How pointed can magnetized ferrofluid tips be?

The tip radius R_tip of ferrofluids deformed by magnetization is typically ∼100 μm, much larger than in conducting electrified liquids, even though in both cases the capillary stress 2 γ /R_tip balances the electrostatic or magnetostatic stress. The reason for the relative dullness of magnetic tips appears to be the saturation of magnetization, M_max=M_sat . This hypothesis lacks quantitative experimental confirmation but is supported theoretically by the recently proposed high field asymptote R_tip→4/γ μ_oM_sat² obtained for isolated drops in strong and uniform magnetic fields. Here we confirm experimentally the controlling role of the magnetic saturation effect under more general conditions, by showing that the dimensionless group μ_oM_sat²R_tip/(4 γ ) is always a quantity of order unity, even for ferrofluid menisci subject to the highly nonuniform magnetic fields created by supporting them at the tip of a ferrous needle facing a strong magnet. Four different ferrofluids are used, with high, medium, and low surface tensions, including highly polar and nonpolar solvents. Substantial changes in M_sat are achieved by letting the suspending solvent evaporate, with record tip radii down to 9.5 μm demonstrated at high volume fractions of magnetic particles.