During the past decade, nanotechnology with its rapid development has grabbed the attention of scientists,
scholars, and engineers. Nano fluids are one of the surprising outcomes of this technology that could increase
the efficiency of thermal systems remarkably. Nano fluids containing solid nanoparticles have a higher viscosity
than common working fluids; hence, measuring the viscosity is necessary for designing thermal systems and estimating the required pumping power. In the current review study, an attempt has been made to cover the latest
experimental studies performed on the viscosity of nano fluids. An experimental investigation is very vital for the
analysis since the theoretical models usually underestimate the nano fluid viscosity. Through experiments, the
real effects of volume fraction, temperature, particle size, and shape on the viscosity of nano fluids will be
determined.