## 2 Materials and Methods
### Emulsification  
The double emulsion was produced in a two-step process, using different emulsification techniques. First, five different \(W_{1}\)/O emulsion were produced with the rotor-stator system \"Megatron MT3000\" (Kinematica AG, Malters, Switzerland) using a two row gear ring at a rotational speed of 20,000 min\({}^{-1}\) (circumferential speed = 27.2 m/s) for 90 s. These emulsions serve as the inner emulsion in the subsequently produced double emulsions.  
The droplet size distribution of the emulsions with 16%, 24%, 33% and 61% ammonium nitrate was measured by static laser light scattering using a particle analyzer HORIBA LA-950 (Microtrac Retsch GmbH, Haan, Germany). The Sauter mean diameter of the inner emulsions are in the range of 2.07 \(\upmu\)m (33% ammonium nitrate) to 2.67 \(\upmu\)m (16% ammonium nitrate). The respective droplet size distributions are shown in Figure 2. The emulsion containing 49% ammonium nitrate in the water phase could not be measured as a result of the similar refractive indices. Since all other emulsions show similar droplet size distributions, it is assumed that the droplet sizes do not deviate in this set.  
Figure 1: Raman spectra of all phases. For the inner and outer water phase, only the lowest and highest concentrations of ammonium nitrate and glycerol are shown.  
Figure 2: Droplet size distribution of the inner emulsions.  
Figure 3 shows the five different inner emulsions. The emulsions become more and more transparent with the increasing concentration of dissolved ammonium nitrate. At the best matching (\(\Delta\)N = 0.002 at 49% ammonium nitrate), the emulsion is optically clear. At 61% ammonium nitrate, the optimum is exceeded, and the emulsion appears milky.  
In order to obtain monodispersed W\({}_{1}\)/O droplets in a double emulsion, these inner emulsions were then emulsified in the outer water phase using a microfluidic glass capillary system [30]. The glass capillaries used are placed in two Teflon blocks, which are connected to each other by a plug-in connection. Syringe pumps (Legato 100, kdScientific Inc., Holliston, MA, USA) were used to inject the inner W\({}_{1}\)/O emulsions and the outer water phases into the microfluidic device. The inner emulsion was pumped at 2 mL/h and the outer water phase at 10 mL/h. This leads to a droplet breakup rate of 3 Hz to 5 Hz with a droplet diameter of 1.15 mm. Figure 4 shows a double emulsion droplet flowing through the larger squared capillary.  
The microfluidic device is designed in a co-flow system, i.e., the inner emulsion and the outer phase flow parallel to each other. When both phases come into contact, the inner emulsion breaks up into droplets in the outer phase. Since the breakup to the double emulsion takes place in a round borosilicate glass capillary (ID: 1.12 mm, WPI, Sarasota, FL, USA), a square quartz capillary (ID: 1.00 mm, CM Scientific Ltd., Silsden, UK) is connected to the round capillary via an internally designed 3D-printed nylon adapter. For online Raman measurements, a square quartz capillary was mounted. Thus, neither the capillary  
Figure 4: An individual double emulsion droplet flowing through the squared capillary. The scale bar is equal to 1 mm.  
Figure 3: The investigated W\({}_{1}\)/O emulsions with the respective ammonium nitrate (AN) concentrations.  
material nor its curvature has a negative influence on the Raman measurement. A schematic of this setup is shown in Figure 5. On the right side, the conical, green, vertical bar indicates the laser beam for Raman excitation. It is emitted by the backscatter probe, which also detects the Raman signal.