Optical Transport and Sorting of Fluorescent Nanodiamonds inside a Tapered Glass Capillary: Optical Sorting of Nanomaterials at the Femtonewton Scale

Christophe Pin, Ryohei Otsuka, and Keiji Sasaki

Nanoparticles from biological, environmental, or industrial sources always show some dispersion in size, shape, composition, and related physical or chemical properties. Sorting nanoparticles according to well-defined criteria is often a crucial but challenging task. While optical forces may be used to target some specific properties such as the size, shape, absorption wavelength, and chirality of nanoparticles, optical sorting techniques usually suffer from the fast diffusion of nanoparticles in comparison to the relative weakness of the optical forces acting on dielectric nanomaterials in liquid dispersion. To achieve high-efficiency optical sorting of an ensemble of nanoparticles in colloidal dispersion, all the nanoparticles to be sorted should be gathered and kept in the light path for a sufficient time. For this purpose, we investigate the use of tapered glass capillaries as optofluidic platforms for optical manipulation and optical sorting applications. While the transparent pipe-like structure of the capillary serves as an optical waveguide that focuses the laser light over a few-millimeter-long distance, the inner part of the capillary forms a microfluidic channel that is filled with a water dispersion of 100 nm fluorescent nanodiamonds (NDs). We first demonstrate power-dependent optical transport of NDs inside few-micrometer-large capillaries. It is observed that NDs located inside the waist of the tapered capillary can be optically propelled at velocities reaching few tens of micrometer per second. We then show how a liquid flow inside the channel enables efficient, size-dependent sorting of a large ensemble of NDs. An analytical model is used to evaluate the influence of the NDs’ size on the optical and hydrodynamic drag forces acting on the nanoparticles, both being in the femtonewton range.

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