On the hydrodynamics of liquid-liquid Taylor flows within mini and micro capillaries

This thesis presents an experimental investigation of the hydrodynamics of liquid-liquid Taylor flows in mini and micro channels, with the objectives of bridging gaps and extending previously conducted research on the subject. Taylor flows have emerged as a highly active research area due to the increasing demand for microscale devices and miniaturized components in order to reduce production costs and improve mobility. The utilization of two-phase flows in microdevices enables the dissipation of remarkably high heat fluxes within a confined space while maintaining minimal temperature variations. In addition, such flows also offer a promising platform for chemical and biomedical processes due to their well-defined high specific interfacial area and enhanced safety resulting from their extremely small dimensions. However, despite this research, the fluid mechanics of Taylor flows are still yet to be fully-determined and understanding the influence of all parameters is a big challenge, which calls for further investigation.

The context of this thesis is divided into three main aspects: the development of a precise measurement technique for slug flow characterization within microchannels; an experimental investigation on the mobility of droplets in liquid-liquid Taylor flows within circular capillaries; and experimentally investigating the pressure drop of liquid-liquid Taylor flows over varying viscosity ratios.

The first aspect introduces a novel technique to easily and reliably measure slug length and velocity. This automated, non-intrusive measurement technique allows for in-line high-frequency droplet/bubble detection and related physical properties based on changes in the light intensity caused by phase shifting in liquid-liquid or liquid-gas flows. This measurement methodology will aid in the determination and analysis of two-phase flow configurations in transparent microchannels, allowing for a better comprehension of the experimental data.

The velocity of individual droplets in liquid-liquid Taylor flow is then investigated experimentally over a wide range of Capillary number (2 × 10−4 to 3.7 × 10−2 ), Bond number (0.05 to 3.2) and carrier to dispersed phase viscosity ratio (0.059 to 23.2) while also varying droplet length. This study provides a greater insight into droplet flows through an analysis of parameters affecting droplet mobility. The results indicate a complex dependency of droplet velocity on various parameters including droplet length, viscosity ratio, Reynolds and Bond number. In all cases, as droplet length exceeds a threshold value, droplet velocity becomes independent of length and is shown to scale with ~ Ca0.5 . Finally, a new expression has been developed to better estimate the velocity of elongated droplets, which has been found to correlate as a function of Capillary, Bond and Reynolds numbers.

Finally, pressure drop in liquid-liquid Taylor flow regimes is empirically explored by means of a reliable experimental set up that ensured high measurement accuracy and repeatability. The experiments were conducted using five different combinations of liquids in a capillary of 800 µm diameter. The strengths and weaknesses of existing models are identified and a more fundamental understanding of predicting pressure drop in Taylor flow regimes is developed. A new expression is presented that more accurately estimates the interfacial pressure drop in liquid-liquid Taylor flows. This correlation fits the experimental data within ±20% by assuming an annular velocity profile across the capillary in the presence of the second phase while accounting for inertial and viscosity ratio effects.

Overall, this study seeks to provide an improved understanding of liquid-liquid Taylor flow hydrodynamics as well as significant insights to facilitate microfluidic device optimisation.