Physics Photo of the Week

October 10, 2008

Magnetic Potentials - Hele Shaw Cell
Michael 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 12, 2005.

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.
Physics Photo of the Week is published weekly during the academic year on Fridays by the Warren Wilson College Physics Department.  These photos feature an interesting phenomena in the world around us.  Students, faculty, and others are invited to submit digital (or film) photographs for publication and explanation.  Atmospheric phenomena are especially welcome.  Please send any photos to dcollins@warren-wilson.edu. 

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