Hydrodynamics of nematic ferrofluids

We derive hydrodynamic equations for nematic ferrofluids (ferronematics) in the limit that the magnetic degree of freedom has relaxed to its equilibri um value. We concentrate on novel dynamic effects linear in the magnetic fi eld. We show that flow alignment, heat conduction, diffusion, thermodiffusi on, viscosity and director reorientation are all modified by the presence o f an external field. In particular, the new effects describe reversible (ir reversible) couplings, where the conventional effects are irreversible (rev ersible). We discuss, how these effects can be measured. In principle, this description is applicable to conventional nematics, too, although huge mag netic fields are expected to be necessary for detecting the new effects in this case.