To stretch a supply of salt generally means using it sparingly. But researchers
from Sandia National Laboratories
and the University of Pittsburgh were startled when they found they had made
the solid actually physically stretch.
Sandia researchers Jack Houston (left) and Nathan Moore examine a tiny salt block while the screen behinds them shows the magnified tip of the Sandia-developed interfacial force microscope (device in the foreground) performing another materials interrogation. The device was developed by Houston. Credit: Sandia National Laboratories
"It's not supposed to do that," said Sandia principal investigator
Jack Houston. "Unlike, say, gold, which is ductile and deforms under pressure,
salt is brittle. Hit it with a hammer, it shatters like glass."
That a block of salt can stretch rather than remain inert might affect world
desalination efforts, which involve choosing particular sizes of nanometer-diameter
pores to strain salts from brackish water. Understanding unexpected salt deformations
also may lead to better understanding of sea salt aerosols, implicated in problems
as broad as cloud nucleation, smog formation, ozone destruction and asthma triggers,
the researchers write in their paper published in the May Nanoletters.
The serendipitous discovery came about as researchers were examining the mechanical
properties of salt in the absence of water. They found unexpectedly that the
brittle substance appeared malleable enough to distort over surprisingly long
distances by clinging to a special microscope's nanometer-sized tip as it left
the surface of the salt.
More intense examination showed that surface salt molecules formed a kind of
bubble — a ductile meniscus — with the exploratory tip as it withdrew
from penetrating the cube. In this, it resembled the behavior of the surface
of water when an object is withdrawn from it. But unlike water, the salt meniscus
didn't break from its own weight as the tip was withdrawn. Instead it followed
the tip along, slip-sliding away (so to speak) as it thinned and elongated from
580 nanometers (nm) to 2,191 nm in shapes that resembled nanowires.
A possible explanation for salt molecules peeling off the salt block, said
Houston, is that "surface molecules don't have buddies." That is,
because there's no atomic lattice above them, they're more mobile than the internal
body of salt molecules forming the salt block.
Salt showing signs of surface mobility at room temperatures was "totally
surprising," said Houston, who had initially intended to study more conventionally
interesting characteristics of the one-fourth-inch square, one-eighth-inch-long
salt block.