Metal Contacts at the Atomic Scale
The atomic process in which two metallic surface contact each other has
been characterized. The formation of a contact between two bodies is a
process which always involves the formation of at least one atomic
contact. Many tribological problems such as adhesion or friction will
then depend on the formation of such a contact. We show that for many
situations this process occurs smoothly without a mechanical instability
that would lead to a jump to contact. We observe that the configuration
and material composition of the electrodes before contact largely
determine the presence or absence of a jump. Through a combination of
experiments, atomistic simulations, and first-principles transport
calculations the first contact in between two metallic surfaces is shown
to be formed, in most of the times, by either a single atom , two atoms
aligned or two parallel atoms.
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Sending a camel through the eye of a needle, the mechanics of DNA
translocation through nanopores
The Bibilical phrase, according to some historians, actually refers
to a certain gate in Jerusalem called
the "Needle's Eye" that was so narrow that a camel could barely squeeze
through and only if unencumbered by baggage. About ten years ago
scientists were quite amazed to
discover that single molecules of DNA will squeeze through holes that
are only just bigger than their diameter,
if a small electrical voltage is applied across it. Since the entry
of a DNA blocks the current that would normally flow
through the hole, the characteristics of the DNA can be inferred by
monitoring the amount of this current.
The intriguing possibility that information about the actual base
sequence can be coaxed out of this current signal
has become the holy grail of a sizable group of researchers pursuing
this as a possible ultrafast DNA sequencing
technology. A major obstacle is that unlike the proverbial camel
trying to go through the Needle's Eye, the DNA
zips through at the galloping clip of a thousand to a million bases
per second -- too fast for scientists to "read" the
sequence. In this paper, we take the first step towards slowing down
the DNA, which is to provide an understanding
of what determines its speed in the first place! It is shown that the
same hydrodynamic resistance that determines
the speed with which a wire can be drawn through a die in the
classical engineering science of metallurgy
also operate at these ultra small molecular scales and is responsible
for determining how fast the DNA
crosses the pore.
