Sedimentation and centrifugation techniques are widely applied
for the separation of biomolecules and colloids but require the presence of
controlled density gradients for stable operation. Here we present an approach
for separating nanoparticles in free solution without gradients. We use
microfluidics to generate a convective flow perpendicular to the sedimentation
direction. We show that the hydrodynamic Rayleigh−Taylor-like instability,
which, in traditional methods, requires the presence of a density gradient, can be
suppressed by the Poiseuille flow in the microchannel. We illustrate the power
of this approach by demonstrating the separation of mixtures of particles on the nanometer scale, orders of magnitude smaller
than the micrometer-sized objects separated by conventional inertial microfluidic approaches. This technique exhibits a series of
favorable features including short analysis time, small sample volume, limited dilution of the analyte, limited interactions with
surfaces as well as the possibility to tune easily the separation range by adjusting the geometry of the system. These features
highlight the potential of gradient-free microfluidic centrifugation as an attractive route toward a broad range of nanoscale
applications