# Studying the Tribological Properties of Coffee Oil-Loaded Water-Based Green Lubricant
## 3 Results and Discussions  
### Physicochemical Properties of Investigated Lubricating Samples  
The viscosity of lubricating fluid plays an essential role in its lubricity. The higher viscosity-possessing lubricants can separate interacting surfaces at lower speeds. Moreover, they can withstand higher loads. However, the lubricant's viscosity must fit the need of the particular application. The kinematic viscosities of investigated samples at two different temperatures are shown in Figure 3. As could be expected, the lubricating fluid's viscosity increased with an increasing amount of coffee oil. However, with increased test temperature, the viscosity sharply decreased, and the impact of coffee oil on viscosity diminished. The reason for such behaviour is that the viscosity of coffee oil, containing saturated fatty acids, varies with temperature [26; 27; 28]. Despite the low concentration, the amount of PIL also influences viscosity. Interestingly, the amount of PIL has a greater effect when the concentration of coffee oil increases. For instance, when the lubricating sample contained 2.5 wt.% CO, the viscosity difference between the two PIL concentrations was only 0.11 mm/s\({}^{2}\). When the amount of CO increased to 10 wt.%, the above-mentioned difference became 0.97 mm/s\({}^{2}\). The higher concentration of PIL may disperse more coffee oil, while at a low concentration, droplets of coffee oil remain in the dispersion and do not increase the viscosity. The density of investigated samples plotted against temperature is shown in Figure 4. An increased amount of coffee oil decreased density, while the amount of PIL had almost no effect. However, the increment in density due to coffee oil was marginal compared to that of temperature. The density was almost linearly dependent on the temperature within the investigated temperature interval. The acidity of the lubricating fluids could result in metal surface corrosion. Therefore, in most cases, it is kept slightly base. This study investigated samples with pH ranging between 8 and 9 (Table 4). Due to a high acid number, coffee oil increased the acidity of the lubricating sample. At the same time, PIL makes it alkaline. The wettability of investigated samples, expressed in contact angle, is presented in Figure 5. The better wettability-possessing lubricants can more easily penetrate the interface and spread on the interacting surfaces. Therefore, wettability is vital in mixed and boundary lubrication regimes, which occur during recipro  
Figure 3: Kinematic viscosity as a function of the amount of coffee oil in investigated lubricating samples at the temperatures of 30 \({}^{\circ}\)C (a) and 70 \({}^{\circ}\)C (b). In the present case, additives improved the wettability of the base fluid, and most of the additive-loaded lubricating samples showed a reduced contact angle. The contact angles measured immediately and after 60 s are different. Time is required to spread the lubricant's droplet on the surface. According to the observed results, polar molecules of PIL governed wettability. Therefore, a higher number of PIL-containing samples showed a higher contact angle than the contact angle. The contact angles measured immediately and after 60 s are different. Time is required to spread the lubricant's droplet on the surface. According to the observed results, polar molecules of PIL governed wettability are more likely to be affected by the contact angle. Therefore, a higher number of PIL-containing samples showed a higher contact angle than the contact angle. \begin{table}
\begin{tabular}{l c} \hline \hline
**Lubricating Sample** & **pH** \\ \hline W + CO2.5 + PIL0.25 & 8.62 \(\pm\) 0.026 \\ W + CO2.5 + PIL1 & 8.78 \(\pm\) 0.046 \\ \hline W + CO5 + PIL0.25 & 8.26 \(\pm\) 0.022 \\ W + CO5 + PIL1 & 8.53 \(\pm\) 0.034 \\ \hline W + CO10 + PIL0.25 & 8.02 \(\pm\) 0.017 \\ W + CO10 + PIL1 & 8.52 \(\pm\) 0.011 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: pH values of investigated lubricating samples.