Size-dependent position of a single aerosol droplet in a Bessel beam trap

Abstract

The equilibrium position of an aerosol droplet trapped in a counter-propagating Bessel beamand gas flow is studied both experimentally and theoretically. This provides an aerosolanalogue to the separation of particles of differing size and refractive index incounter-propagating laser beam and liquid flow, referred to as optical chromatography. Usingthe model system of a pure glycerol droplet it is found that, as evaporation takes place and thesize of the droplet decreases, the size-dependent equilibrium position does not change in asimple monotonic fashion. Instead, the position of the droplet is greatly affected by theexcitation of whispering gallery modes. This leads to sharp peaks in the equilibrium positioncurve, not unlike those that occur in single particle spectroscopy. The conditions necessary toexcite whispering gallery modes are thoroughly investigated.