The optical trapping of airborne hydrocarbon droplets from an oil mist

Abstract

This paper describes the optical trapping of hydrocarbon droplets in air using a high numerical aperture objective lens (NA = 1.4) to tightly focus a continuous wave Nd:YAG laser (λ = 1064 nm). The trapping efficiency (Q = 0.42) was sufficiently robust to enable capture of droplets directly from the turbulent flow of an oil mist generated using a commercial ultrasonic nebulizer. Droplets could then be manipulated in three dimensions for periods in excess of 30 minutes. Ultrasonic nebulizers are commonly used for respiratory therapy; hence the captured oil droplets were in the respirable size range of 1–7 µm. To enable preliminary studies of droplet interactions two optical traps were created using acousto-optic deflection techniques to multiplex the laser beam. These traps were independently manipulated to force a collision that resulted in rapid droplet coalescence where the larger droplet was retained in the optical trap. Information on the chemical nature of the trapped droplets was also acquired by changing the laser wavelength to 514.5 nm and collecting the Raman spectra. © 2006, Taylor & Francis Group, LLC.