DSR, a further application?
The Paper Chase
Focused blasts of sound may be the key to
saving trees and reducing the flow of garbage.
The computer revolution has brought many improvements to the workplace, but the long-promised "paperless office" is not one of them. In theory, people can read information on the Web, communicate via email, and shuttle files by disk or network, performing all of their tasks electronically. In practice, the paper that once went into legal pads and typewriter sheets has just been replaced by even larger stacks of paper that spew out of laser printers and copying machines. All that paper translates into a lot of landfill garbage and a lot of lost trees.
Paper recycling can reduce that waste, but photocopier and printer inks contain water-repelling compounds like styrene, making them difficult to dissolve. The harsh chemicals required to remove these kinds of ink for bulk recycling are themselves environmentally unfriendly. One solution is to develop a gentler chemical process. Sushil Bhatia of Imagex Technologies in Framingham, Mass., is developing a "decopier" that uses nontoxic compounds to strip the ink from paper right in the office. His invention was a finalist in the 1998 Discover Technology Awards.
Sameer Madanshetty, a mechanical engineer at Kansas State University, has come up with another way. He has devised a recycling procedure that dispenses with chemicals entirely; it also does so little damage to the paper fibers that pages can be reused multiple times. And all he needs to do it is some water and precisely controlled pulses of sound.
As a sound wave travels, it creates a ripple of expansion and compression: a low-pressure pulse followed by a high pressure pulse, and so on. Tiny bubbles that can sometimes form in water and other liquids react to this change in pressure. The low pressure pulse expands the bubbles and the high pressure pulse makes them contract.
With certain sound waves, the contraction caused by the high pressure part of the sound wave can be strong and fast enough to make the bubble collapse in on itself, creating a surprisingly large concentration of energy. Called cavitation, this process is powerful enough that it can damage things like ship propellers and water pipes, eventually eroding them away. Engineers therefore usually try to create cavitation-proof designs, but this goal is hampered by the unpredictable nature of cavitation.
Madanshetty does not see cavitation as a bad thing. "It's such a fascinating energy manifestation, but we are removing it because we don't understand it and we can't control it," he says. "If you think of fire, any fire is devastating, but if you contain it you can do some good cooking." Indeed, Madanshetty has found a way to control when and where cavitation will occur. Along with a strong sound pulse that causes the bubbles to expand and contract, he uses a weaker background sound wave that destabilizes the bubbles so that they collapse at a lower pressure; the result is that they release less energy and their effect is more localized.
When a piece of paper is placed in water, tiny bubbles are most likely to form around the inked parts of the page, because the ink repels water. Madanshetty's sound system blasts these microbubbles; when they collapse, they literally explode the ink off the page. "It's like micro-jackhammers," he explains. "It chisels away the ink and throws it up." The process can strip the ink from a page in a few seconds, and the ink particles can then be filtered out of the water.
Although the bubbles have enough energy to destroy the ink, Madanshetty's control process keeps them weak enough that they do not seriously damage the paper fibers below. In most cases, the paper need not be mashed up to be recycled--the microbubbles erase the ink effectively enough that the original paper is as good as new, Madanshetty says.
"Present practice mechanically pulps the paper, and in the churning process they damage the paper fibers, so it limits the recycleability to three times," he notes. "In this case, you are not touching the paper fibers at all, so the number of times you can recycle the paper would be endless."
--Fenella Saunders
Posted 12/9/98 |
