Skip to main content
U.S. flag

An official website of the United States government

Official websites use .gov
A .gov website belongs to an official government organization in the United States.

Secure .gov websites use HTTPS
A lock ( ) or https:// means you’ve safely connected to the .gov website. Share sensitive information only on official, secure websites.

Long Working-Distance Optical Trap for in Situ Analysis of Contact-Induced Phase Transformations



Joshua A. Gordon, Ryan D. Davis, Sara Lance, Margaret A. Tolbert


The development and characterization of a long working-distance optical trap to analyze a diverse range of particle phase transformations and crystal growth processes is described. Utilizing an upward propagating Gaussian beam and a down-ward propagating Bessel beam, aqueous droplets ranging from 1-25 µm in diameter and irregularly shaped crystalline parti-cles as large as ~15 µm in diameter have been stably levitated for indefinite lengths of time. This range of particle diameters and morphologies is possible without changing optical configurations. An image analysis is described to detect particle phase transitions using a template-based autocorrelation of imaged far-field elastically scattered laser light. The instrumental capabilities are validated with observations of deliquescence and homogeneous efflorescence of well-studied inorganic salts, including (NH4)2SO4, NaCl, NaBr, Na2SO4, and K2SO4. Utilizing the long working distance of the optical trap and the far-field image analysis, 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. The long working distance of this setup (55 mm) allowed high gas flow rates to deliver sub- or super-micron seed crystals to the aqueous phase at any desired level of supersaturation. 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.
Journal of Analytical Chemistry


optical tweezers, optical trap, bessel beam, droplet, efflorescence


Gordon, J. , D., R. , Lance, S. and Tolbert, M. (2015), Long Working-Distance Optical Trap for in Situ Analysis of Contact-Induced Phase Transformations, Journal of Analytical Chemistry, [online], (Accessed June 24, 2024)


If you have any questions about this publication or are having problems accessing it, please contact

Created May 11, 2015, Updated January 27, 2020