Ferrous Fluid is a viscous liquid which is magnetic.  Ferrous Fluids are made by mixing minute iron particles whose size is of the order of nanoamperes with a viscous fluid like oil. The particles which constitute the ferrofluid are less than 10 nanometers in size.   This results in a colloidal solution.    A surfactant such as oleic acid or citric acid is applied to the filings to prevent clumping.  When the ferrous fluid is exposed to a magnetic field.

When the ferrofluid is exposed to a magnetic field, it gets magnetized and assumes a range of geometric patterns depending on the magnetic lines of force.  The pattern assume the form of  corrugations due to the effects of gravity and surface tension on the liquid mass.  

FerroFluids find wide application in liquid seals, high end speakers and computer disk drives.