3 and 4) and used to determine their hydrodynamic radii (R h ) according to the Stokes-Einstein equation (Eq. 5) . Here, we exploit this phenomenon to separate non-diffusive aggregates from the diffusive monomers to determine the stability of amyloid fibrils. To better understand the behaviour of the diffusive and nondiffusive particles during FIDA, we first numerically simulated the experiment using the COMSOL software (COMSOL Multiphysics® v. 6.1. COMSOL AB, Stockholm, Sweden). In the simulations, a small plug (20 s, 75 mbar) containing a mixture of nanoparticles (NPs, D = 2.5×10 -12 m 2 .s corresponding to R h = 100 nm) and protein monomers (D = 7.0×10 -11 m 2 .s -1 , corresponding to R h = 3.5 nm derived from FIDA measurements of αSyn monomer) was injected into the one meter-long capillary with internal diameter of 75 µm, and their time-dependent distribution in 1500 mbar flow was analyzed (Figure d and e; Supplementary movie). Expectedly, the radially diffusing monomers formed a wide uniform zone which travels with the average flow speed (Figure d and e; Supplementary movie). In contrast, the non-diffusive NPs follow the parabolic shape of the flow speed with their major fraction being pushed ahead of the plug, followed by the gradually decreasing fraction moving in the slower layers. This translates to a highly skewed distribution at the point-of-detection (84 cm from the injection) with sharp increase in signal followed by its slow gradual decrease over time (Figure d and e; Supplementary movie). A COMSOL simulation of different particle sizes are shown in SI figure .