Abstract
The equilibrium position of an aerosol droplet trapped in a counter-propagating Bessel beam and gas flow is studied both experimentally and theoretically. This provides an aerosol analogue to the separation of particles of differing size and refractive index in counter-propagating laser beam and liquid flow, referred to as optical chromatography. Using the model system of a pure glycerol droplet it is found that, as evaporation takes place and the size of the droplet decreases, the size-dependent equilibrium position does not change in a simple monotonic fashion. Instead, the position of the droplet is greatly affected by the excitation of whispering gallery modes. This leads to sharp peaks in the equilibrium position curve, not unlike those that occur in single particle spectroscopy. The conditions necessary to excite whispering gallery modes are thoroughly investigated.
Highlights
The manipulation of nano- and micro-objects using light is at the heart of many mature and emerging areas of research
Minima in the potential energy curves are located at positions that approximately correspond to intensity maxima in the rings of the Bessel beam, suggesting that droplets will preferentially adopt equilibrium positions in regions of maximum light intensity
The only equilibrium position for the a = 1.5 μm curve is at x0 = 0 which coincides with the center of the core of the Bessel beam
Summary
The manipulation of nano- and micro-objects using light is at the heart of many mature and emerging areas of research. The use of light to levitate particles [1] and the realization of optical tweezers [2] demonstrated that precise optical control of micro-particles was possible These tools have found enormous success in biological applications such as in the manipulation and mechanical characterization of cells [3], and in single molecule studies such as monitoring the movements of motor proteins [4,5,6] and mapping the energy landscapes of nucleic acids [7, 8]. The equilibrium position (or retention distance) of aerosol droplets in a system such as that shown in figure 1 will be studied here. For the systems of curent interest, this propagation distance will be several millimeters Such a beam has an inherent appeal for use in optical chromatography. We demonstrate that WGMs, droplet position, and the non-ideal nature of the Bessel beam in the region where the droplet is trapped greatly influence experimental results
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