High-speed tracking of coalescence and mixing of optically controlled droplets

We are presenting an all-optical platform for on-demand and nonintrusive studies of coalescence of micron-sized liquid droplets in air. The platform has the purpose to mimic the airborne conditions occurring in typical environments where droplet interactions are involved. By means of the Förster resonance energy transfer mechanism, we have been able to track the dynamics of two coalescing glycerol droplets, with a radius of <a:math xmlns:a="http://www.w3.org/1998/Math/MathML"><a:mrow><a:mo>∼</a:mo><a:mn>10</a:mn><a:mspace width="4pt"/><a:mi>µ</a:mi><a:mi mathvariant="normal">m</a:mi></a:mrow></a:math> and a temporal resolution of <d:math xmlns:d="http://www.w3.org/1998/Math/MathML"><d:mrow><d:mn>86</d:mn><d:mspace width="4pt"/><d:mi>µ</d:mi><d:mi mathvariant="normal">s</d:mi></d:mrow></d:math>. At the onset of coalescence, a fast mixing is observed. Such an effect depletes the fluorescence of the donor fluorophore to half its initial intensity on a timescale below the temporal resolution of our platform, whereas a complete mixing is observed to occur on a timescale of seconds. The experimental platform design and findings facilitate an increased understanding of the microphysics of collisions, coalescence, and the subsequent mixing/diffusion of droplets on the micrometer lengthscale and the microsecond timescale. Published by the American Physical Society 2024