Ms. Megan McGrory Profile

Megan McGrory

at Royal Holloway Univ of London

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Proceedings Article | 9 September 2019 Paper

Spectroscopic studies of optically trapped aerosol at elevated temperatures

Andrew Ward, Megan McGrory, Martin King

Proceedings Volume 11083, 1108323 (2019) https://doi.org/10.1117/12.2530088

KEYWORDS: Refractive index, Aerosols, Particles, Polymers, Optical tweezers, Raman spectroscopy, Spectroscopy, Atmospheric particles

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A prototype sample cell has been designed and tested with the long-term aim of following heterogeneous catalysis chemistry and aerosol synthesis on optically trapped aerosols at temperatures approaching 500°C. Liquid aerosol droplets that contain high molecular weight molecules that gel, and then solidify, at higher temperatures have become of increasing interest to the catalysis community. To address the challenges of performing spectroscopic studies on individual airborne particles at high temperatures a sample cell was designed to localise heating at the optical trapping position whilst maintaining the objective lenses at close to ambient temperature. The heating cell was tested using polystyrene beads (2.0 μm diameter) that were trapped in air between opposed 1064 nm laser beams, and illuminated with a broadband white LED. Backscattered light from the trapped particle was collected to obtain a Mie spectrum over the 450-620 nm wavelength range. Mie spectral fitting was used to determine particle radius and wavelength dependent refractive index. In addition, a 514.5 nm laser beam was used to illuminate the particle to generate a Raman spectral signal. Raman spectroscopy enables the measurement of conformational changes in the polymer sample. The trapped particle was heated within the aperture of a ceramic heating element and retained through the melting point of polystyrene (~240°C). The changes in size and refractive index were measured. Both the glass transition temperature (T_g), melting point and the thermal expansion coefficient of a single bead were determined in comparison to literature values.

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