Physicists have a pretty good idea of what to expect
when friction and adhesion occur in the visible world. You
jam on the brakes, for instance, and your tires and the
highway interact to stop your car. You glue two pieces of
wood together, and they stick.
But how slippery or sticky are things that are too
small to see? When solid surfaces no more than a thousand
atoms across brush past each other, will they respond like
the rubber and the road? Will they adhere like the wood and
the glue?
The answer turns out to be "it depends," according to
Johns Hopkins physicists who used computer modeling to
examine how friction and adhesion operate on the atomic
level.
"Any surface made of individual atoms has 'bumps' of
atomic dimension, and being able to vary the placement of
atoms [in the computer models] allowed us to quantify the
influence of atomic structure," said Mark O. Robbins, a
professor in the Henry
A. Rowland Department of Physics and Astronomy in the
Krieger School of Arts and Sciences.
The modeling showed that surfaces from a few to a
thousand atoms across, with the same shape but different
local structures, or "bumps," behave quite differently,
even if those surfaces are made of the same material,
Robbins said. Local stresses and adhesion forces can vary
by a factor of two or more, and friction can change
10-fold, he said.
The research is reported in the June 16 issue of the
journal Nature by Robbins and graduate student
Binquan Luan. Their findings could one day help in the
successful design of nanomachines, the name given to
devices built by manipulating materials on an atomic
scale.
"Everyone knows that matter is made up of discrete
atoms, yet most models of mechanical behavior ignore this
and think of atoms as being 'smeared' into an artificial
continuous medium," Robbins said. "This approach works well
when describing the behavior of larger machines, but what
happens when the whole machine is only a few to a thousand
atoms across? The answer is crucial to the function of
man-made nanomachines and many biological processes."
Robbins and Luan examined contact between solid
surfaces with "bumps" whose radii varied from about 100 to
1,000 atomic diameters. Bumps that size might be typical of
nanomachine surfaces or the tips of atomic force
microscopes used to measure mechanical properties at the
atomic scale.
Using computer simulations, the team followed the
displacements of up to 10 million atoms as the solid
surfaces were pushed together. They then compared these
displacements and the total adhesion and friction forces to
calculations of the same forces using the standard
"continuum theory," the model that views matter as having
smeared rather than discrete atoms.
"Knowing the exact atomic structure and how each atom
moved allowed us to test the two key assumptions of
continuum theory," Robbins said. "While it described the
internal response of solids down to nearly atomic scales,
its assumption that surfaces are smooth and featureless
failed badly" at the atomic level.
In a "News and Views" paper accompanying the Nature
article, Jacob Israelachvili of the University of
California, Santa Barbara, noted that these results have
fundamental implications for the limits of theories that
try to "smear out" atomic structure, as well as indicating
"how surfaces might be tailored in desirable ways ... if
atomic-scale details are taken into consideration."
This work is important because of the growing interest
in nanotechnology, in which unwanted adhesion and excessive
friction can cause devices to malfunction or just not to
work, Robbins said. "Hopefully, this will help in the
creation of new tools needed to guide the design of
nanotechnology," he said.
The study was funded by the National Science
Foundation.