Surface tension measurement techniques of magnetic fluids at an interface between different fluids using perpendicular field instability

Two measurement techniques to determine the surface tension of ferrofluids using the perpendicular field instability are described. Four ferrofluid layers were examined with magnetic field applied perpendicularly to the surface of s1d oil-based ferrofluid in air; s2d water-based ferrofluid in air, s3d oil-based ferrofluid, and s4d fluorocarbon-based ferrofluid, both below a blend of 50% n-Propyl alcohol and 50% deionized water spropanold. Surface tension was accurately calculated by utilizing the measured Taylor wavelength from measurements of incipient fluid instability peaks and the measured densities of fluids. For cases s1d and s2d, the calculated surface tension values were in good agreement with a tensiometer measurement. No accurate tensiometer measurements were conducted for the superposed liquids s3d and s4d since accurate tensiometer measurements are difficult for a two fluid layer system. The second less accurate method used the ferrofluid’s nonlinear Langevin magnetization characteristics to compute the surface tension from incipience of interfacial instability conditions. Discrepancies between the surface tensions measured by the two methods were probably due to the ferrofluid particle size distributions and the strong dependence of the ferrofluid magnetization on particle size. © 2005 American Institute of Physics. fDOI: 10.1063/1.1861374g