Does Magnetic Field Influence the Surface Tension of Ferrofluid?

Studying
the physical properties of ferrofluids is a challenging
task, especially when conventional experimental techniques are adapted
to the presence of a magnetic field. To date, there has been no definitive
understanding of how the magnetic field affects the surface energy
of ferrofluid interfaces. In this study, we perform a direct experimental
investigation to assess the effect of magnetic fields on the surface
tension of ferrofluids. For this purpose, a modified capillary wave
technique was modified for use in the presence of an external magnetic
field. A decrease in the wavelength of the capillary wave was observed
when the magnetic field was oriented perpendicular to the ferrofluid
surface, and an increase was recorded when the magnetic field was
parallel. We note that the capillary wave pattern elongates along
the magnetic field force lines. The observed effect is attributed
to the varying influence of the magnetic field along and across the
propagating capillary wave. Analysis of the dispersion relation and
evaluation of the impacts of various mechanisms influencing capillary
waves revealed, that the changes in the surface tension of ferrofluids
in the presence of a magnetic field are responsible for the observed
behavior. It is shown that the surface tension of the MK 8-40 ferrofluid
gradually increases with the applied magnetic field and reaches a
grouth up to 10% in a magnetic field of ∼10 kA/m. Thus, the
surface tension is found to be influenced by an external magnetic
field.