature and humidity switching kinetics. The connection between chamber and lid was changed to a flat geometry in order to facilitate the accurate positioning of the chamber lid, on which the setup for spectral reflectance measurements is mounted. The windows have a height of 30 mm and a width of 90 mm to enable NR measurements. The angle between a window edge and the furthest sample edge (of a 70 \(\times\) 70 mm\({}^{2}\) sample placed in the center) measures to 8.2\({}^{\circ}\), well above the typical scattering angles needed for both NR and GISANS measurements. This ensures that the 3D printed material is not hit by the incident nor the scattered neutron beam (which would produce unnecessary background). It was shown with the previous chamber design that the 3D printed material yields a strong scattering signal when directly hit, but it generates no detectable background otherwise [43; 45; 46]. As such, NR and GISANS measurements with no measurable background contribution from the chamber itself were already demonstrated. Within the 10-mm-thick walls of the chamber, a pathway of a 6-mm-diameter fluidic channel for temperature control is centered and runs around the chamber, as shown in Figure 1b. The total path length is 2.2 m, thus providing ample contact area for a thorough heat transfer. Figure 1: (**a**) Wireframe model with important measures indicated in mm. (**b**) Chamber with the outer surface hidden in order to reveal the fluid pathways embedded within the chamber walls. (**c**,**d**) Chamber printed out of \(\text{AlSi}_{0}\text{Mg}\) with all contact surfaces polished. The chamber is 3D printed from an AlSi10 Mg aluminum alloy powder via selective laser melting (SLM) using a metal 3D printer (EOS M290, PROTIQ GmbH, Blomberg, Germany). During the layer-by-layer printing process, the construction chamber was flooded with an inert gas to prevent the oxidation of the material. Afterwards, a heat treatment ensured the homogenization of the intermetallic phase and reduced grain boundaries within the bulk material, improving the direction-independent tensile strength of the chamber. Lastly, the contact surfaces were polished flat to ensure the airtight sealing of the chamber, thereby yielding the printed chamber and lid shown in Figure 1c,d. The lid of the chamber is designed to accommodate the optical lens system of the spectral reflectance setup, while the holes at the side of the chamber are designed as connections to either the internal liquid flow channel or the inside of the chamber, providing in and outlets for the gas flow system and additional electronics. For measurements, the three pillars at the bottom of the chamber, visible through the chambers window in Figure 1d, are used to mount an aluminum plate with three vertical pins that can be adjusted to accommodate various sample sizes from 10 \(\times\) 10 to 70 \(\times\) 70 mm\({}^{2}\).