Introduction to Foams
A Child’s View of Physics, by Marc in het Panhuis
"Are you a grown-up?"
I looked up, or more precisely down, into the eyes of a child. He had a good reason to ask this, since I was playing with bubbles at an exhibition for young scientists. What attracts them? What does not? Physicists have been arguing whether or not we should consider it as our duty to encourage children into physics. I would like to describe the actual experience of standing amongst the young and curious.
At our stand we were displaying various ways of studying foams, such as wire frames and cylindrical structures. To the physicist (who, at least, according to age is a grown-up) these display interesting foam phenomena. But not to a child. They do not see that wire frames can be used to verify the equilibrium rules laid down by the Belgian scientist J.A.F. Plateau over 100 years ago. Children do not want to hear about symmetric angles between touching films. Instead, they ask if it is possible to blow a square bubble using a cubic frame. The fact that Plateau was blind while working with these elegant frames appeals to their imagination and you can see them wondering how this could be possible.
The typical foam, in your kitchen sink or your bath tub, is disordered, however to make an ordered foam is simple and straightforward. All you need is a cylindrical glass tube and a nozzle connected to an air pump. Place the cylindrical glass tube about 5 to 10 cm under water and subsequently blow air bubbles into it through the submerged nozzle. Through variation of the gas flow rate one can make a large range of ordered structures. My friend and colleague Stefan Hutzler uses them to study foam drainage, twists and transitions between different types of structures. Children find it hard to get interested in these beautiful experiments. The Plateau borders in the tubes are too small for a child's eye, although adding liquid at the top thickens the network of Plateau borders, enhancing their visibility. I used these tubes to explain to the interested young listeners that a Plateau border is a channel formed where 3 films meet. By adding liquid it can be shown that a Plateau border can also transport liquid.
If you want to attract large crowds of children at your stand, follow these two rules. Firstly, give them hands-on experience by letting them play around with your experiments. Secondly, the bigger the objects used in the experiments the better. In accordance with this second rule, we used a giant bubble maker in addition to the smaller frames and glass tubes experiments.
Children, parents and people from other stands were highly fascinated by the giant bubble maker. It consists of a bottle of soap solution hanging upside down. From a hole in the top, two strands of monofilament are attached to a weight. Soap solution runs down along the wires. Pulling these wires apart slowly enough creates a single soap film. In our case, I could make a film with a length and breadth of 5 to 6 feet. The originators of this trick in the United States made one of 45 feet length. It was developed to study flow pattern around objects.
To challenge the young I would show two simple experiments and then invite everybody to try it themselves. In experiment one, I pulled the wires as far apart as possible. Then by gently rocking the film, I created an oscillation. As the film moved away towards the close standing public I gently blew against the film. The effect is amazing, the film can be stretched up to 12 feet. Sometimes a bubble would loose itself from the film and float in the air for a minute before hitting the ground. In experiment two, I pulled the wires roughly 3 feet apart and made a half circular movement with both hands. Followed by closing the wires again, which created a bubble of roughly 3 feet diameter. The champion bubble maker could earn a chocolate bar, which also happened to be an excellent example of a solid foam.
Am I a grown-up? When standing amongst 30 children with the strands of the giant bubble maker in my hands, ready to produce the biggest bubble they have ever seen, certainly not.