Scientists love foam, says Giselle Weiss, removing the froth from a vital fabric
Ever since Robert Boyle studied his own urine, scientists everywhere have been fascinated by foam, says Denis Weaire, author of The Physics of Foam. The material that makes up a large part of our bones, forms cuckoo spit and cork and creates sea sponge attracts a "roll call of interested scientists" that reads like a science Who's Who according to the Trinity College, Dublin professor who has studied foam for 20 years.
In 1887, William Thompson (later Lord Kelvin) suggested that the universal ether was a foam, which led him to wonder what shape would allow its bubbles to pack most economically. His solution could not be proved, but neither was it bettered until Weaire and Robert Phelan presented a new structure in 1994.
Physicists love foams because at one level they are simple and geometrical objects that are easy to define. They occur so frequently in nature that they are a useful model for studying very different kinds of structures, for example dried mud or the skins of mammals and cucumbers.
By the same token, they are challenging materials for the theorist, with unusual properties and complex behaviours associated with their disordered structure. That may be one reason why, until recently, the study of foams had lagged behind the study of other areas of materials science, such as solid-state physics.
Another reason may be their relatively low-tech applications in products such as shampoo and polyurethane cushions. Whatever the cause, foams have largely been the province of chemical engineers.
But movement of the subject closer to the centre stage of science is notable, says Weaire, and that is indicative of a change in physics.
Eurofoam 2000, a five-day conference held in Delft, in Holland, last June, brought together the major players in the field from around the world in both pure and applied science.
"People are finally getting the research tools to seriously attack what is really an extraordinarily difficult material to analyse," affirms Sidney Perkowitz, professor of physics at Emory University, United States, and author of Universal Foam: From Cappuccino to the Cosmos. Massive computer modelling, magnetic resonance imaging and even null-gravity experiments, he says, are enabling work that could not have been done a decade or so ago.
Foam occurs whenever a liquid and a gas are churned up together. The composition of the liquid stabilises the foam to a greater or lesser degree, which is why plain water bubbles are ephemeral and the foam in a bubble bath persists. Another way of making a foam is to create gas inside a liquid, for example the carbon dioxide that is generated in making beer. Pouring the beer into a glass releases the carbon dioxide as foam.
Solid foams are valued in construction and packaging for their lightness, stiffness and strength, a combination not easily duplicated in other materials. Liquid foams add aesthetic appeal to personal and household products, and contribute to the "mouth feel" of foods such as ice cream and the flavour of others, such as espresso coffee. Where foam in food is hard to stabilise, for example in low-fat food products, the feel is very different.
Stability in a foam has its drawbacks. "All over industry," says Weaire, "where you have liquid mixed up with gas, you're in danger of a foam. And in many cases, you don't want it. Because you can imagine, you've got this big plant, and foam starts to form. It could fill the whole factory, and it will get into turbines or whatever. It's a tremendous nuisance."
Anti-foaming agents are big business. But stable foams also play an important role in industry. Jan Cilliers directs the Foam and Froth Research Group in the department of chemical engineering at the University of Manchester Institute of Science and Technology (UMIST). He studies foams for their application in foam flotation, a process used widely in the mining industry to concentrate the valuable parts of minerals. The technique is also used to separate ink from paper in recycling and to purify drinking water.
For example, says Cilliers, copper is generally mined as copper sulphide mineral mixed with junk. Maybe 2 per cent of that mixture is the mineral you want. In foam flotation, the mixture is ground and then chemically treated so that the desired mineral attaches to bubbles in a foaming liquid and rises to the surface, where it can be collected. Three billion tonnes of sulphide minerals such as nickel and platinum are processed worldwide each year using foam flotation.
The problem is that undesirable particles still make it into the froth on the surface by a process called entrainment, and removing them is expensive and time-consuming. Moreover, as minerals become depleted, they need to be ground ever finer for separation, which means that even more unwanted material enters the froth.
Up to now, operators in flotation plants have controlled froth performance through a process of trial and error. Based on experience, operators adjust the amount of foaming agents, air and froth height to get a froth that "looks right". But grinding minerals finer affects the structure of the froth. And here operator experience proves an unreliable guide.
Cilliers and his group have turned to physicists such as Weaire to understand how liquid moves in and out of a foam through the gaps between the bubbles. "We believe that the critical part of separation happens in the froth phase," says Cilliers. "And that is because when the liquid drains out, it's taking the unwanted material with it."
Because draining of liquid is so fundamental to the formation and performance of foams, it is a focus of much research at the moment. It is particularly relevant to investigations into some of the new classes of foams, such as metallic foams, which drain so quickly that they may collapse before they can solidify, and so-called wet foams, which do not stay wet long enough to study in normal circumstances. A programme planned by the European Space Agency offers the opportunity to experiment with these materials in zero gravity.
In the artistic imagination, bubbles are symbols of transitory beauty and of mortality. If you look at those allegorical paintings with a skull in them, Weaire reflects, there is always a bubble in there somewhere. The scientific imagination finds plenty to ponder as well, in the elegant constructions of soap bubbles.