Optofluidic Force Induction Scheme for the Characterization of Nanoparticle Ensembles

Momentum transfer from light to matter provides the basic principle of optical tweezers, which have been awarded the Nobel Prize in Physics 2018.[1] Most studies have hitherto employed this principle for trapping and manipulation of single nanoparticles. However, in a microfluidic channel one can also monitor the effect of optical forces exerted on ensembles of dielectric nanoparticles, to acquire knowledge about various nanoparticle parameters, such as size, shape or material distributions. In this paper we present an optofluidic force induction scheme (OF2i) for real-time, on-line optical characterization of large ensembles of nanoparticles.[2] Our experimental setup builds on precisely controlled fluidics as well as optical elements, in combination with a focused laser beam with orbital angular momentum. By monitoring the single-particle light scattering and nanoparticle trajectories, we obtain detailed number-based information about the properties of the individually tracked particles. We analyse the trajectories using a simulation approach based on Maxwell’s equati­ons and Mie’s theory, in combination with realistic laser fields and fluidic forces.[3] We discuss the basic physical principles underlying the OF2i scheme and demonstrate its applicability using standardized Latex particles with a pre-determined size distribution as calibration reference. Our measurement scheme is applied to different particle systems and evaluated within our theoretical framework, where we also monitor evolutionary processes over large time scales. Our results prove that OF2i provides a flexible work bench for numerous pharmaceutical and technological applications, as well as medical diagnostics. [1] Ashkin A., PNAS 1997, 94, 4853-4860. [2] C. Hill. (2020). EU Patent No. 3422364B1. European Patent Office. [3] A. D. Kiselev and D. O. Plutenko, Phys. Rev. A 2014, 89, 043803