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.