spin microscopic objects
Technology Research News
A group of scientists from St. Andrews
University in Scotland have found a way to rotate microscopic objects
using the interference pattern produced by two laser beams.
The method, which allows the researchers to closely control how quickly
an object spins, could prove useful in developing new rotational elements
for micromachines and
should make it easier for biologists to study the effects of rotational
forces on large molecules.
The method works because objects as small as a few microns are largely
transparent to light, and when lightwaves pass through an object, the
lightwaves bend. The equal and opposite force required to counteract the
bend isn't something that makes much difference at the macro level, but
it is enough to move micron-size objects. A micron is one-thousandth of
a millimeter; an E. coli bacteria is about two microns long and one micron
Using lasers to trap and move small objects is something scientists regularly
The researchers' key idea was to use the interference pattern of two light
beams to rotate the objects.
They used an interferometer to split a beam of light, then changed the
phases of the beams so the beams interfered with one another in a certain
pattern when recombined. "The idea is to use a special combination of
two light beams that results in a spiral [wave] pattern," said Kishan
Dholakia, a lecturer at St. Andrews University in Scotland. "By varying
one of the light beams we can cause the pattern to rotate. The particles
are trapped in the bright regions of the pattern -- the spiral arms --
and rotate too," he said.
Because the method is relatively simple, it is easy to control, allowing
the researchers to rotate tiny objects at different speeds. "We can control
the pattern rotation exactly and therefore the rotation of the particles
with high precision," said Dholakia.
The researchers have used the method to set microscopic glass rods, chromosomes
and clusters of silica microspheres spinning from one to five hertz, or
rotations per second. The glass rods were five microns long and the sphere
clusters ranged in length from one to five microns. A single chromosome
weighs about one millionth of one millionth of a gram.
"So far we have only a very basic set up to demonstrate that the idea
works, but... rotation rates of several tens or even hundreds of hertz
should be easily obtainable," said Jochen Arlt, a research fellow at St.
Andrews University in Scotland.
The researchers "have devised a very elegant method of rotating a wide
variety of micron-sized objects [optically]," said Chris Meiners, an assistant
professor at the University of Michigan. "It's beauty lies in the use
of a helical interference pattern as [a] force generating field that can
be rotated simply by changing the phase in one of the interferometer arms,"
The technique "promises to be very useful [because] it can be used to
rotate virtually any nonspherical object that can be optically trapped,"
he said. This would allow microelectromechanical systems researchers to
more easily develop tiny rotors, he said. The technique itself is not
likely to be used in micro mechanical products, however because the system
-- with the laser, interferometer and microscope -- is too complex and
bulky, he added.
The method also has potential biological uses, Meiners said. "I think
we will soon see single molecule experiments using it to study biological
macromolecules under an applied torque," he said. Until now, optical trapping
techniques have allowed biologists to study molecules like DNA by applying
and measuring the effects of linear forces, but there have been few experiments
studying the equally important effects of rotational forces because rotating
molecules is difficult, said Meiners.
The researchers plan to further develop the method for both micromachine
and biological applications, said Dholakia. The method should work well
with biological objects because "many biological things are transparent
at some wavelength, and one can even attach transparent beads... to help,"
he said. Possible biological applications include orienting biological
objects to get active sites to latch onto one another, and microstirrers
that mix things on a microscopic scale, he said.
The method could be applied practically in less than two years, said Arlt.
Arlt and Dholakia's research colleagues were Lynn Paterson, Michael P.
McDonald, Wilson Sibbett and Peter E. Bryant. They published the research
in the May 4, 2001 issue of the journal Science. The research was funded
by The UK Engineering and Physical Sciences Research Council.
Timeline: < 2 years
TRN Categories: Materials Science and Engineering; Optical
Computing, Optoelectronics and Photonics
Story Type: News
Related Elements: Technical paper, "Controlled Rotation
of Optically Trapped Microscopic Particles," Science, May 4, 2001.
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