## 2 Experimental Methodology
### Measurements of the Convective Heat Transfer Coefficient  
The forced convective thermal loop (Figure 1a,b) consisted of a hot and a cold section through which the working fluid circulates. The components in both sections were connected with flexible PVC tubing (Nalgene(r), \(d_{o}=\)5/16\" and \(d_{i}=\)3/16\"; Thermo Fisher Scientific Inc., Waltham, MA, USA). The flow lines were thermally insulated with a standard rubber tubular pipe insulator. The grip and seal on all PVC tubing connections were provided by worm drive clamps. The hot section was constituted by the heating bath circulator utilized for the \(k_{NF}\) and \(\rho_{NF}\) measurements (see Section 2.2) operating at 70 \({}^{\circ}\)C, as well as the CFI test section, as shown in Figure 2. The CFI set was constructed from malleable copper tubing (wall thickness 0.030\" and \(d_{o}=\)\(\frac{1}{4}\)\"; Mueller Industries, Memphis, TN, USA). The set consisted of four helical arms, each constituting four turns with a radius of curvature of 0.031 m (\(\lambda=12.8\)) and a pitch of 0.01 m. The \(\lambda\) closely approximated the optimum value suggested by [PERSON] and [PERSON] [33]. The tubing was bent to form a 90\({}^{\circ}\) angle between two consecutive arms. The approximate internal volume of the CFI set based on its dimensions (i.e., internal diameter and total length), was 0.064 L. The total volume of working fluid necessary to fill the whole circuit was 0.45 L.  
The temperatures of the working fluids (i.e., deionized water and the NFs described in Section 2.1) were measured by 14 calibrated thermocouples (Type K, \(d=\)0.5 mm, \(\pm\)0.4%; OMEGA Engineering, Norwalk, CT, USA), placed as indicated in Figures 1a and 2. The inlet and outlet temperatures of the working fluid in the CFI set were measured by thermocouples \(T_{0}\) and \(T_{13}\) in Figure 1a, respectively. The temperatures at the intermediate points along the test section (Figure 2) were recorded by 12 thermocouples inserted through the cooper tubing into approximately the centerline of the tube. These thermocouples were fixed with cyanoacrylate adhesive (Loctite(r), Henkel, Rocky Hill, CT, USA), paste epoxy (PC-11(r), PC-Products, Allentown, PA, USA), and conventional silicon sealant. All thermocouples were connected to a data logger (OMB-DAQ-56 and OMB-PDQ2, OMEGA Engineering, USA) for data acquisition. The cold section consisted of three positive displacement pumps connected in parallel (two gear pumps (models PQ-12 DC, Cole Parmer, USA) and a peristaltic pump (model Masterflex L/S Easy-LoadII Head coupled with a Masterflex L/S Variable-Speed Drive, Cole Parmer, Vernon Hills, IL, USA)), a rotameter (0.2-1.9 L per minute (LPM), \(\pm\)2% full scale; model H205B-005 Hedland, Budger Meter Inc., USA), a benchtop recirculating chiller (750 W at 0 \({}^{\circ}\)C; model LS51 MX1A110C, Polyscience Inc., USA), a jacketed glass beaker (500 mL; model CG-1103-04, Chemglass Inc., Vineland, NJ, USA), and a helical section (six turns, radius of curvature = 0.063 m, and pitch = 0.02 m) made of the same copper tubing as the CFI set. The working fluid passed from the test section to the cooling loop through the helical section immersed in the chilled water. From there, it was received in the jacketed glass beaker, which served as the reservoir for discharge and suction (tap water was circulated through the jacket). Thus, the working fluid was auctioned and circulated through the test section by the pump system. The flow rate was manipulated by the analog speed controller of the peristaltic pump and the plug-in class-2 transformers implemented in the gear pumps. Five flow rates that provide \(N_{Rc}\) values ranging from 1400 to 9500 were selected. The rotameter was calibrated for each working fluid. The temperature of the chilled water was adjusted to maintain the inlet temperature of the working fluid to the CFI set within a maximum variation of \(\pm\)0.5 \({}^{\circ}\)C for all working fluids at the designated flow rate. Trials were conducted at least twice and all systems reached steady state.