Amazing Physics: Ferrofluid
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A
fluorocarbon-based ferrofluid, with about 400-G saturation
magnetization and low field magnetic susceptibility of 3, is placed
within a glass Hele-Shaw cell of 1.1-mm gap. Magnetic fields are
applied that have in-plane clockwise rotating and dc axial magnetic
fields. The ferrofluid is surrounded by a 50/50 mixture of isopropyl
alcohol and deionized water, which prevents ferrofluid wetting of
the glass plates. In all cases, the rotational field strength is 20
G rms, and the frequency is 25 Hz. The dc magnetic field will be
increased from 0 to 100 to 200 G. The first experiment uses a 50-??l drop of ferrofluid. The dc
axial field is first increased to 100 G, and then the clockwise
rotating field is turned on. The ferrofluid drop is circular before
the magnetic field is applied. When the dc magnetic field is
applied, the ferrofluid drop forms a spiking labyrinth pattern. Then
the clockwise rotating field is applied, and the spikes begin to
curl in on themselves, forming a smooth spiral pattern after some of
the spikes are absorbed into the larger structure. The smooth
spirals form from viscous shear as the clockwise rotating magnetic
field causes counterclockwise flow on the outside ferrofluid
surfaces, which return on the inside surfaces. The second experiment uses a 200-??l drop of ferrofluid. First,
the clockwise rotating field is applied, which causes a
counterclockwise flow that holds the circular fluid drop together
without spikes. Then a 100-G dc axial field is gradually applied.
This results in the ferrofluid drop appearing to expand before a
phaselike transition at a critical dc magnetic field strength around
100 G. Careful observations show that the pattern forms at slightly
less than 100-G dc field under a thin ferrofluid coating on the top
glass plate, which then abruptly peels away at slightly increased dc
axial magnetic field. The magnetic field is then increased from 100
to 200 G to form an intricate internal pattern surrounded by a
circle of ferrofluid with spiraled arms. The second experiment is
repeated again three more times under essentially identical
conditions, with common features but it appears that the fine
features are different each time. Acknowledgments: This research is supported by National
Science Foundation Grant No. CTS-0084070. Ferrofluidics Corp.,
now FerroTec Corp., contributed the ferrofluid used in these
experiments.
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