## 2 Materials and Methods
### Raman Measurement  
The RNX1-532 Raman spectrometer (Kaiser Optical Systems, INC., Ann Arbor, MI, USA) is used for the Raman measurements. It is equipped with a laser at a wavelength of 532 nm and a laser power of 150 mW. The associated backscatter probe (Kaiser NCO-0.5-VIS) irradiates the sample with a half-inch lens, producing a focal diameter of 55 um with a focal length of 500 um.  
Spectra of all phases have been measured, see Figure 1. These were measured at 1s integration time with 3 accumulations in a standard spectroscopy cuvette (Type: 100-10-40, Hellma Analytics, Mullheim, Germany), allowing the spectra to be quantitatively compared. Ammonium nitrate shows one dominating peak in the fingerprint region [38] at 1047 cm\({}^{-1}\)[39], which is the highest peak of all phases in this area. The oil phase has only minor peaks in the shorter shifted fingerprint region at 489 cm\({}^{-1}\) and 707 cm\({}^{-1}\)[40], while the outer water phase shows multiple smaller peaks in the range between 800 cm\({}^{-1}\) to 1200 cm\({}^{-1}\) which are attributable to glycerol [41]. Specifically, there is a peak at 1060 cm\({}^{-1}\), which overlaps significantly with the ammonium nitrate peak at 1047 cm\({}^{-1}\). Therefore, a baseline correction is applied in the evaluation to reduce the influence of glycerol.  
For the different concentrations of both water phases, the linearity of the peak height at 1047 cm\({}^{-1}\) (ammonium nitrate) and 1060 cm\({}^{-1}\) (glycerol) is investigated. The respective Pearson coefficients are R\({}^{2}\) = 0.992 (ammonium nitrate) and R\({}^{2}\) = 0.983 (glycerol).  
For the baseline correction, the two endpoints are determined from a spectrum of ammonium nitrate solution. These are located at 1030 cm\({}^{-1}\) and 1060 cm\({}^{-1}\). First, the total integral is calculated in this range. Subsequently, the slope of the baseline is determined using the measured values at 1030 cm\({}^{-1}\) and 1060 cm\({}^{-1}\) and the integral between the x-axis and the baseline is calculated. The difference of the two integrals gives the integral of the baseline corrected ammonium nitrate peak.  
To measure a significant number of droplets per experiment, each experimental set is measured for 400 s in total. This time is divided into five individual measurements of 80 s, which in turn are divided into 4 accumulations of 20 s. In the end, five spectra are obtained of each experimental set, which are combined into one data point.  
\begin{table}
\begin{tabular}{c c c c} \hline \hline  & **Phase** & **Mean Value** & \(\Delta\)**N** \\ \hline \multirow{8}{*}{W\({}_{2}\)} & 00.00\% Glycerin & 1.3385 & 0.0642 \\  & 10.00\% Glycerin & 1.3513 & 0.0514 \\  & 20.01\% Glycerin & 1.3635 & 0.0392 \\  & 30.01\% Glycerin & 1.3782 & 00245 \\  & 40.00\% Glycerin & 1.3917 & 0.0110 \\  & 49.04\% Glycerin & 1.4044 & \(-\)0.0017 \\  & 60.00\% Glycerin & 1.4189 & \(-\)0.0162 \\  & 70.00\% Glycerin & 1.4317 & \(-\)0.0290 \\ O & 2.02\% Dowsil Resin XR 0497 & 1.4027 & \\  & 16.33\% Ammonium nitrate & 1.3569 & 0.0458 \\  & 24.50\% Ammonium nitrate & 1.3662 & 0.0365 \\ W\({}_{1}\) & 32.46\% Ammonium nitrate & 1.3771 & 0.0256 \\  & 48.91\% Ammonium nitrate & 1.4007 & 0.0020 \\  & 61.25\% Ammonium nitrate & 1.4200 & \(-\)0.0173 \\ \hline \hline \end{tabular}
\end{table}
Table 1: Summary of the refractive indices for each phase. Each mean value is based on three measurements. Each set of measurements has a standard deviation of less than 0.0001. \(\Delta\)N is the difference between the refractive index of the oil phase compared to the water phases.