  A recent area of interest in thermal applications has been nanofluids, with
extensive research conducted in the field, and numerous publications expounding
on new results coming to the fore. The properties of such fluids depend on
their components, and by the use of combined and complex nanoparticles with
diverse characteristics, the desirable qualities of multiple nanoparticle
suspensions can be obtained in a single mixture. The paper covers these hybrid
nanofluids, the techniques for their synthesis, their properties,
characteristics, and the areas of debate and outstanding questions.
