Five aerosol water droplets are trapped in a ring. The droplets can be rotated, using
holographic optical tweezers, to move through the “interrogation zone” denoted by the
dashed box on the left-hand side. Here a Raman spectra is taken (right-hand image)
and can be used to analyze the droplet composition as well as to gain information on
its size. Using this technique, one can make comparative measurements between the
different droplets.
to trap such particles, but heating may
compromise this approach. Research
that evaluates small, opaque particles
such as soot would help shed light on ice
nucleation and aerosol aging.
The past four decades have seen this
optical subfield mushroom into a widely
used technique across all the sciences.
This explosion of work is unlikely to
slow down as more and more physicists
become interdisciplinary scientists and
move into applied areas in biophysics
and chemistry. Technology development continues as well. However, it is
the applications—in biology, medicine,
atmospheric science, and other areas—
that are driving this exciting field. Look
for more groundbreaking discoveries in
the microworld over the next 40 years. t
and it may lead to novel strategies for
deploying and developing biosensors.
Trapping aerosols
To end, we come full circle. In some of
the very first papers published on optical
forces in the early 1970s, Ashkin looked
at trapping particles in air: aerosols. This
optical levitation work did not receive
much attention when optical tweezers
were developed, and it has remained a
niche area that was far removed from
mainstream optical manipulation
research. It was not until very recently
that any airborne particle was actually
trapped within a conventional (
high-numerical-aperture) optical trap.
Clearly, however, as our need to
understand climate change grows, the
work on airborne particles is becoming
increasingly relevant. Optical tweezers
provide a unique way of studying aerosol properties. They are able to localize
a range of particle sizes in a simple
way that other techniques, such as the
electrodynamic balance, cannot match.
They are particularly useful in the study
of particle dynamics, as they are a nondestructive mechanism—unlike, say, a
mass spectroscopy measurement.
In the figure above, we show the
combination of some of the techniques
we discussed to form an optical carousel
for aerosols. Here, a holographic optical
tweezers system is used to rotate trapped
aerosols through a Raman spectroscopy
probe region, so that particles can be
analyzed in a comparative fashion, keep-
ing the environment the same but the
aerosol size different, for example. Our
groups have led the way in analyzing
aerosol dynamics, both in understand-
ing the basic physics and technology
and in developing techniques to probe
atmospherically interesting aerosol
properties, such as size, temperature,
hygroscopicity, chemical aging and
mass accommodation.
David McGloin ( d.mcgloin@dundee.ac.uk) is
with the electronic engineering and
physics division at the University of
Dundee, Dundee, United Kingdom. Jonathan P.
Reid is with the School of Chemistry, University
of Bristol, Bristol, United Kingdom.
Member
[ References and Resources ]
>> Micro Tetris: www.youtube.com/
watch?v=jCdnBmQZ6_s
>> Smallest Strip the Willow in the World:
www.physics.gla.ac.uk/Optics/projects/
tweezers/movies/Strip The Willow.mov
>> A. Ashkin. “Acceleration and Trapping of
Particles by Radiation Pressure,” Phys.
Rev. Lett. 24, 156 (1970).
>> A. Ashkin et al. “Observation of a single-beam gradient force optical trap for
dielectric particles,” Opt. Lett. 11, 288
(1986).
>> M.J. Lang and S.M. Block. “Resource
letter: LBOT-1: Laser-based optical tweezers,” Am. J. Phys. 71, 201 (2003).
>> D. McGloin. “Optical Tweezers: 20 years
on,” Phil. Trans. Roy. Soc. A 364, 3521
(2006).
>> K. Dholakia et al. “Optical Micromanipula-tion,” Chem. Soc. Rev. 37, 42 (2008).
>> M. Dienerowitz et al. “Optical manipulation of nanoparticles: a review,” J. Nano-photon. 2, 021875 (2008).
>> H. Zhang and K.K. Liu. “Optical tweezers
for single cells,” J. Roy. Soc. Interface 5,
671 (2008).
>> T. Perkins. “Optical traps for single
molecule biophysics: a primer,” Laser &
Photon. Rev. 3, 203 (2009).
>> J.B. Wills et al. “Optical Control and Characterisation of Aerosol,” Chem. Phys. Lett.
481, 153 (2009).
26 | OPN Optics & Photonics News
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