he nonlinear magnetization characteristics of recently developed ferrof l u i d s complicate studies of wave qynamics and s t a b i l i t y . A general formulation of t h e incompressible ferrohydrodynamics of a f e r r o f l u i d w i t h nonlinear magnetization c h a r a c t e r i s t i c s i s presented which distinguishes c l e a r l y between e f f e c t s of inhomogeneities i n the f l u i d properties and saturation e f f e c t s from nonuniform f i e l d s . The formulation makes it c l

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... e a r t h a t w i t h uniform and nonuniform f i e l d s t h e magnetic coupling w i t h homogeneous f l u i d s i s confined t o interfaces; hence it i s a convenient representation f o r surface interactions. Detailed attention is given t o waves and i n s t a b i l i t i e s on a planar i n t e r f a c e between f e r r o f l u i d s stressed by an a r b i t r a r i l y directed magnetic f i e l d . The close connection w i t h related work i n electrohydrodynamics i s c i t e d and emphasis i s given t o t h e effect of the nonlinear magnetization c h a r a c t e r i s t i c s on o s c i l l a t i o n frequencies and conditions f o r i n s t a b i l i t y . investigated using quasi-one-dimensional models f o r the imposed f i e l d s i n which either a perpendicular o r a tangential imposed f i e l d varies i n a direction perpendicular t o t h e interface. Three experiments are reported which support t h e t h e o r e t i . c a l models and emphasize t h e i n t e r f a c i a l dynamics as w e l l as t h e s t a b i l i z i n g e f f e c t s of a t a n g e n t i a l magnetic f i e l d . surface waves are measured as a function of magnetization w i t h fields imposed first The effects of nonuniform f i e l d s are The resonance frequencies of ferrohydrodynamic perpendicular, and second t a n g e n t i a l , t o t h e unperturbed interface. In a t h i r d experiment, t h e second configuration i s augmented by a gradient i n the imposed magnetic f i e l d t o demonstrate the s t a b i l i z a t i o n of a f e r r o f l u i d surface supported against gravity over a i r ; t h e ferromagnetic s t a b i l i z a t i o n ofARayleigh-Taylor i n s t a b i l i t y . a