Studying aerosol in the atmosphere for climate science involves being able to characterise properties such as radius and refractive index. The light scattering instruments used to do this are normally calibrated with polystyrene beads and therefore knowledge of the bead’s refractive index as a function of wavelength is crucial for this purpose.
A paper published in Physical Chemistry Chemical Physics (link opens in a new window) (article available as open access) describes a novel method designed by PhD student Stephanie Jones (Royal Holloway University of London, RHUL) and her supervisors Andy Ward (CLF) and Martin King (RHUL) for measuring “the refractive index dispersion with wavelength of optically trapped solid particles in air”.
The laser optical trap/tweezers set-up. A 6-micron diameter bead can be seen levitating in the right-hand image (click to open larger version in new window)
The laser tweezers were used to capture individual polymer spheres from a mist of ~ 3 micron diameter polystyrene beads. The beads were then illuminated with a white light source under which they acted as optical cavities and the resulting spectra of the elastically scattered white light were collected. The spectra were then used to infer the beads' radii as well as their wavelength-dependent refractive indices.
Interestingly, the work also revealed a significant variation in the refractive index as a function of wavelength, for each bead in a commercial sample, indicating that the beads are chemically slightly different and therefore caution should be taken when using polystyrene beads to calibrate light scattering instruments that are used for aerosol size measurements.
Related articles:
CLF tweezers and ISIS neutron source revealing cloud chemistry (link opens in a new window)
