Both the nanofluids show characteristic absorption around λ ≈ 360 nm, which is the absorption edge for ZnO. For the ZnO nanofluid without PVP, the absorption initially decreases with time as shown in Figure 1b. The decrease is rapid initially and then slows down considerably after 30 min, when the absorption MK0683 supplier decreases by about 3% in 1 h. This stability is long enough to carry out the thermal measurements over a period of 2 h. The addition of the PVP leads to a very stable nanofluid that is stable over few weeks as can be seen in Figure 1c where there is no perceptible change in the UV-visible absorption

even after 2 weeks. Figure 1 TEM image of ZnO nanocrystal used. (a) Time dependence of UV–vis spectra for ZnO nanofluids, (b) without and (c) with PVP stabilizer. Thermal measurements using 3ω technique The

thermal measurements were done using a 3ω technique [19–21], where we use a platinum film both as a thermometer and a heater. The method, as applied to nanofluids, is explained elsewhere [15]. Here, we provide a small gist for quick reference. In this method, the Pt film (width of 300 μm, thickness of 50 nm, and length of 5 mm grown on a glass substrate by magnetron sputtering) carrying a current at frequency f is immersed in the liquid in which measurements have to be made [19]. The periodic heating of the film, due to the sinusoidal current, makes the temperature oscillate around the average GSI-IX supplier with an amplitude δT 2ω at a frequency 2ω (ω = 2πf).

This leads to resistance oscillations of amplitude δT 2ω at frequency 2ω around the mean, where δR 2ω  = αR 0 δT 2ω, α is the temperature coefficient of resistance (TCR) of the heater, and R 0 is the average resistance of the heater. The resistance oscillation δR 2ω at frequency 2ω mixes with the current at frequency ω to produce a potential drop ( ) with a component at 3ω (sum band). The experiment measures the complex voltage with its phase and amplitude, using a phase-sensitive detection technique. The thermal properties of the heater-on-substrate (S) and surrounding liquid (L) are given by two parameters Z and the phase φ. These parameters are obtained PAK5 experimentally from the observed 3ω eFT-508 signal , the area of the heater (A), the power dissipated (P), and the measured TCR (α) of the Pt film using the equation [19] (1) where the thermal parameter is the effusivity given as ξ ≡ C p κ. L and S refer to the liquid and the substrate, respectively. The Pt film has a resistance of ≈ 100 Ω and a measured temperature coefficient of resistivity α ≈ 3.5 × 10−3/K. The relative size of the heater width and the thermodiffusion length (D = thermal diffusivity) determines the low-frequency range of the experiment. In our case for the base liquid ethanol (D ≈ 9 × 10−8 m2/s), the working frequency is for the width of the heater used (approximately 300 μm). At high-frequency range, the limit arises due to the low value of the signal.