Liquid Magnetic Froths
A froth is a solid or liquid containing homogeneously dispersed solid particles.
In recent experiments, researchers at the University of Paris created a liquid magnetic froth,
a mixture of magnetic material and oil. Unlike regular
froths, such as soap-water mixtures, magnetic froths
can display patterns which are reversible by magnetic fields, making them a convenient
type of froth to study. In fact, one can even use these froths to study ideas about the surfaces
of other physical systems, such as the space-time fabric of the early universe.
In the experiments photographed above, the researchers put their magnetic froth between two glass plates, allowing them
to make two-dimensional patterns on its surface and study them. In the above figure,
an alternating magnetic field (H) is applied perpendicularly to the plane of the froth. (a) Initially, a drop of magnetic fluid (black) is surrounded by oil (white).
The magnetic fluid (MF) consists of
cobalt ferrite particles suspended in a water based solution. (b) Seven minutes later,
some oil bubbles appear within the MF drop at the edges. (c) Fifteen minutes later, the number of bubbles increases with time
and the holes already formed at the edges grow. (d) At t=97 minutes, an equilibrium pattern forms. The scale is given by the millimeter grid in the first photograph.
Different patterns form when there are different percentages of magnetic fluid in the mixture of oil and magnetic
fluid. In (a), there is 13% magnetic fluid by volume, in (b) the percentage is 21% and in (c) the percentage
is 35%. In (a), the froth is "dry," meaning that the oil cells (white) are polygons and are separated by thin
boundaries. Photograph (c) represents a wet froth, in which the cells are circular. Photograph (b)
represents an intermediate case.
When neighboring cells form an intersection with a four-fold vertex, it is inherently unstable..
Figure (a) shows a four-fold vertex which is created by placing a small piece of iron above the vertex
of a cell. Figure (b) was taken just
after the piece of iron above the vertex was removed, and Figure (c) shows the froth several seconds
later. The four-fold vertex splits into two three-fold vertices, which turns out to be more energetically stable
for all values of the magnetic field.
When the strength of the magnetic field is decreased, the edge between bubbles 2 and 4 disappears
and a new perpendicular edge is created between cells 1 and 3. In other words, the
edge between two cells has shrunk until it disappears and another perpendicular edge
forms to avoid an energetically unfavorable four-fold vertex. This process is sketched
below the photographs.
Click here to download a high-resolution TIFF version (20 K) of this image
In this sequence of images, an edge between two cells breaks when the magnetic field strength is decreased. Consequently, the two cells coalesce to form a larger cell.
The photographs represent the
situation prior to (left), during (middle), and after (right) the destruction of the wall. This
process typically takes just a second.
This work is described in a paper by Florence Elias, Cyrille Flament, Jean-Claude Bacri, Olivier Cardoso, and Francois Graner in the September 1997 issue of Physical Review E. The figures and captions were
adapted from this paper. For more information, you can contact the group at flament@aomc.jussieu.fr.
Go to related Physics News Update item
