Multiphase flow and control of fluid path in microsystems

Abstract

Miniaturized chemical-systems are expected to have advantages of handling, portability, cost, speed, reproducibility and safety. Control of fluid path in small channels between processes in a chemical/biological network is crucial for connecting process elements. We show complete separation of individual phases (phase routing) from two-phase gas-liquid and liquid-liquid (aqueous-organic) mixtures on microscale. To provide for robust interfacing of operations in a network, we demonstrate this ability over a wide range of two-phase flow conditions, including transient ones. Enabled by the technique for complete separation of individual phases from two-phase mixtures, we show mixing of liquids by introduction of a passive gas-phase and demonstrate integration of mixing, reaction and phase separation on a single platform. Additionally, we use the principles developed for phase routing to design microfluidic valves that do not rely on elastic deformation of material. Such valves can be used in a variety of chemical environments, where polymer-based deformable materials would fail.

(cont.) We show a concept for realization of logic-gates on microscale using appropriate connections for these valves, paving the way for design of automation and computational control directly into microfluidic analysis without use of electronics. Further, we use the phase separation concept for sampling liquid from gas-liquid and liquid-liquid mixtures. Such sampling ability, when coupled with a suitable analysis system, can be used for retrieving process information (example mass-transfer coefficients, chemical kinetics) from multiphase-processes. We provide evidence of this through estimation of mass-transfer coefficients in a model oxygen-water system and show at least an order-of-magnitude improvement over macroscale systems. Controlled definition of fluid path enabled by laminar flow on microscale is used in a large number of applications. We examine the role of gravity in determining flow path of fluids in a microchannel. We demonstrate density-gradient-driven flows leading to complete reorientation of fluids in the gravitational field.

(cont.) We provide estimates of the time and velocity scales for different parameter ranges through two-dimensional and three-dimensional finite-element models, in agreement with experimental observations. We believe this thesis addresses a number of both: system and fundamental issues, advancing applications and understanding of microfluidic networks.