Yesterday, I told you about virtual beer. Today, let's follow two North America researchers who are studying the physics of real beer bubbles. 'Singly scattered waves form the basis of many imaging techniques such as radar or seismic exploration.' But pouring beer in a mug involves multiply scattered acoustic waves. They are more complex to study, but they can be used to look at various phenomena, such as predicting volcanic eruptions or understanding the movement of particles in fluids like beer. They also could be used to monitor the structural health of bridges and buildings or the stability of food products over time.
The photo above shows how the researchers were monitoring beer bubbles. "A one-microsecond-long ultrasound pulse, at a center frequency of 1 MHz, is generated near the top of a mug of beer using an immersion transducer (top left) and detected with a needle-shaped hydrophone (top center). After many reflections off the mug's walls, the detected waves are sensitive to the evolution of the bubbles even when the bubble concentration is very low. (Credits: photo courtesy of Valentin Leroy, University of Manitoba; caption by John Page and Roel Snieder)
This research work has been led by two scientists. John Page worked on the project with the members of his Ultrasonics Research Laboratory at the University of Manitoba, Winnipeg, Canada. And Roel Snieder, from the Department of Geophysics, Colorado School of Mines, Golden, Colorado, used the resources of the Center for Wave Phenomena to collaborate with Page.
Page started to study multiply scattered acoustic waves about 15 years ago. "That's a remarkably complicated problem,' he said. "You might imagine that if you took a handful of sand and dropped it into a beaker and watched it sediment that 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."
I've already mentioned above a variety of applications in the geosciences sector, but here is what Page says about this imaging technique can do for the food industry. "Many foods are very heterogeneous on a number of different length scales, and some contain gas bubbles, like ice cream, for example. 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."
OK, but what can Page tell us about beer? "I know that 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 said. "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."
This research work has been published by Physics Today under the name "Multiple Scattering in Evolving Media" (Volume 60, Issue 5, Pages 49-55, May 2007). Here is a link to the abstract. "Singly scattered waves form the basis of many imaging techniques that are used, for example, in radar and in seismic exploration. If one knows the arrival time of a singly scattered wave, along with its velocity and direction of propagation, one can determine the location of the scatterer. For multiply scattered waves it is more difficult to determine the locations of the scatterers because the waves propagate over many possible scattering trajectories involving a number of scatterers. Thus, multiply scattered waves are not so useful for imaging."
Luckily, they're good enough to model beer bubbles... For more information, here is also a link to the full article (PDF format, 7 pages, 759 KB). The above picture has been extracted from this document.
Sources: University of Manitoba, August 1, 2007; and various websites
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