title: Long Working-Distance Optical Trap for in Situ Analysis of Contact-Induced Phase Transformations.
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.
authors.affiliation: †Cooperative Institute for Research in Environmental Sciences (CIRES), University of Colorado, Boulder, Colorado 80309, United States.|‡Department of Chemistry and Biochemistry, University of Colorado, UCB 216, Boulder, Colorado 80309, United States.
authors.forename: Ryan D
authors.initials: RD
authors.lastname: Davis
authors.affiliation: †Cooperative Institute for Research in Environmental Sciences (CIRES), University of Colorado, Boulder, Colorado 80309, United States.
authors.forename: Sara
authors.initials: S
authors.lastname: Lance
authors.affiliation: ∥National Institute of Standards and Technology (NIST), Communications Technology Lab, Boulder, Colorado 80305, United States.
authors.forename: Joshua A
authors.initials: JA
authors.lastname: Gordon
authors.affiliation: †Cooperative Institute for Research in Environmental Sciences (CIRES), University of Colorado, Boulder, Colorado 80309, United States.|‡Department of Chemistry and Biochemistry, University of Colorado, UCB 216, Boulder, Colorado 80309, United States.
authors.forename: Margaret A
authors.initials: MA
authors.lastname: Tolbert