A LIQUID MAGNETIC FROTH, created and studied by University of Paris researchers (Cyrille Flament, flament@aomc.jussieu.fr), can display reversible, magnetically controlled patterns on its surface. Perhaps the most common example of a froth is a soap-and-water mixture. In general, a froth is any solid or liquid containing uniformly dispersed solid particles as well as homogeneously distributed gas molecules. Combining oil with a magnetic fluid (cobalt ferrite particles suspended in a water-based solution), and placing the resulting froth between two glass plates, the researchers applied magnetic fields to create 2-D patterns consisting in some cases of 4-to 7-sided "cells" of oil separated by magnetic fluid. Decreasing the intensity of the magnetic field could change, for instance, the number of 5-sided cells. Unlike other froths, such as a soap-and-water mixture, the topological features of the pattern could return by increasing the field again. One could conceivably use these films to test ideas about the surfaces of other physical systems, such as the space-time fabric of the early universe. For example, placing a bar magnet on top of the froth could introduce an instability; once the magnet is removed, the evolution of the system could then be studied. (Florence Elias et al., Physical Review E, Sept. 1997; figures at Physics News Graphics.)