## 2 Instrumental Concept and Performance
### Temperature Control  
Temperature change of the foams is achieved by two AlMg\({}_{4.5}\)Mn\({}_{0.7}\) thermostating jackets at both sides of each foam cylinder. The thermostating jackets of all three sample cells are connected via appropriate distributors to a Julabo FP50-HL circulating thermostat (Julabo GmbH, Seelbach, Germany) with a temperature range of \(-50\,\mathrm{\SIUnitSymbolCelsius}\) to \(200\,\mathrm{\SIUnitSymbolCelsius}\) when the appropriate thermofluid is used. For studying aqueous foams, however, an achievable temperature range of \(10\,\mathrm{\SIUnitSymbolCelsius}\) to \(80\,\mathrm{\SIUnitSymbolCelsius}\) is typically sufficient, allowing the use of water as thermofluid. To validate the performance of the heat input by the thermostating jackets, tests with a foam stabilized by C\({}_{14}\)TAB at its critical micelle concentration of \(c=3.5\) mM were performed [34]. Therefore, \(12\,\mathrm{mL}\) of the surfactant solution were foamed at ambient temperature with a synthetic air flow of \(15\,\mathrm{mL}\,\mathrm{min}^{-1}\). After the foam reached the top of the quartz cylinder, the gas flow was stopped and the setpoint of the water bath thermostat was adjusted to \(50\,\mathrm{\SIUnitSymbolCelsius}\). The evolution of the temperature was monitored by three Pt-100 temperature sensors (model PT-102-35-QT, Lake Shore Cryotronics, Inc., Westerville, OH, USA) at different positions at a height of \(11.5\,\mathrm{cm}\) inside of the foam cylinder. The positions were chosen in a way to reflect the asymmetric shape of the thermostating jacket around the foam cylinder as indicated by A, B, and C in Figure 4. One sensor was put in the center of the cylinder (C), while the remaining two sensors were placed on the rim of the foam column, one in close proximity to the thermostating jacket (A) and one right behind the neutron window slit (B). The evolution of the temperature and a schematic sketch of the different positions are shown in Figure 4a.  
Following an initial increase, the temperature reaches a plateau at all three positions after around \(20\,\mathrm{min}\). The final temperatures were \(48\,\mathrm{\SIUnitSymbolCelsius}\) at position A (close to thermostating jacket), \(42\,\mathrm{\SIUnitSymbolCelsius}\) at position B (behind neutron window slit), and \(45\,\mathrm{\SIUnitSymbolCelsius}\) at position C (center). This temperature gradient is explained by the asymmetric shape of the thermostating jacket around the foam cell and the low heat conductivity of foams. As explained in Section 2.1, this asymmetric shape was chosen to allow SANS measurements at any height along the foam cylinder, accepting the drawback of a potential temperature gradient. The temperature jumps observed at every position are most likely due to air bubbles passing by the sensors, changing the heat conductivity next to them.  
In order to reduce this temperature gradient, the experiment was repeated with the foam cylinder wrapped in aluminum foil (see Figure 4b). Again, the temperature reaches a plateau after around \(20\,\mathrm{min}\) with final temperatures of \(49\,\mathrm{\SIUnitSymbolCelsius}\) at position A, \(47\,\mathrm{\SIUnitSymbolCelsius}\) at position B, and \(48\,\mathrm{\SIUnitSymbolCelsius}\) at position C. Wrapping the cylinder in aluminum foil decreases the temperature gradient inside the foam because of the improved thermal contact between the quartz cylinder and the thermostating jackets. In addition, aluminum is almost transparent to neutrons and is also often used as a material for neutron windows. A drawback of this approach is that it is no longer possible to observe the foam during a SANS experiment with a camera. Detecting the formation of holes at the measuring position is sometimes beneficial, especially when dealing with rather unstable foams where holes may form randomly. Depending on the requirements regarding the accuracy of temperature control, one of the two methods described can be used.