On-chip optical characterization of flowing nanoparticles with coherence-gated DLS

Dynamic light scattering (DLS) is the gold standard for measuring the hydrodynamic size of nanoparticles (NPs) in suspension, in samples at rest, based solely on the particles’ diffusion. Here, we present a DLS-based optofluidic platform for on-chip characterization of flowing NPs which consists of a microfluidic channel instrumented with coherence-gated DLS (CG-DLS). CG-DLS is an all-optical-fiber technique and its incorporation into a microfluidic platform enables non-contact DLS measurements on flowing samples. We performed proof-of-concept experiments in a microfluidic device made of polymethyl methacrylate (PMMA), whose geometry allows for straightforward, plug-and-play integration of the endoscopic-like fiber probe of CG-DLS, avoiding the need for complicated alignment. Using model aqueous colloidal suspensions of monodisperse polystyrene NPs, we demonstrate that particle sizing and velocimetry can be performed independently, if one of them is known, and simultaneously, under certain conditions, without a priori information. The proposed strategy decouples advective and diffuse contributions of particles’ transport over an extended flow speed range and good sensitivity to lower concentrations, while capturing local spatial information of the velocity profile in regimes where velocity gradients are non-negligible.