Two-phase flow in micro and nanofluidic devices

Abstract

This thesis provides experimental data and theoretical analysis on two-phase flow in devices with different layouts of micrometer or nanometer-size channels. A full flow diagram is presented for oil and water flow in head-on microfluidic devices. Morphologically different flow regimes (dripping, jetting and threading) are observed over a broad range of capillary numbers: 10-6~10-1. At extremely low flow rates, we obtained a new droplet formation regime in which a droplet formation due to the device geometry is solely determined by capillary instability. This head-on device provides us with the choice to generate droplet-based flow in regimes that are determined either by flow-rate or device geometry. To investigate the role of interfacial tensions in micro- and nanofluidics, we modulated solid-liquid and liquid-liquid interfacial forces. In microchannels, the emulsion types are mainly determined by the solid-liquid interfacial forces. By combination of different interfacial forces, we could obtain the designed emulsion type, determine droplet deformation, obtain coalescence and even emulsion conversion. Surface-determined flow phenomena may therefore be important to explain fluid transfer in some biological processes and to estimate interfacial properties. The strong confinement and flow resistance in nanochannels challenges high pressure and low flow rate fluidic interconnections. We developed a flow control method to smoothly manipulate liquid in nanochannels with flow rates as low as pL.s-1. Using this method, we investigated the mixing of two miscible liquids and flows of two immiscible liquids in nanochannels. In devices with nanochannel/microchannel interfaces, the creation of stable and monodisperse attoliter to femtoliter droplets was obtained at the nanochannel-microchannel interface in a wide range of flow rates and ratios. Furthermore, microfluidic manipulation of fluids at the microscale enabled us to form and control liquid microspheres in three-dimensional lattices which are highly applicable in many fields: photonic crystal structures, liquid chromatography packings, data storage and so on. Micro- and nanofluidic devices can not only create small droplets but also control them to flow or organize into stable structures. Therefore, for different purposes, it is highly required to precisely understand and design suitable micro- and nano-structures (or networks).