| Summary: | Narrow residence time distributions (RTDs) are extremely desirable in many chemical engineering processes where plug flow behaviour is requested. However, at low Reynolds numbers the flow is laminar resulting in strong radial velocity gradients. This in turn causes spreading of fluid particles, usually referred to as hydrodynamic dispersion. Such problem is particularly relevant to microfluidic devices operated in laminar regime due to the reduced dimension and low operating flow rates. Many solutions have been proposed to reduce the hydrodynamic dispersion: static mixers, segmented flow, secondary flow, etc. The latter relies on the action of centrifugal force inducing transversal mixing in helically coiled tubes. Further mixing and therefore reduced dispersion can be achieved by introducing geometrical disturbances, generating chaotic advection. Coiled flow inverters (CFI) exploit the beneficial effects of secondary flow and chaotic advection. They consist of sections of helically coiled tubes with 90-degree bends placed at regular intervals along a cylindrical support. Despite being a very promising solution, they have not been extensively adopted. This is due to the lack of experimental data and correlations relating the design parameters and operating conditions to the reduction of hydrodynamic dispersion. In this thesis, a flexible and reliable experimental procedure was developed to investigate RTD in microfluidic devices. It resorts to step input injections and UV-vis inline spectroscopy for detecting the concentration of tracer. The procedure was validated using Taylor’s dispersion for straight tubes. The platform was then employed to perform experiments on CFIs, constructed with microfluidic capillaries, varying operating conditions and a geometrical parameter. A similar characterization was carried out on helically coiled tubes. A significant reduction of axial dispersion was observed as compared to straight pipes, confirming the available data in the literature. It was also demonstrated that the curvature ratio primarily defines the strength of radial mixing in CFIs and therefore represents a crucial design parameter.
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