  The ever-growing field of microfluidics requires precise and flexible control
over fluid flow at the micro- and nanoscales. Current constraints demand a
variety of controllable components for performing different operations inside
closed microchambers and microreactors. In this context, novel nanophotonic
approaches can significantly enhance existing capabilities and provide new
functionalities via finely tuned light-matter interaction mechanisms. Here we
propose a novel design, featuring a dual functionality on-chip: boosted
optically-driven particle diffusion and nanoparticle sorting. Our methodology
is based on a specially designed high-index dielectric nanoantenna, which
strongly enhances spin-orbit angular momentum transfer from an incident laser
beam to the scattered field. As a result, exceptionally compact, subwavelength
optical nanovortices are formed and drive spiral motion of peculiar plasmonic
nanoparticles via the efficient interplay between curled spin optical forces
and radiation pressure. The nanovortex size is an order of magnitude smaller
than that provided by conventional beam-based approaches. The nanoparticles
mediate nano-confined fluid motion enabling nanomixing without a need of moving
bulk elements inside a microchamber. Moreover, precise sorting of gold
nanoparticles, demanded for on-chip separation and filtering, can be achieved
by exploiting the non-trivial dependence of the curled optical forces on the
nanoobject size. Altogether, this study introduces a versatile platform for
further miniaturization of moving-part-free, optically driven microfluidic
chips for fast chemical synthesis and analysis, preparation of emulsions, or
generation of chemical gradients with light-controlled navigation of
nanoparticles, viruses or biomolecules.
