Aug. 4, 2007 Many of us look forward to a cold beer at the end of a hot summer day, but physicist John Page can bring beer in to work. For him, the bubbly beverage is a perfect medium for demonstrating a scientific technique pioneered by his group at the University of Manitoba.
John Page is a leading expert on the use of multiply scattered acoustic waves to study changes in physical systems and the movement of particles through a medium — in this case, bubbles in a glass of beer. He directs the university’s ultrasonics research laboratory, one of only a handful in North America focused on this technique. A summary of major work in this field appeared in the May 2007 edition of Physics Today, co-authored by Page.
Standard imaging technologies, including everything from optical microscopes to radar, use singly scattered waves. The idea is that the wave scatters once off the thing you’re trying to image, so you can tell exactly where it is. Unfortunately, if you have a medium that results in multiple scattering, where the waves sequentially scatter or reflect off many different things before leaving the medium, direct imaging no longer works.
“We’ve been interested in multiple scattering of waves for about 15 years,” Page says. “We were initially interested in learning more about how multiply scattered waves propagate and travel through some strongly scattering material, and one of things that we started working on fairly early was the development of a technique that we call diffusing acoustic wave spectroscopy.”
Page’s initial work was aimed at using this technique to get a better understanding of the movement of particles within fluids.
“It’s a remarkably complicated problem,” he explains. “You might imagine that if you took a handful of sand and dropped it into a beaker and watched it sediment, the particles would just go straight down and that would be the end of it. But it’s not as simple as that. There are interactions between the particles, and if one particle moves, it moves the fluid, and that moves another particle, and so on. We were able to make some contribution to understanding this, which is important to a number of fields and applications.”
Diffusing acoustic wave spectroscopy, Page notes, could enable researchers to obtain new kinds of information about food properties.
“Many foods are very heterogeneous on a number of different length scales, and some, like ice cream, contain gas bubbles. Multiply scattered acoustic waves could work well for studying some of these kinds of porous food materials, and they would be particularly useful if you are developing a food with an appealing pore structure. You could potentially use this technique to monitor the product to make sure it remains stable over time.”
And that brings us back to beer. There’s no doubt that bubbles are a large part of the drink’s appeal, and Page’s technique can provide precise information about their evolution.
“A lot of effort has gone into figuring out how to get just the right concentration and size of bubbles, and how to produce the perfect head on a glass of beer,” he says. “There are people who work in that industry who know much, much more about that than I do. Could diffusing acoustic wave spectroscopy be useful to them? Maybe. But for me, beer is just a good example of the kind of thing you can do using this technique.”
Multiply scattered acoustic waves can also be used to study changes over time in a range of different materials and environments. For example, some of Page’s collaborators are working with this method as a potential way to predict volcanic eruptions and earthquakes by monitoring the environment around volcanoes and fault lines. Others are experimenting with it as a way to monitor the structural health of bridges and buildings.
Page says the technique also has potential use in the food industry, particularly for foods whose structure makes them hard to study using light. He already has a long-standing collaboration with University of Manitoba food scientist Martin Scanlon, aimed at characterizing food materials using traditional ultrasonic techniques.
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