## 3 Results and Discussion  
The results from the pressure-viscosity measurements of [Choline] [Proline] at three different temperatures are shown in Figure 3. The curves in the figure are from the improved \begin{table}
\begin{tabular}{l c} \hline \hline \multicolumn{1}{l}{Lubricant} & IL and paraffin \\ Temperature & 40 \({}^{\circ}\)C \\ Contact load, \(F\) & 100 and 300 N \\ Maximum hertzian pressure & 1.25 and 1.95 GPa \\ Entrainment speed, \(U_{e}\) & 1 and 3.14 m/s \\ Slide to Roll Ratio, SRR & 0.0002 to 1.05 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Frictionest conditions. Yasutomi correlation, a pressure modification of the Williams-Landel-Ferry model for temperature dependence [36]. The parameters for the improved Yasutomi correlation can be seen in Table 2.  
Based on these pressure-viscosity measurements, pressure-viscosity coefficients by three different definitions were calculated based on the McEwen model [25]. The results are shown in Table 3. These definitions and calculations assume that [PERSON]'s integral will converge for the liquid, a fact not in evidence. These measured values are substantially lower than pressure-viscosity coefficients usually measured for common hydrocarbon oils that are usually in the range of 15-25 Gpa\({}^{-1}\) at room temperature [23; 37; 38]. The pressure-viscosity coefficients of [Choline] [Proline] are also compared to some other ionic liquids [39; 40; 41; 42; 43; 44; 45], summarized in [46] and shown in Table 4. In this comparison, [Choline] [Proline] IL has the lowest pressure-viscosity response of the investigated ILs. Other researchers have found even lower \(\alpha_{film}\) for [C\({}_{2}\)C\({}_{1}\)Im][C\({}_{2}\)SO\({}_{4}\)] [47]. The large variation in pressure-viscosity coefficients between different kinds of fluids are connected to the differences in molecular structure. The literature is surprisingly sparse in studies on this relationship. [PERSON] [48] has shown that the general trend is that the pressure-viscosity coefficient is increased when going from a few long branches to many short branches. An example is given in Figure 4 where the pressure viscosity response of two isomers of dodecane is shown. \begin{table}
\begin{tabular}{l c} \hline \(\mu_{g}\) [Pas] & \(10^{12}\) \\ \(T_{g0}\) [\({}^{\circ}\)C] & \(-76.20\) \\ \(A_{1}\) [\({}^{\circ}\)C] & \(111.47\) \\ \(A_{2}\) [GPa\({}^{-1}\)] & \(0.5664\) \\ \(b_{1}\) [GPa\({}^{-1}\)] & \(3.240\) \\ \(b_{2}\) & \(-0.1422\) \\ \(C_{1}\) & \(16.625\) \\ \(C_{2}\) [\({}^{\circ}\)C] & \(39.14\) \\ \hline Std. Dev & \(1.5\%\) \\ \hline \end{tabular}
\end{table} 
Table 2: Parameters of the Improved Yasutomi correlation for [Choline] [Proline]. Figure 3: Viscosity pressure curves for [Choline] [Proline] at various temperatures. Here it is clear that the highly branched 2,2,4,6,6-Pentamethylheptane has a significantly higher pressure viscosity response than 5-propylnonane. The same is true when going from linear to cyclic and then to polycyclic structures [48]. It has also been discussed that an increase in side chain will increase the pressure viscosity coefficient [46]. Generally, this seem to hold when observing the values in Table 3. The ILs having a [C\({}_{4}\)C\({}_{1}\)im] cation seem to scale in pressure-viscosity in the order of the branch densities of the anions BF\({}_{4}\), Tf\({}_{2}\)N and PF\({}_{6}\). In general, an increase in chain length of the cation with the same anion also increases the pressure viscosity coefficient for the ILs in Table 3. The [Choline][Proline] with a lower branch density than the ILs in Table 3 therefore has lower-pressure viscosity coefficients. A lower pressure-viscosity coefficient means that the viscosity will not increase as much with pressure compared to a higher pressure-viscosity fluid. This has primarily two effects. The first one