Definition of Magnetic Fluid
Definition
A homogeneous mixture formed by chemically combining two or more substances is called a solution. A colloid is a system in which particles are large enough that it cannot be regarded as a solution, yet the dispersed particles are not easily distinguishable from the solvent to the naked eye.
In the context of fluid mechanics, a ferrofluid is a colloid that exhibits strong magnetism.
Explanation 1
A ferrofluid is different from conducting fluids. One interesting property of ferrofluids is that the magnetic field affects the fluid flow, while the flow in turn affects the magnetic field. In that sense, studies of conducting fluids often do not consider the reverse effect of the flow back on the magnetic field.
As implied by the term colloid, a fluid does not need to be intrinsically magnetic; it can become magnetic through mixing, i.e., by adding magnetizable particles. Historically, research into these fluids reportedly began from attempts to control liquid rocket propellant using magnetic fields, which I personally consider a clear and intuitive example of both the application and the underlying principle.
Altmeyer, S., Hoffmann, C., Leschhorn, A., & Lücke, M. (2010). Influence of homogeneous magnetic fields on the flow of a ferrofluid in the taylor-couette system. Physical Review E—Statistical, Nonlinear, and Soft Matter Physics, 82(1), 016321. https://doi.org/10.1103/PhysRevE.82.016321 ↩︎