It is not (too our knowledge) possible to simulate with ordinary laboratory fluids the ß-effect in a continuously stratified fluid; for, the rest-state potential vorticity, just 2omega/h where omega is the rotation rate and h the thickness between two paraboloidal constant-density surfaces, is independent of position. We want instead a RADIAL gravity field, and this can be simulated using ferromagnetic fluid.
'Ferrofluid' is a colloidal suspension of small (~100A) magnetic dipoles in water or oil. The dipoles (in our case of magnetite) are coated with a lignin-based surfactant to prevent them from glomming together. The result is a fluid which like very 'soft' iron, is magnetized in the presence of an external fluid (which orients the dipoles). The force experienced by the fluid is proportional to the gradient in the applied field, and to the field itself (in the linear part of the magnetization curve), hence a force proportional to grad(h^2).
With the collaboration of Eric Lindahl in our laboratory, we molded permanent magnets into a ceramic sphere, and coated it with ferrofluid; this in turn was immersed in low-viscosity silicon oil, mixed with Freon to match its density to that of the ferrofluid. The result was a close analog to the rotating, centrally gravitating, spherical Earth. We successfully excited equatorial Kelvin waves and Rossby waves, which had not heretofore been seen (except in Nature).
More generally, the development of this fluid by R.Rosenzweig of Exxon, and colleagues, gives a new tool for fluid dynamics research: we suddenly have the ability to exert 'tidal' forces on the fluid remotely. We have carried out numerous experiments in which permanent magnets create a 'warped' gravity field and an electromagnet, pulsed periodically, excites 'gravity' waves.
We are particularly in debt to the Office of Naval Research for their support of this work through an Ocean Sciences Educator's Award to P. Rhines, and more generally for their long-term support of the GFD laboratory.
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