Many
recent advances in microtechnology and nanotechnology depend on
microscopic spherical particles self-assembling into large-scale
aggregates to form a relatively limited range of crystalline structures.
Directed assembly is a new branch of this field, where scientists
figure out how to make particles assemble to form a broad range of
structures at given locations.

Current
techniques for directed assembly typically use an applied field, such
as an electric or magnetic field, to move particles and to assemble them
into well-defined structures. Now, researchers at the University of
Pennsylvania have identified a simple new method to direct particle
assembly based only on surface tension and particle shape.

The
research, led by Kathleen J. Stebe, professor in the Department of
Chemical and Biomolecular Engineering and the school’s Deputy Dean for
Research, was performed by a team of researchers in her laboratory,
Marcello Cavallaro Jr., Lorenzo Botto, Eric P. Lewandowski and Marisa
Wang. It was published in the Proceedings of the National Academy of
Sciences.

Their results rely on the simple fact that a liquid surface will tend to minimize its surface area. 

“It’s
the same reason that surface tension makes a drop of water want to be a
sphere,” Stebe said. “But we can tune that phenomenon to do astonishing
things.”

Self-assembling
spherical particles have been used to make new materials with unique
optical and mechanical properties, but non-spherical, or anisotropic,
particles may hold even greater promise. By having a definable
directionality, the properties of the materials the particles make up
can be altered based on their orientations.    

In
the study, Stebe’s lab used cylindrical particles made out of a common
polymer. When placed on the surface of a thin film of water, the
cylinders produce a saddle-shaped deformation: the water’s surface dips
at each end of a particle and rises up along their sides.

The
Stebe lab had previously demonstrated that this saddle-shape can be
used to orient two cylindrical particles end-to-end.  As the depressions
at their ends come in contact, surface tension causes the area of the
space between them to contract, bringing the ends together.

In
the new study, instead of two particles interacting, particles interact
with a stationary post. The post pokes through the water’s surface,
causing the surface to curve upward around it. The interaction between a
particle’s deformation and this curve is governed by the same
phenomenon of surface tension shown in the earlier study; the particles
move so as to make the surface area as small as possible.

“This
means that as soon as the particles hit the surface of the water, they
change their alignment and start moving rapidly uphill toward the post,”
Cavallaro said. “We were also able to predict the lines they would
travel for three different post shapes.”  

By
changing the cross-sectional shape of the posts, the researchers were
able to show fine control over how the particles moved and oriented. A
circular post attracted particles in straight lines, whereas an
elliptical post drew particles to the elongated ends. A square post
produced the most complex behavior, drawing particles strongly to the
corners, leaving the sides open. 

The
lab’s choice of particle shape and material was only to help the
researchers observe the particles’ orientation and position; any
non-spherical particle, on any liquid-liquid or liquid-vapor surface,
would be governed by the same principles and produce the same type of
deformation. This makes this research particularly powerful: it does not
depend on the particle having a certain shape or being made from a
certain material.

Surfaces
studded with strategically placed and shaped posts could direct and
orient particles into almost any configuration. And because the
mechanism behind the particles’ movement is simply the surface
curvature, their movement could be “programmed” by changing the
arrangement of the posts or the shape of the interface.

“I
could go in with needle, for example, and dynamically pull the surface
up at different locations, or over different times,” Stebe said.

“Very
often when we think about using micro- or nanotechnology, we’re not
thinking about properties on that tiny scale: It’s going to be the
organized structure made from micro- or nanoparticles that’s going to be
useful, perhaps as a lens or a smart surface,” she said. “This
phenomenon could be used to make new structures by sending particles to
certain locations. We could define paths and say ‘here’s a docking site:
go there’ or ‘here’s a spot where we want nothing; don’t go there.’

“This
is a clear demonstration of directed assembly. Like self-assembly,
things come together from the bottom up, but here they come together
exactly where we want them to.”

The research was supported by the National Science Foundation.

Source: University of Pennsylvania