Droplet-based microfluidics has proven to be a useful tool to investigate heterogeneous reactions that occur between multiple phases due to the reproducible flow patterns and effective mass transport between phases. Nowadays, droplet-based microfluidics is showing its potential for systematic study of the transport across interfaces between different micro reactors since the surfactant layers around micro droplets are in principle permeable to small molecules. In order to study such bio/chemical assays and medium transfer from one droplet to another, droplet paring system in continuous-flow channels and even static arrays has been developed and demonstrated. However, a more desirable, but more difficult unit operation for complex assays is to cluster multiple-droplet containing different reagents/samples for various biological and chemical experiments. The objective of this thesis is to study clustering of multiple-droplet in a double-layered microfluidic device integrating multiple functions such as droplet generation, manipulation, trapping, guiding, and storing. The use of guiding tracks and simple forward/backward flows has been incorporated to improve trapping/storing efficiency. The primary goal is to investigate the fluid dynamics and adopt a method to design and simulate the device. The secondary goal is to learn the ability to fabricate the device and demonstrate a large array of different multiple-droplet clustering. Finally, the future goal is to apply the tested device for a wide range of biomedical applications. We expect the proposed strategy will be valuable to study transfer of molecules across the droplets, enzymatic reaction and high throughput bioassay reaction.

The droplet based microfluidic technology has become indispensable in many chemical, biomedical research and high-throughput assay applications. The ability to controllably merge droplets within flow systems is of high importance when performing complex chemical or biological analysis. However, in order to perform controlled fusion reaction one needs to perform controlled droplet trapping and pairing. Recent microfluidic systems are capable of pairing the droplets by using unstabilized flow pattern. Controlled droplet pairing and fusion, especially for same-sized droplet pairing, is still a challenge, mostly because of the difficulty to manipulate droplets. It is also seen that it requires to control the droplet generation along with the flow rate control simultaneously which is also difficult to realize.^In our research, a serial flowing microfluidic system and an obstruction based microfluidic system are presented for checking the droplet flow pattern along the system using hydrodynamic resistance phenomenon. In addition to this, we also checked the device working for droplet generation along with sequential trapping and pairing of aqueous micro-droplets of different liquids. It is more robust as compared to the prior research done in this area. These systems are competent of accomplishing multiple functions including droplet generation, transportation, trapping and merging on a single integrated device. These devices consist of three different functional regions: flow focusing droplet generator; a single droplet trap region and pairing region. Our designs were based on the principle of exploiting hydrodynamic resistance of the columnar structure in the microfluidic channel. The device designs include two inlets for oil and water.^Similar structure was embedded at the outlet for the generation of second droplet of different liquid. In a typical scenario, droplets would be generated at the T-junction and would travel through the microfluidic channel to enter the single droplet trapping area. During the reverse flow, the trapped droplets in the first phase would be released and would enter the pairing chamber. These droplets would be held until another droplet of different liquid to combine with it. Second droplet would travel in the reverse flow direction and would be trapped in the pairing chamber along with the first droplet to combine with it. Deionized water and gel were used as the aqueous phase and mineral oil as the oil phase. 2% (w/w) Span-80 was used as surfactant. These devices were also simulated using PSpice and COMSOL Multiphysics to verify the droplet trapping and pairing sequences before fabrication.^Finally, we designed and tested the double droplet trapping system in a serial flowing microfluidic device along with the obstruction based microfluidic device. The efficiency for single droplet trapping in forward flow was about 99%, single droplet trapping in reverse flow direction was about 90-95% for both serial and obstruction based microfluidic device. For droplet pairing, the serial microfluidic device had an efficiency of 40-45% where as the obstruction based microfluidic had 60-65% efficiency. These devices were very simple and could very efficiently trap two different liquid droplets in a chamber without merging and with the help of an external electric field they could be selectively merged.

Droplet-based microfluidics has been considered as a prospective tool for high throughput screening analysis, which is highly demanded in a wide range of areas including but not limited to life science research, drug discovery, material synthesis and environmental monitoring. Low sample consumption, reduced reaction time, high throughput manipulation, fast mixing, and prevention of cross contamination at channel walls are just some of the benefits of droplet-based microfluidics. Although extensive research efforts have been reported in the study of droplet-based microfluidics over the past decades, it has yet to be widely commercialized. One of the challenges that limit droplet microfluidic chips from being commercialized is the difficulty in integrating multiple functions robustly without increasing the device footprint. Major functionalities of interest include generating droplets with controlled volume and frequency, and precisely controlling and manipulating each individual droplet such as sorting, detecting, merging, splitting, pairing, mixing, trapping, releasing, long term and short term storing, etc. Since many of these functionalities rely on the accuracy of droplet generation which is the first step, it is crucial to investigate the droplet formation process and understand how to design microfluidic structures to manipulate each individual droplet effectively. To this end, this thesis started with a fundamental study of droplet generation in a flow focusing geometry based on extensive experimental data, from which a physical model was developed to describe droplet formation processes, then move on to study droplet generation in a geometry with two junctions in series, with the goal of improving single encapsulation (one particle per droplet) efficiency. Later, droplet merging towards whole genome amplification and drug screening applications was investigated, and finally a microfluidic chip integrated with multiple functionalities was developed, and its robustness was validated. The first project studied the fundamental principles of liquid-liquid droplet generation in a flow focusing device. This work presents a 3D physical model with less fitting parameters than existing ones. The model describes droplet formation process in flow focusing devices operating in the squeezing regime, where droplet size is usually larger than the channel width, and was developed based on a systematic and extensive experimental study. In particular, it incorporates an accurate geometric description of the 3D droplet shape during the formation process, an estimation of the time period for the formation cycle based on the conservation of mass, and a semi-analytical model predicting the pressure drop over the 3D corner gutter between the droplet curvature and channel walls, which allows droplet size, spacing and formation frequency to be determined accurately. The model takes into account change in channel geometry (height to width ratio), viscosity contrast, flow rate ratio and capillary number with a wide variety. In the second project, liquid-liquid droplet generation in a flow focusing device with two junctions in series was investigated using experimental approach. Extra emphasis was placed on the device's ability to encapsulate single cell and particle. . This study employs glycerol solutions with different concentrations as the dispersed phase, which tends to form stratified flow at the first junction due to viscosity contrast. The stratified flow proceeds to form droplets in oil stream at the second junction. To obtain a comprehensive understanding of the droplet formation dynamics involving stratified flow, five different scenarios were considered. These include a single stream of 10%glycerol aqueous solution, a single stream of 80%glycerol aqueous solution, as well as the simultaneous flow of multiple streams of the above mentioned solution. Droplet size and formation period for these cases were compared and analyzed considering the same geometric and flow conditions. It is found that stratified flow structure strongly influences droplet formation dynamics such as droplet size and formation frequency and the scenario with 80%glyc surrounded by 10%glyc in the first junction generates the largest droplet size. Each structure finds its own applications. For the purpose of single encapsulation, the scenario with 80%glyc surrounded by 10%glyc in the first junction is most suitable because the high viscosity of 80%glyc allows particles to be focused into a thin stream and spaced out before entering droplets. On the other hand, the scenario with two fluids side by side in the first junction generates droplets with high monodispersity for a larger range of flow ratios, which is useful for high throughput reactions involving different reagents. After understanding the fundamentals of the droplet generation process, several designs for practical use were proposed to generate or manipulate droplets. These designs include: i) a flow focusing device that improve droplet size uniformity through changing junction angle; ii) a system for droplet generation on demand, which is essential to controlling droplets of specific reagents; iii) a geometry for generating droplet pairs with uniform droplet sizes and controlled droplet spacing , and to study the interaction between two nearby droplets; iv) a simple droplet merging chamber for controlled reagent volume; and v) a droplet trapping and releasing on demand system for drug screening. The final part of this thesis presents a complex microfluidic system that integrates multiple functionalities, including droplet generation, pairing, trapping, merging, mixing, and releasing. The criterion of this design was analyzed and verified by experiments. This design does not require any synchronization of droplet frequency, spacing or velocity, which makes the microfluidic chip work robustly, and is controlled entirely by liquid flow eliminating the needs for electrodes, magnets or any other moving parts. This design can be applied to many chemical or biological reactions, such as drug screening, chemical synthesis, and cell culture, etc.

Droplet microfluidics has dramatically developed in the past decade and has been established as a microfluidic technology that can translate into commercial products. Its rapid development and adoption have relied not only on an efficient stabilizing system (oil and surfactant), but also on a library of modules that can manipulate droplets at a high-throughput. Droplet microfluidics is a vibrant field that keeps evolving, with advances that span technology development and applications. Recent examples include innovative methods to generate droplets, to perform single-cell encapsulation, magnetic extraction, or sorting at an even higher throughput. The trend consists of improving parameters such as robustness, throughput, or ease of use. These developments rely on a firm understanding of the physics and chemistry involved in hydrodynamic flow at a small scale. Finally, droplet microfluidics has played a pivotal role in biological applications, such as single-cell genomics or high-throughput microbial screening, and chemical applications. This Special Issue will showcase all aspects of the exciting field of droplet microfluidics, including, but not limited to, technology development, applications, and open-source systems.

Droplet microfluidics offers tremendous potential as an enabling technology for high-throughput screening. It promises to yield novel techniques for personalised medicine, drug discovery, disease diagnosis, establishing chemical libraries, and the discovery of new materials. Despite the enormous potential to contribute to a broad range of applications, the expected adoption has not yet been seen, partly due to the interdisciplinary nature and the fact that, up until now, information has been scattered across the literature. This book goes a long way to addressing these issues. Edited by two leaders, this book has drawn together expertise from around the globe to form a unified, cohesive resource for the droplet microfluidics community. Starting with the basic theory of droplet microfluidics before introducing its use as a tool, the reader will be treated to chapters on important techniques, including robust passive and active droplet manipulations and applications such as single cell analysis, which is key for drug discovery. This book is a go-to resource for the community yearning to adopt and promote droplet microfluidics into different applications and will interest researchers and practitioners working across chemistry, biology, physics, materials science, micro- and nano-technology, and engineering.

The combination of microfabrication and microfluidics has enabled a variety of opportunities in making new tools for biological and diagnostic applications. For example, microdroplets-based systems have attracted lots of attentions in recent years due to potential advantages in controlled environments with fast reaction time, high-throughput and low noises. This work presents a number of advanced microfluidic systems in process, control and manipulation of microdroplets, including finger-powered pumps to generate microdroplets, continuous-flow rupture reactors for the rupture and content retrieval of microdroplets, and magnetic microcapsules for drug delivery applications. Prototype `finger-powered' pumping systems have been designed and constructed and integrated with passive fluidic diodes to pump microfluidics, including the formation of microdroplets. No electrical power is needed for pumping by using a human finger as the actuation force to generate pressure heads. Both multilayer soft lithography and injection molding processes have been successfully utilized to make the pumping systems. Experimental results revealed that the pressure head generated from a human finger could be tuned based on the geometric characteristics of the system, with a maximum observed pressure of 7.6±0.1 kPa. In addition to the delivery of multiple, distinct fluids into microfluidic channels, the finger-powered pumping system is also employed to achieve rapid formation of both water-in-oil droplets (106.9±4.3 [mu]m in diameter) and oil-in-water droplets (75.3±12.6 [mu]m in diameter), as well as the encapsulation of endothelial cells in microdroplets without using any external or electrical controllers. To advance the technology of microdroplets in microfluidic systems, the technique to rupture microdroplets via the continuous-flow micropost array railing systems has been developed. The key step is to transport water-in-oil microdroplets with surfactant into the pure oil microchannel to wash away the surfactant and allow the washed microdroplets to transport to the next water microchannel and rupture at the oil-water interface boundary. Microdroplets-based nanoparticle synthesis systems have been fabricated to demonstrate synthesis and retrieval of iron oxide nanoparticles without the need of an external centrifuge machine. In a second demonstration, a rapid solution alteration system for the bead-in-droplet microreactors has been demonstrated via the continuous flow micropost array railing technique. The prototype system has accomplished: (i) the retrieval of microbeads in water-in-oil droplets by the 'rupture' of the droplets, (ii) transfer of the released microbeads into a second solution, and (iii) the formation of new water-in-oil droplets containing the original microbeads and a different, second droplet solution. In these experiments, a total of four different microdroplets generation systems have been fabricated and different designs and operation conditions result in different sizes of microdroplets, including 41.1 [mu]m for the basic microdroplets rupture demonstration, 67.5 [mu]m for nanoparticle synthesis experiments, 61.1 [mu]m in the original solution, and 38.6 [mu]m for the new solution in the bead-in- droplets alternation experiments. In the last example, a new class of magnetic microcapsules with aqueous core and polymer shell containing magnetic nanoparticles has been demonstrated for possible drug delivery applications. The combination of multi-layer flow-focusing methodology and an optofluidic polymerization process is employed to form double emulsions of water-in-photocurable polymer microdroplets. A subsequently polymerization process cure the magnetic polymer shells and encapsulates drug materials in the core. Experimentally, remote manipulations of the magnetic microcapsules by applying an external magnetic field have been achieved. As such, the proposed microcapsules have the potential to overcome a number of hurdles associated with current state-of-art technologies: (1) magnetic shells can be guided by DC magnetic field for location control; (2) magnetic particles can be heated by AC magnetic field to break or change the porosity of the shells for active drug release control; and (3) encapsulated microdroplets can prevent the possible degradation and contamination of the drug materials during the transportation processes.