Separation-dependent near-field effects in Mie scattering spectra of two optically trapped aerosol droplets

The backscattering of ultraviolet and visible light by a model organic (squalane) aerosol droplet (1.0<r<1.4 μm) is investigated upon approach of a second isolated droplet at varying separations. Illumination and collection of light is along the interparticle axis. The conditions replicate typical broadband light spectroscopy studies of atmospheric aerosol. T-Matrix near-field modelling, which includes near-field effects, predicts separation-dependent changes in the intensity of the backscattered light on close approach of neighbouring spheres. However, the experimental results show no evidence of separation-dependent near-field effects on the scattering. The results are best replicated by modelling the droplets as individual Mie scatterers.