Bubble wrap

时间:2019-03-08 08:14:04166网络整理admin

By Catherine Zandonella in San Francisco ALUMINIUM foam in the body of your car could help you survive a car crash that would otherwise be fatal, by absorbing the energy of the collision. But can you persuade the foam to absorb more energy? The answer, say researchers at Harvard University, could lie in how fast the head on your beer disappears. Foams are just big masses of gas bubbles separated by narrow channels through which fluids, such as soapy water or beer, can flow. Gravity drains the fluid down through the channels leaving the bubbles to dry out and collapse. But modelling this process is far from easy. Foam bubbles come in many shapes and sizes and they have an annoying tendency to disappear while researchers are trying to study them. In 1993, a team of researchers headed by physicist Denis Weaire at Trinity College, Dublin, injected soapy water into the top of a tube filled with dishwashing soap bubbles. From their results they developed a model that described how foams collapse (New Scientist, 21 May 1994, p 34). But Stephan Koehler of Harvard University found his foams collapsed faster than the model predicted. So he tried a similar experiment, except he fed new bubbles in through the bottom of the tube to replace those that popped or shrank. The Dublin model predicted a relatively slow collapse, because the air bubbles create a drag which slows up the fluid draining through the foam. But Koehler found instead that the liquid drops relatively freely through the channels. The only significant drag happens where the flows from different channels merge, at the junctions between four touching bubbles (Physical Review Letters, vol 82, p 4232). Of his rivals in Dublin, Koehler says, “It is easy from a limited data set to come to the wrong conclusion.” But the Dublin researchers stand by their model, which they say has been replicated by a number of other groups. “I don’t think we’ve yet come to the bottom of this,” says Trinity College physicist Stefan Hutzler. Apart from the potential for foam to make safer cars, people like Steven Fishman of the US Navy’s Office of Naval Research, want to make solid foams out of metals such as aluminium to use on aircraft carriers. If foam drains too fast during manufacture, Fishman explains, you get a foam solid that is too dense on the bottom and too light on the top. “The question is how do I control my process so I can get more perfect foam cells,” says Fishman. Richard Rusiniak, president of Cymat, a Canadian-based company that makes aluminium foams, says they might change their manufacturing processes because of the new model. “By understanding how drainage occurs we can make a decision on process improvement, or at least it will point us in the right direction.” But the research is unlikely to lead to a better head on your beer. Enhancing the protein content is a better way of making the foam last longer, says Charles Banforth, professor of brewing science at the University of California,