Thesis title: On the effective transport approach at the microfluidic scale and its applications for size-based separations of nanometric particles
In this thesis, transport of diluted solutions and suspensions in microfluidic separation devices is investigated through Brenner macrotransport theory and Lagrangian-stochastic approach. These methods have been employed to analyze the interaction between convection and diffusion, aiming at increasing the performance of two separation techniques, namely Open Tubular Liquid Chromatography (OTLC) and Hydrodynamic chromatography (HDC), the first used to separate molecular solutes embedded in a homogeneous (single-phase) solution, the second targeting the size-based separation of particle suspensions. In both cases, the separation performance depends on the ratio between the selectivity and the analyte dispersion bandwidths. Both techniques are affected by the Taylor-Aris effect, an undesired phenomenon that impacts on any pressure-driven micro-separation processes causing the increase of species dispersion, especially when high flow rates are enforced. In the case of OTLC, the efficiency improvement is achieved through the reduction of Taylor-Aris dispersion obtained by triggering transversal vortices alongside the pressure driven axial flow. Different convective transport conditions are investigated, where the flow kinematics ranges from helical to chaotic streamlines. As regards the improvement of HDC columns, two alternative approaches have been considered. In the first, the performance enhancement is obtained by boosting the column selectivity. This is achieved through an altogether new separation mechanism, which has been termed Browian sieving, which makes use of a multichannel geometry combined with a size-based sieve. Improvement of the efficiency up to a thirtyfold factor above standard HDC has been demonstrated. As an alternative separation-enhancement strategy, electroosmotically driven cross-sectional vortices have been investigated. In this case, 50 fold enhancement factors have been theoretically predicted. The results discussed in this thesis should therefore motivate new lines of experimental research in novel microfluidics-assisted channel geometries that could be applied to both liquid chro- matography and hydrodynamic chromatography.