Number 826, May 30, 2007
by Phil Schewe and Ben Stein
Microfluidic Accelerator
Microfluidics is the science of carrying out fluid chemical processing on a chip whose channels are typically millimeters or microns across. In such a constricted space, viscosity becomes large, and the fluid flow can slow way down, thus limiting the kind of mixing or testing that can be done. Physicists at the University of Twente in the Netherlands, however, use tiny exploding bubbles to speed things up.
The bubbles are produced by shooting laser light into the fluid. (See movie at http://stilton.tnw.utwente.nl/people/ohl/controlled_cavitation.html ) The light brings a tiny volume of fluid above its boiling temperature, causing a local bubble explosion, which accelerates the surrounding fluid along the channel, now at speeds of up to 20 m/sec, twenty times higher (and still another factor of 10 within reach) than would be the case without the bubble. (The same researchers have produced sonoluminescence in the same way.)
An extra advantage of using flexibly positioned laser light is that for transparent microfluidic chips fluid pumping can be accomplished without external connections to the chip.
Besides being the first to apply such a cavitation technique for speeding up fluids on a chip, the Twente scientists are the first to achieve flow visualization at rates of a million frames per second at a size scale of 100 microns.
The leader of the Twente group, Claus-Dieter Ohl (c.d.ohl@utwente.nl, 31-53-489-5604) says that he and his colleagues are currently using the bubble acceleration technique for improving mixing in various enzyme reactions and in producing tiny pores in membranes. (Zwaan et al., Physical Review Letters, upcoming article)
Warm the World, Shrink the Day
Global warming is expected to raise ocean levels and thereby effectively shift some ocean water from currently deep areas into shallower continental shelves, including a net transfer of water mass from the southern to the northern hemisphere. This in turn will bring just so much water closer to the Earth’s rotational axis, and this-like a figure skater speeding up as she folds her limbs inward-will shorten the diurnal period.
Not by much, though. According to Felix Landerer, Johann Jungclaus, and Jochem Marotzke, scientists at the Max Planck Institute for Meteorology in Hamburg, the day should shorten by 0.12 milliseconds over the next two centuries. (Recent issue of Geophysical Review Letters.)
Listening to Muscle Noise
Muscles make noise. For example, you can hear the sound of the masseter muscle-a jaw muscle used in chewing food-by propping your head (ear down) in the palm of your hand. The low rumbling comes from the shortening of the actomyosin filaments in the muscle fibers. Muscle noise can be measured using various sensors, such as microphones and even skin-mounted accelerometers.
Scientists at the Scripps Institute of Oceanography listen to muscle noise in order to detect muscle stiffness, which in turn can provide information about neuromuscular disease, such as muscle dystrophy. Muscle stiffness was traditionally measured using external radiation sources (such as a vibrating piston).
But the Scripps researchers use a process called passive elastography, a low-cost, in-vivo, non-invasive technique in which an array of surface sensors follow the passing of natural shear waves traveling along the muscle fibers. The new results will be delivered next week by Karim Sabra (ksabra@mpl.ucsd.edu) at the meeting of the Acoustical Society of America (ASA) taking place June 4-8 in Salt Lake City.
By the way, the Scripps scientists were originally interested in underwater noise effects and only later adapted their work to noise in muscle. (ASA paper 2pUW9; meeting website at http://www.acoustics.org/press)
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