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18 Sep
Physics Bits #448

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From: Peter Langston <psl>
Date: Sat, 18 Sep 99 12:43:51 -0700
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Subject: Physics Bits #448

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The American Institute of Physics Bulletin of Physics News
Number 448  September 16, 1999  by Phillip F. Schewe and Ben Stein

LIQUID CRYSTAL ACOUSTICS.  Penn State physicist Jay Patel seeks to
understand the optical properties of liquid crystals which, consisting of
rod shaped molecules with the ability to polarize light, are regularly
employed in electronic displays; an applied voltage lines up the rods and
shuts off or turns on transmitted light.  So it came as a big surprise when
Patel discovered that liquid crystals also have acoustic properties.  To be
precise, an applied voltage imparts energy to the rod molecules which in
turn cause the cavity in which the liquid crystal resides to vibrate.  The
cavity resonates with an audible frequency that could be heard with the
unaided ear.  (An analogy: the stings of a violin aren't what make sound;
rather they transmit the energy of the bow to the body of the violin whose
vibrations are source of the music we hear.) Unsure of the implications of
liquid crystal sound (tiny speakers, delay lines for circuits?), Patel and
his colleagues suspect that this discovery will lead to a fruitful new
research area.  (Kim and Patel, Applied Physics Letters, 27 Sept. 1999;
contact Patel at; 814-863-8999.)

CLAY OSCILLONS.  Nature often sorts energy into certain preferred forms such
as the unique spectrum of colors emitted by heated atoms or the
characteristic note sounded by an organ pipe.  This energy sorting can even
turn up in a granular material.  For example, a few years ago (Update 286)
scientists discovered that collections of tiny metal balls, when shaken
slightly up and down, vested some of their energy in the form of tiny
waterspout heaps called "oscillons"  Now physicists at the Hebrew University
in Jerusalem (Jay Fineberg, have observed a similar
effect in a colloid, a fluid material (e.g., milk) in which tiny particles
(in this case small bits of clay) are suspended in a solvent (see  Granular media and suspensions are very
different in nature---grains are discrete objects that collide directly with
each other whereas the particles in colloids interact via the medium of the
solvent fluid---so the appearance of oscillons in both materials might
represent some universal manifestation of driven nonlinear systems. The
researchers  are not yet sure where localized oscillon states would turn up
in the natural world. One possibility is earthquakes.  Oscillon-like states
may explain the localized and highly variable damage (or intense ground
acceleration) which, in many cases, occurs in poorly consolidated sediments
(in analogy to the clay sediments used in the experiments)  at relatively
large distances from an earthquake's epicenter. (Lioubashevski et al.,
Physical Review Letters, tent. 11 Oct.)

VISUALIZING ELECTRONIC ORBITALS. The image of an atom is really the image
of its outermost electrons or, to be more precise still, the image of the
averaged likelihood that the electrons will be at various places.  For any
but the innermost electrons, the shape of this likelihood surface (or
orbital) will be non-spherical in shape.  Physicists at Arizona State have
now actually imaged these orbitals for the first time and shown that they
look just the drawings used in quantum textbooks for decades.  Using a
combination of x-ray diffraction and electron microscopy the ASU scientists
produced a 3D map of the orbitals of copper atoms and their bonds with
neighboring atoms in a cuprite (Cu2O) compound (see figure at  The images of Cu-O and Cu-Cu bonds might
provide insight into the workings of high temperature superconductors, in
which the whereabouts of electrons and holes (the voids left by vacated
electrons) are crucial.  (J.M. Zuo et al., Nature, 2 Sept. 1999.)

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