Airborne microparticle manipulation

Airborne microparticles are an integral part of our environment. They are commonly encountered in large quantities, then called "aerosol". These aerosols form the clouds in the sky as well as the smog over some of our cities, thereby critically affecting our daily life. A full understanding of these phenomena necessarily requires the understanding of the involved physical and chemical processes on a microscopic scale. It requires the separation of single particles out of these aerosols to allow a undisturbed study of their basic interactions.
Our work aims to create handy and simple yet powerful optical devices that perform this task.

Levitation of microparticles with laser light

The levitation of a microparticle by laser-light mediated forces is a fascinating phenomenon. It was demonstrated for the first time by Arthur Ahskin and coworkers in 1970. Since then, a variety a of "optical traps" have been invented. The video on the right shows a glycerol-water droplet levitated in a fiber-based counterpropagating beam trap. In this model, the trapping site is located inside the aerosol injection chamber. Remaining (untrapped) aerosol particles obviously influence the trapped one. Precise and sensitive measurements are often hindered by such unwanted interactions.

Integrated fiber-capillary trap

For this reason, we chose to design a novel optical airborne microparticle trap, separating the injection chamber from the actual trapping site.

Schematic of the trap setup:
An optical fiber emits green laser light into the injection chamber. Particles drifting into this laser beam inside the injection chamber are propelled through a glass-capillary into the sample chamber. The counterpropagating beam trap in the sample chamber is formed by the light radiating out of the capillary and the red light emitted by another single-mode optical fiber.

The transportation of a particle through a capillary into a counterpropagating beam trapping site can be observed in the video below.

Note that this is a true-color recording! Green laser light is used to push the particle through the capillary, while the red light from the right side is used to balance the axial optical forces and create a stable trapping position. The blue illumination is necessary to obtain a sharp microscope image of the trapped particle.

Measurement of size and refractive index

Measurement of size and refractive index of a trapped droplet.

Particles in our trap scatter the laser light that holds them in a characteristic way. We collect this light with a microscope objective placed perpendicular to the fiber axis and image it onto a camera. The counts at a specific pixel position can then be related to the intensity scattered into a respective solid angle. The green line in the figure on the right represents such a measurement for a trapped glycerol-water droplet.
The scattering pattern produced by a spherical droplet of diameter d and refractive index n can be calculated using Mie theory. We successively alter these parameters until the calculated pattern agrees with the measured one. This way we are able to precisely determine the size and composition of trapped droplets.

Droplet evaporation

The size of an airborne droplet decreases continously due to evaporation of the liquid. The video on the right shows a time lapse of an evaporating glycerol-water droplet. Pattern matching software was used to automatically determine the droplets size evoution.

At the same time, the scattered laser light is collected, recorded and evaluated as described above. We cross-checked the results with the ones from the pattern matching procedure and found them to agree well. Note that the light scattering measurements are substantially more precise (approximately one order of magnitude).

Comparison of the experimentally measured to the theoretically calculated Mie scattering spectrum of a evaporating glycerol-water droplet is shown below.

Theoretically predicted Mie scattering spectrum.

Experimentally measured Mie scattering spectrum.

References

• M. Horstmann, K. Probst and C. Fallnich, An integrated fiber-based optical trap for single airborne particles, Appl. Phys. B
  
• Ashkin, A. Acceleration and Trapping of Particles by Radiation Pressure, Phys. Rev. Lett., 1970, 24, 156-159
• Renn, M. J.; Pastel, R. & Lewandowski, H. J. Laser Guidance and Trapping of Mesoscale Particles in Hollow-Core Optical Fibers, Phys. Rev. Lett., 1999, 82, 1574-1577
• Davis, E. & Schweiger, G. The Airborne Microparticle, Springer, 2002

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