Anisotropic thermal conductivity in ferrofluids induced by uniform cluster orientation and anisotropic phonon mean free path

Researchers have made great efforts to study the anisotropic thermal conductivity in ferrofluids when an external magnetic field is imposed. However, the underlying mechanism is still confusing because some important factors, such as phonon ballistic transport, thermal resistance at solid-liquid interface, were omitted in the many previous studies. In the present work, we develop a three-level homogenization model based on phonon theory to describe the thermal conductivity in ferrofluids, where liquid adsorbed layer, Kapitza resistance, and phonon size effect are considered. The model is validated by previous experiment data in literature. Our results show the thermal conductivity in clusters becomes anisotropic due to the enhancement of phonon MFP along the chain direction. Other than a well-known reason, i.e. uniform cluster orientation, we discover that the anisotropic thermal conductivity in clusters can also induced anisotropic thermal conductivity in ferrofluids. The former is the main contributor to the anisotropy, however, the effect of the latter is also significant for the cases of small particle size and low concentration. (C) 2019 Elsevier Ltd. All rights reserved.