| Physics
Update - January 2000 |
|
| Microfluidic
flows driven by thermal gradients. Microfluidics is to the
mixing of fluids what integrated circuits are to the processing
of electrical signals: Transactions occur quickly, controllably,
and in a very small region of space. A common technique involves
excavating nano-sized channels in a substrate and then propelling
tiny fluid volumes around the system of aqueducts with electric
fields. Such systems allow for turbulence-free mixing and analysis
of small samples of, for example, blood or DNA. At the November
1999 meeting of the American Physical
Society’s division of fluid dynamics, Princeton University
professor Sandra Troian reported moving tiny parallel rivulets
along the surface of a silicon wafer using mild temperature
gradients rather than electric fields. In such a system, the
liquid experiences a variable liquid surface tension, which
causes it to seek out a relatively colder region. The pathways
were “drawn” by chemical lithography or UV ablation of a monolayer.
With thermocapillary microfluidics, high electric fields and
the precision carving of channels are not necessary, and everything
happens on an open planar surface, which could simplify the
fabrication of “labs-on-a-chip.” (D. E. Kataoka, S. M. Troian,
Nature 402, 794, 1 |
|
| Ultrasound
imaging without physical contact between device and patient
has been achieved, providing a potential solution to a pressing
medical need—determining the depth and severity of burns in
a convenient, accurate, and pain-free fashion. Currently, physicians
usually diagnose burns by visual inspection, but that cannot
provide direct information on whether there is damage to underlying
blood vessels, a condition that requires surgery. Technologies
such as magnetic resonance imaging or conventional ultrasound
are too slow, too cumbersome, or too painful if they require
direct contact with the burn area. In particular, conventional
US requires direct contact because of the great impedance mismatch
between air and the transducer; without contact, most of the
sound would bounce right back into the device without having
penetrated any tissue. At the November 1999 meeting of the Acoustical
Society of America, Joie Jones (University of California,
Irvine) reported passing ultrasound through a multilayered material,
with each succeeding layer having an impedance value closer
to that of air; the technique is similar to using antireflective
coatings on optical systems. He and his colleagues were then
able to image burns by holding their device about 5 cm away
from the skin for about a minute or so. Having tested this device
on over 100 patients, the researchers plan to begin larger clinical
studies and develop a device that can take images in real time.
(A preprint is available from Jones.) |
|
| Laser
light in, streams of protons out. At the November 1999 meeting
of the American Physical Society’s
division of plasma physics, three groups independently announced
their ability to generate intense streams of energetic ions
by shining ultrashort laser pulses on tiny spots of solid foil
targets. Researchers from Lawrence Livermore National Laboratory
reported using a single pulse of light from the Petawatt laser
to generate 3 x 1013 protons with energies of up
to 50 MeV. A group from the University of Michigan reported
using a tabletop terawatt (“T-cubed”) laser, having one-thousandth
the power of the Petawatt, to produce 109 protons, confined
to an exit cone of about 40°, and having energies of up to about
1.5 MeV. Lastly, researchers from Imperial College (University
of London, UK) revealed that, using the Vulcan laser at the
Rutherford Appleton Laboratory, they had accelerated lead ions
up to 420 MeV and protons up to 30 MeV. The physics is similar
in all three demonstrations: The laser pulse drives electrons
to the back of the target, whereupon the ensuing electric field
accelerates ions out the back. Such laser ion acceleration might
someday be used to produce ions for cancer therapy and electronics
manufacturing. |
|
| Nonthermal
melting of germanium has been observed. In general, a solid
becomes a liquid when its thermal energy exceeds its atomic
binding energy—the atoms rattle around so much that they break
free of their lattice. But one can envision breaking all the
bonds between atoms in a cold solid so quickly that the atoms
will never have a chance to move first. In fact, there has been
indirect evidence for that very process occurring in germanium
when exposed to an intense laser pulse. Some subpicosecond effects
looked like melting, which normally would take tens to hundreds
of picoseconds. Now, a group of researchers led by Craig Siders
(University of California, San Diego) has used ultrafast x-ray
diffraction to follow the structural changes in laser-irradiated
germanium in real time. They found that a thin surface layer
homogeneously lost all its crystalline order within a few picoseconds,
a clear indication of nonthermal melting. That disorder then
propagated by thermal melting, both outward along the surface
and into the bulk. For more on ultrafast chemistry, see our
story on last year’s Nobel Prize (December 1999, page 19). (C.
W. Siders et al., Science 286, 1340, 1999.) |
|
Previous
Physics Updates:
|
|
©
1999 American Institute of Physics
[an error occurred while processing this directive]
|
