Long Working-Distance Optical Trap for in Situ Analysis
of Contact-Induced Phase Transformations
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Abstract
A novel optical trapping technique
is described that combines an
upward propagating Gaussian beam and a downward propagating Bessel
beam. Using this optical arrangement and an on-demand droplet generator
makes it possible to rapidly and reliably trap particles with a wide
range of particle diameters (∼1.5–25 μm), in addition
to crystalline particles, without the need to adjust the optical configuration.
Additionally, a new image analysis technique is described to detect
particle phase transitions using a template-based autocorrelation
of imaged far-field elastically scattered laser light. The image analysis
allows subtle changes in particle characteristics to be quantified.
The instrumental capabilities are validated with observations of deliquescence
and homogeneous efflorescence of well-studied inorganic salts. Then,
a novel collision-based approach to seeded crystal growth is described
in which seed crystals are delivered to levitated aqueous droplets
via a nitrogen gas flow. To our knowledge, this is the first account
of contact-induced phase changes being studied in an optical trap.
This instrument offers a novel and simple analytical technique for
in situ measurements and observations of phase changes and crystal
growth processes relevant to atmospheric science, industrial crystallization,
pharmaceuticals, and many other fields