Towards a Brownian sieving device for nanoparticle size separations.

We report on a preliminary study to test the liquid flow and particle diffusion properties in a device developed to deliver a proof-of-principle for the so-called Brownian sieve-enforced hydrodynamic chromatography (BS-HDC). This is a recently proposed novel separation principle to enhance the selectivity and efficiency of conventional HDC by combining it with a size sieving step through a micro- or nano-perforated separating wall interfacing a central feed channel to a peripheral auxiliary channel, characterized by a different average velocity that can only accept the small particles in the sample. We demonstrate the feasibility to fabricate a proof-of-principle device with a 400 nm sieving gap, and report on the flow and diffusion tests that were carried out with a fluorescent marker. These tests demonstrate the magnitude of the velocity fields and the transport across the nano-gaps between the central and peripheral auxiliary channel, which perfectly match with theoretical expectations. Specifically, transport across the nano-gaps is dominated by molecular diffusion. Nanosieving experiments with 235 and 563 nm particles showed the ability to selectively transport the smaller particles through the nano-sieving gaps while the larger particles remained in the central feed channel with 100% selectivity. However, the observed fluxes were found to be strongly influenced by local convective flows opposing the diffusive transport near the inlet and outlet of the channels, a phenomenon we think can be owed to the presence of persistent air bubbles in the flow distributors creating undesirable pressure fluctuations.