Physics Photo of the Week
Magnetic Potentials - Hele Shaw Cell
Snyder, a graduate student in Physics at Murray State University, sent
this photo of a magnetic ferrofluid in a Hele Shaw cell. Mr.
Snyder has discovered interesting optical effects of ferrofluids made
visible by means of this technique. A Hele Shaw cell consists of
two optically flat glass plates that enclose a very thin layer of
fluid. In this case the fluid thickness between the glass plates
is less than 1 micron (10-6 m). The diameter of the
cell is about 15 cm - a little larger than the reproduction of the
photograph. Light-emitting diodes (LED's) around the perimeter
inject green light radially inward into the fluid.
The fluid in this Hele Shaw cell is a ferrofluid. A ferrofluid
contains nanoscale ferromagnetic particles such as nanosized iron or
hematite particles. The nano particles are suspended in a fluid
(mineral oil) along with a surfactant. The extremely small size
of the magnetic particles together with the surfactant and normal
thermal molecular agitation (Brownian motion) keep the magnetic
particles from coelescing as would much larger iron filings.
Small quantities of ferrofluids are available for a few dollars from
various Internet sites.
Readers are probably familiar with magnetic field lines made visible by sprinkling iron
filings on a paper laid on top of magnets. With the iron filings,
the filings clump up and form lines along the directions of the
magnetic fields tracing the field lines through a 3-dimensional
configuration. See the photo by Jessenia Majias at right and
featured in Physics Photo of the Week for May
A ferrofluid resembles a fluid containing iron filings except the
magnetic particles are so small that Brownian motion and the
surfactants keep the particles separated, even in the presence of
strong magnetic fields. In Michael Snyder's photograph the bright
curved lines are perpendicular to the the magnetic field lines - much
different from the field lines produced by conventional iron
filings. The central circle in Michael Snyder's photograph is the
sillouette of a circular permanent magnet. The N-S poles of the
magnet are at the top and bottom of the circle. The
magnetic field lines typically radiate outward from the one pole, bulge
at the "equator", and re-converge at the opposite pole. The
curved lines in the ferrofluid in the Hele Shaw cell radiate instead
from the magnetic equator. These curves are everywhere
perpendicular to the magnetic field lines. They are called
"magnetic potential contours". The drawing at left shows the
magnetic potential curves for a magnetic dipole calculated by
Michael. The black circles in Michael's model
represent the N and S poles of a magnet. I have drawn
in blue the magnetic field lines. Iron filings show the magnetic
field lines (the blue dotted lines). Michael's Hele Shaw cell of
the ferrofluid shows the potential contours instead of the field lines.
What makes the light in the Hele Shaw cell follow the potential
contours rather than the field lines? This is an interesting
phenomenon of "magneto optics" not fully understood. Michael
believes that the alignment of the nano-scale nano particles - each
particle is considerably smaller than the wavelength of light - forms
an optical waveguide (optical fiber?) - that guides light in the
directions perpendicular to the magnetic fields. The alignment of
the nanosized magnetic particles causes a preferred path for the light
that is perpendicular to the field directions. Michael has
discovered a very interesting visualization of magnetic potentials.
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