Prof. Erika Eiser, University of Cambridge

Optical tweezers have been established as indispensable tool for the manipulation of micro- and
nano-sized objects. We show that colloids anchored to a water-oil interface via DNA-tethers
will crystallize when only one of the particles is trapped with optical tweezers [1,2]. These
DNA-anchored colloids are fully immersed in the water phase, thus they do not disturbe the
oil-water interface, but allow the tethered colloids to diffuse freely along the oil-droplet surface
in the absence of a tweezing laser [3,4]. Our combined experimental and theoretical analyses
show that local temperature gradients induced by optical tweezers cause a thermophoretic force
pushing the trapped particle towards the colder oil phase, causing an attractive long-ranged
hydrodynamic flow towards the laser focus that promotes the observed out-of equilibrium
crystallization of the DNA tethered colloids around the trapped particle. The crystallization is
further enhanced by scattering forces known as optical binding.

[1] A. Caciagli, R. Singh, D. Joshi, R. Adhikari, E. Eiser, PRL accepted (2020)
[2] A. Caciagli, D. Joshi, J. Kotar & E. Eiser, arXiv 1703.08210 (2017)
[3] D. Joshi, D. Bargteil, A. Caciagli, J. Burelbach, J. Xing, A. Nunes, D. Pinto, N. Araujo, J. Bruijc & E. Eiser,
Science Advances, 2 : e1600881 (2016)
[4] A. Caciagli, M. Zupkauskas, A. Levin, T.P.J. Knowles, C. Mugemana, N. Bruns, T. O’Neill, W. J. Frith & E.
Eiser, Langmuir 34, 10073 (2018)

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