Table 2 Corner frequency, relaxation time, and estimated length scale of local

agglomeration obtained from the data Nanofluid system f c (Hz) τ (ms) L A (μm) ZnO 23 ± 1.5 4 ± 3 18 ± 2 ZnO + PVP 43 ± 2.3 2 ± 1 13 ± 2 The thermally driven local aggregation, which would enhance the local thermal transport and hence the value of the thermal conductivity, would lead to solid-like aggregated region in the nanofluids. It is proposed that the response of the type shown in Equation 5 is a manifestation of this local aggregation. The local aggregates respond to an oscillating temperature field δT 2ω with a characteristic thermal relaxation time τ c . This will be related to the characteristic length scales of the local aggregate L A through the thermal diffusivity D by the relation τ c  ≈ D −1 L A 2. The

relaxation eFT508 supplier time will determine the corner frequency f c  ≈ (4πτ c )−1 (the GS-1101 order extra factor of 2 arises because the temperature oscillation is at frequency 2f). For frequencies larger than 2f c , the temperature oscillation is too fast for the aggregate to respond leading to a decrease in the enhancement of heat transport. In Table 2, we show the characteristic time τ c as well as the aggregation length L A as derived from the data. We find that the addition of the stabilizer leads to the reduction of the aggregation length L A by 25% to 30%. The corresponding reduction in effusivity or the thermal conductivity is around 40%. This agrees well with the hypothesis that the local aggregation can control the enhancement of the thermal transport as well as the frequency response. Conclusions We have investigated the dynamical thermal property (effusivity and thermal conductivity) of ZnO nanofluids containing ZnO nanocrystals with an average

diameter of 10 nm with and without PVP stabilizer. This was done to investigate the role of the stabilizer in the enhancement of thermal transport properties of nanofluids. It had been suggested that thermodiffusion-assisted ‘solid-like’ local aggregation of the nanoparticles PAK5 in the nanofluids can be the origin of enhancement of thermal conductivity in nanofluids. The investigations carried out on bare ZnO nanofluids as well as PVP-stabilized nanofluids show that addition of a stabilizer, which inhibits diffusion-assisted local aggregation due to attached moiety, leads to reduction in the enhancement of thermal parameters that are observed in bare ZnO nanofluids. It has also been shown, from characteristic time scales of the dynamic thermal measurements, that the scale of aggregation gets reduced in the addition of stabilizers. The experimental results provide evidence that the origin of enhancement of thermal conductivity in nanofluids can arise from local aggregation that occurs by thermodiffusion.