The influence of particle clustering on the rheological properties of highly concentrated magnetic nanofluids

[Display omitted] ► The highest solid volume fraction of the magnetite nanoparticles was 21%. ► DLS and magnetogranulometry revealed weak particle clustering. ► Viscosity versus solid particle concentration was fitted with Krieger–Dougherty formula. ► 1.3 part./cluster and 1.4nm surfactant layer thickness were estimated from the fit. In this paper the particle volume fraction and temperature dependence of the dynamic viscosity of highly concentrated transformer oil based magnetic nanofluids was investigated in the absence of an external magnetic field. The solid particle volume fraction dependence of the relative viscosity was found to be very well fitted by the Krieger–Dougherty formula, whence the mean ellipticity of the colloidal particles and the effective surfactant layer thickness were obtained. Using the information on the particles’ size and shape statistics obtained from TEM, DLS and magnetogranulometry investigations, it was concluded that the magnetite nanoparticles agglomerate in small clusters of about 1.3 particles/cluster, due to the van der Waals interactions. The effective thickness of the oleic acid surfactant layer was estimated as about 1.4nm, in very good agreement with the value resulted from previous SANS investigations.