Magnetic Soap Bubbles

Lay-language version of "Reverse Draining of a Magnetic Soap Film: Experiment"

Presented at the 61st APS Division of Fluid Dynamics Meeting in San Antonio
At 10:30 a.m. on Sunday, November 23, 2008 in Room 003B of the Gonzales Convention Center
Gray arrow Abstract


John A. Pelesko
University of Delaware
Modeling, Experiment, and Computation Laboratory
Department of Mathematical Sciences
406 Ewing Hall
Newark, DE 19716

Derek Moulton
University of Arizona
Department of Mathematics
617 N. Santa Rita Ave.
P.O. Box 210089
Tucson, AZ 85721–0089

Soap bubbles are a source of curiosity for children and mathematicians alike. Studied by scientists such as Isaac Newton and Leonardo da Vinci, "play" with soap bubbles and films has led to a surprising array of discoveries in both mathematics and physics. In this work, we add a new dimension to these studies by creating soap bubbles that are magnetic and can be manipulated with the magnetic field produced by an ordinary refrigerator magnet.

Children are attracted to soap bubbles by their perfect shapes and iridescent colors. Scientists are attracted to soap bubbles for exactly the same reasons. The study of the shapes of soap bubbles and films was pioneered by the Belgian physicist, Joseph Plateau. Dipping wire frames into a soap solution, Plateau created the study of what mathematicians call minimal surfaces. A soap film spanning a wire frame will do so in a way that makes its surface area as small as possible; it minimizes its surface area. Mathematicians have long recognized the Plateau Problem, the problem of predicting the shape a soap film on a wire frame will assume, as one of the most difficult and beautiful problems in mathematics. In the hundred odd years since Plateau did his work, the study of this problem has led to advances in multiple branches of mathematics.

The colors of soap bubbles are also of scientific interest. Produced by light waves reflecting and interfering as they strike the surface of a soap film, the remarkable colors of a bubble tell us precisely how thick the bubble is at any given point. It’s exciting to realize that a child holding a bubble wand is playing with a film only a few hundreds of nanometers thick. If the bubble wand is held perpendicular to the ground, the beautiful banded structure seen in the photograph below appears. The color bands indicate that the film is becoming thicker near the bottom and thinner near the top as the liquid in the film flows downward under the influence of gravity.

Soap film draining under the influence of gravity.
Photo courtesy of the MEC Lab.

Soap film draining under the influence of gravity.

In our laboratory, we have created soap bubbles and films that display the remarkable shapes and colors of ordinary soap films, but are also magnetic, allowing us to manipulate these colors and shapes using an ordinary permanent magnet. To make our soap solution, we begin in much the same way as you would at home. We mix one part commercial dish soap with twelve parts of water and add a touch of glycerin to minimize evaporation. But, to our solution, we also add magnetic nanoparticles, essentially tiny iron spheres about ten nanometers in diameter. If you bring a magnet near a collection of small iron particles, they will be attracted toward the magnet. With our particles, since they are contained inside of a soap film, they drag fluid along with them as they try to move toward the magnet. This affects both the shapes that a soap film will assume and the way the liquid within a film will flow.

In our flow experiments, we simply place a magnet at the top of a soap film held perpendicular to the ground. You can imagine simply holding a bubble wand vertically with your left hand and with your right hand holding a magnet at the top of the wand. What we find is that the pattern of colors that appear in the film can be dramatically altered by the presence of the magnet and the magnetic particles. If the magnet is strong enough, the liquid in the film can even be made to flow upwards against the action of gravity, an effect that we call reverse draining. The photo below shows the results of one such experiment. In this picture, the black region indicates where the film is thinnest, while the gold regions at the top indicate a thick film.

A magnetic soap film draining upwards. Photo courtesy of the MEC Lab.Photo courtesy of the MEC Lab.

A magnetic soap film draining upwards.

A close up of a draining magnetic film showing reverse marginal regeneration. Photo courtesy of the MEC Lab.Photo courtesy of the MEC Lab

A close up of a draining magnetic film showing reverse marginal regeneration. .

One goal for this work is to better understand what is called marginal regeneration. This is the process by which the thinnest, or black, regions of a draining film grow. In a typical draining soap film, new regions of black film are formed near the bottom of the film and move upwards adding to previously existing black film. In our system, we find that this process can be reversed and that light regions of new black film can "swim" downward against gravity and add to the growing black region.

In our laboratory, we also study how the shapes of magnetic soap bubbles can be manipulated and how they differ from those of ordinary soap bubbles. This adds a new twist to the Plateau Problem. Magnetic bubbles not only attempt to minimize their surface area, but also try to balance this with an attempt to minimize magnetic energy in the system. Our experiments in this area are just beginning, but as can be seen in the video below, some unusual behavior can be expected.