Microfluidics plays a vital role in modern technologies, such as chemical synthesis, drug screening, emulsification reactors, oscillating heat pipes and biomedical diagnostics. Most of these microfluidic applications are various types of multiphase flow. Among the many flow patterns, slug flow, compared to other flow regimes in micro-reactors, has been proven to bear several advantages: such as better mass transfer efficiency, reduced residence time distribution and so on. However, the hydrodynamic effects of wetting in micro-channels remain unexplored. In this thesis, a non-invasive optical measurement technique is developed to validate the contact line dynamics theory in micro-channels. This thesis focuses on the development of a new laser interferometry measurement technique for pro...[ Read more ]

Microfluidics plays a vital role in modern technologies, such as chemical synthesis, drug screening, emulsification reactors, oscillating heat pipes and biomedical diagnostics. Most of these microfluidic applications are various types of multiphase flow. Among the many flow patterns, slug flow, compared to other flow regimes in micro-reactors, has been proven to bear several advantages: such as better mass transfer efficiency, reduced residence time distribution and so on. However, the hydrodynamic effects of wetting in micro-channels remain unexplored. In this thesis, a non-invasive optical measurement technique is developed to validate the contact line dynamics theory in micro-channels. This thesis focuses on the development of a new laser interferometry measurement technique for probing the interfacial film displacement of gas-liquid slug flow in a circular microchannel. The fundamental physics of the meniscus development of gas-liquid two phase flow in micro-channels, as well as an investigation of double layer liquid film displacement in immiscible liquid-liquid fluid flows is presented.

Liquid thin film is an essential phenomenon of slug flow in micro channels for industrial applications such as micro-reactors, micro-mixers, etc. Real-time measurement of whole profile liquid thin film in an accurate and reliable way is still a challenging task. The newly developed technique here uses light scattering from different liquid/gas interfaces to form interference fringes and the spatial frequency of interference fringes in a receiving plane is a function of the liquid film thickness according to geometrical optics. Experiments are conducted for demonstrating this novel measurement technique utilizing a laser, a capillary tube, a high-speed camera and an optics system. The fringes generated by the liquid film thickness are recorded by a high-speed camera located at a certain angle from the incident laser beam and the image data are calculated using FFT and the Levenberg-Marquardt algorithm for solving a non-linear least squares problem. The signal-to-noise-ratio (SNR) of this technique is above 37dB, resulting in the accuracy and the uncertainty the measurements being 50 ?? at zero point and 118.4 ??, respectively . The entire film thickness profile of an air bubble can be measured at the same time.

The air-liquid interface meniscus motion equations based on the lubrication theory in a circular channel were formulated. These equations are used for predicting the liquid thin film formation in a microchannel. When air presses a fluid interface moving forward in a microchannel, the interface around the contact line can be described by the Navier-Stokes equation with a slip condition. After being simplified by the lubrication theory, the Navier-Stokes equation with a slip condition can be integrated. Then a developed nonlinear high order differential equation can degenerate to a solvable high order differential equation when the film tends towards the contact line. We solve the high order nonlinear differential equation using perturbation theory when the film tends towards the macro scale away from the contact line. As a result, a critical capillary number can be determined depending on the wall surface wettability above which a liquid film is generated after the moving contact line. Utilizing the newly developed measurement technique mentioned above, gas-liquid experiments are conducted to validate the critical capillary equation. After that we extend the equation of the air-liquid model into a liquid-liquid model.

Finally, we demonstrate a special double layer liquid film generation phenomenon for two immiscible liquids in a micro channel. We measure the upper layer liquid film thickness and the velocity field in it. When a short liquid slug is pushed forward in another immiscible liquid phase faster than the critical velocity, two liquid films will be generated from the two contact lines of the short liquid slug at the same time. These two liquid films form a double layer film in the capillary. The length of double layer film is dozens of capillary diameter. A measurement method is developed to measure the upper liquid film layer thickness based on Bill-Lambert’s law. To achieve the velocity field in the upper liquid film, a PIV measurement is conducted. These two measurement methods are performed at the same time to detect the flow in real time.