# Investigation of the Behaviors of Methanol Spray Impingement and Wall Wetting
## 2 Experimental Setup and Methodology
### RIM Calibration  
Before analyzing the wall wetting images, according to previous studies [25, 29, 30], it is the premise that a quantitative relationship is established between the change of the scattered light and the local film thickness at each pixel. The local scattered light change \(\Delta I(x,y)\) at the coordinate point (x, y) is calculated by Equation (2),  
\[\Delta I(x,y)=1-\frac{I_{wet}(x,y)}{I_{dry}(x,y)} \tag{2}\]  
where \(I_{wet}(x,y)\) and \(I_{dry}(x,y)\) are, respectively, the local intensity of the images with and without adhesive liquid.  
At 293 K and 94.7 kPa, the mixed solution of methanol and n-octanol was used to calibrate the RIM method. Figure 4 showed the curve of the average light intensity with time after 0.5 uL solution (99%vol methanol and 1%vol n-octanol) was deposited on the surface of the glass. As illustrated, the process from the beginning of the curve to the lowest intensity was dominated by the solution spread. The rise of the curve indicated that the evaporation was in dominant control [25]. Methanol evaporates faster than n-octanol,  
Figure 3: The impinging spray image from this procedure and impinging spray height and width were defined.  
resulting in the rising process of the curve undergoing two stages; in stage \(I\), the average light intensity of ROI was increasing rapidly with the evaporation of methanol. At 7.2 s, the growth rate of ROI average light intensity curve slowed down significantly. It could be considered that the methanol completely evaporated at this time, and the subsequent increase of ROI average light intensity was caused by the evaporation process of n-octanol (in stage _II_). The intersection between the two segments was the calibration point.  
Further, the experiment was repeated with deposited different total volumes of mixed solution, many times. The volume and mixing ratio of mixed solution used are given in Table 3.  
As shown in Figure 5, the relationship curve between the adhesive fuel film thickness \(T_{f}\) and the change of light intensity was obtained. Then, the fitting formula was given in Equation (3), and the R\({}^{2}\) was 0.9817.  
\[T(x,y)=7.89872\times\Delta I^{2}(x,y)+1.12507\times\Delta I(x,y) \tag{3}\]  
In addition, in the early stage of the formation of the adhesive fuel film, the images were overexposed because the light was scattered by droplets in spray, which caused deviations in the image processing results. Therefore, results of adhesive fuel film from RIM only, from 10 ms or 20 ms ASOI, were available for analysis.  
Figure 4: Evaporation curve of methanol and n-octanol mixed solution after deposit.  
\begin{table}
\begin{tabular}{c c c c} \hline \hline \multirow{2}{*}{**Mixed Solution**} & **Proportion of** & **Proportion of** & \\  & **Methanol/\%vel** & **n-Octanol/\%vel** & **Deposited Volume/\(\mu\)L** \\ \hline M95O5 & 95 & 5 & 0.5, 0.6, 0.7, 0.8, 0.9, 1.0 \\ M97O3 & 97 & 3 & 0.5, 0.6, 0.7, 0.8, 0.9, 1.0 \\ M99O1 & 99 & 1 & 0.5, 0.6, 0.7, 0.8, 0.9, 1.0 \\ \hline \hline \end{tabular}
\end{table}
Table 3: The volume and mixing ratio of mixed solution.