Gas-liquid droplet microfluidics under confined 3D flow-focusing geometries for droplet generation under the jetting regime

Abstract

The purpose of this thesis is to study an alternative method for droplet generation using in-air confined flow focusing microfluidic chips. Conventional methods for droplet generation use oil as the continuous phase in a microfluidic chip to generate highly uniform droplets. Previous studies proved the possibility of using highly inertial gases as the continuous phase to generate liquid droplets. Other studies have generated liquid droplets by dispersing the liquid in an open gaseous environment using non-microfluidic formats. This work investigates a novel method for droplet generation within the Dripping and Jetting regimes in a confined microchannel. It identifies relationships between the geometry of the microchannel and the flow rates in terms of dimensionless numbers to then relate this to the physical characteristics of the droplets and the jet. This study uses multilayered 3D microfluidic chips using SU8-photolitography to fabricate the mold to later replicate it in PDMS. Several geometries are fabricated to study the influence of air, liquid and output channels width and height as well as the size of the neck before the output. These parameters are studied against each other to understand their physical meaning over the jet formation and its resulting droplets. The result of this experiment is represented in several flow regime maps which allow to explain the physical requirements for the Jetting to occur and how different parameters affect it. A study of the passive control of the droplet generation is made. Indicating that is possible to control the droplet size and generation frequency by adjusting the flow rates of the continuous (gas) and the dispersed (liquid) phases. For the Dripping regime results are obtained with frequencies in the order of 10kHz and droplet sizes between 160 µm and 50 µm. On the other hand, for the Jetting we obtain droplets between 50 µm and below 15 µm at frequencies higher than 100 kHz. The outcomes of this work are useful in many areas, namely pharmaceutical and food industries where uniform droplets can be generated purely in air and without any extra liquid contaminants. Also, the ability to create these droplets within a confined microchannel allows for additional modules for developing integrated Lab-on-a-chip platforms suitable for material synthesis and nanotechnology.