A critical analysis of clustering phenomenon in Al₂O₃ nanofluids

In the present work, an investigation on the relationship between clustering phenomenon and thermal conductivity of nanofluids is presented. Particularly, an experimental campaign was carried out to correlate mean dimension of cluster, ranging from 168 to 20,933 nm, to nanofluid's thermal behavior. A further objective of this study was to evaluate how the stability of nanofluid can affect thermal conductivity measurements, carried out by means of hot-wire technique. Experimental results showed that thermal conductivity, measured at constant volume concentration of nanoparticles as a function of cluster dimension, first decreases and then augments, as it was under a dual effect: negative in case of small clusters and positive with big clusters within nanofluid. Actually, further measurements of zeta potential and backscattered light demonstrated that clustering reduces nanofluid's thermal conductivity, while its increment can be related to sedimentation of clusterized particles, which produces convective motion around the hot wire, generating overestimated measurements.