Microfluidics is the science of processing microliters or less of fluid at a time in a channel with dimensions on the order of microns. The small size of the channels allows fluid properties to be studied in a world dominated by viscosity, surface tension, and diffusion rather than gravity and inertia. Microfluidic droplet generation is a well studied and understood phenomena, which has attracted attention due to its potential applications in biology, medicine, chemistry and a wide range of industries. This dissertation adds to the field of microfluidic droplet studies by studying individual droplet deformation and the process of scaling-up microfluidic devices for industrial use. The study of droplet deformation under extensional and mixed shear and extensional flows was performed within a microfluidic device. Droplets were generated using a flow-focusing device and then sent through a hyperbolic contraction downstream of the droplet generator. The hyperbolic contraction allowed the smallest droplets to be deformed by purely extensional flows and for the larger droplets to experience mixed extensional and shear flows. The shear resulted from the proximity of the droplet to the walls of the microfluidic channel. The continuous phase in all of these devices was oil and the dispersed phase was water, an aqueous surfactant solution, or an aqueous suspension of colloidal particles. Droplet deformation dynamics are affected by the use of surfactants and colloidal particles, which are commonly used to stabilize emulsion droplets again coalescence. Microfluidic droplet generating devices have many potential industrial applications, however, due to the low output of product from a single droplet generating device, their potential has not been realized. Using six parallel flow-focusing droplet generators on a single chip, the process of microfluidic droplet formation can be scaled up, thus resulting in a higher output of droplets. The tuning of droplet size and production frequency can be achieved on chip by varying the outlet tubing lengths, thus allowing for a single device to be used to generate a range of droplet sizes.

Deformation-based cell separation has emerged recently as an effective approach to isolate cells that have similar size but different deformability and thus exhibits potential applications in disease diagnostic including circulating cancer cells and sickle cell anemia. However, key physical parameters that regulate deformation-based cell separation remain unclear. Here we developed a microfluidic approach in a droplet-based model system to explore the effect of physical parameters of droplets including size, viscosity, and velocity on deformation-based separation. We fabricated microfluidic devices that had a straight flow-focusing channel, a cylindrical post, two inlets and three outlets and studied droplet sorting at different channel dimensions and flow rate. We showed that decreasing viscosity or increasing velocity of droplets would result in decreasing the effective size of droplets in droplet sorting. The results showed that droplets with a large size or high viscosity were sorted to side outlets at low velocity in the microfluidic device whereas droplets with a small size or low viscosity exited through the center outlet at high velocity. Such separation was determined by the characteristic distance ([delta]) and impact angle ([theta]) during a two-step sequential droplet deformation process. The droplets were sorted to the side outlets when [delta] ≥ 0.542 or [theta] ≥ 28°, and the droplet exited through the center outlet when [delta] ≤ 0.525 or [theta] ≤ 28°. We then further tested the dependence of [delta] and [theta] on cell sorting using RPF-HUVECs and showed that with cells up to the characteristic distance [delta] = 0.419 tested in the experiment, all exited through the center outlet. [theta] measurement was skipped for all the cells exiting through the center outlet because it was not applicable. Future studies of the role of [delta] and [theta] in cell sorting however is needed by optimizing the geometric parameters of the microfluidic device, i.e., gap distance, channel width, post diameter. With properly designed microfluidic device, this microfluidic approach is expected to provide a way for conducting fast, low-cost, and efficient cell analysis that would benefit future disease diagnostics.

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.

In both engineering and medical applications it is often useful to use the knowledge of the conditions under which adhering liquid droplets appear, deform and interact with surrounding fluids, in order to either remove or create them. Examples include the de-wetting of aircraft surfaces and the process of injecting glue into the bloodstream in the treatment of aneurysms. In this study, we look at various methods of modelling a particular class of droplets - those attached to a wall in the presence of an external shear flow.

Microfluidics have aroused a new surge of interest in recent years in environmental and energy areas, and inspired novel applications to tackle the worldwide challenges for sustainable development. This book aims to present readers with a valuable compendium of significant advances in applying the multidisciplinary microfluidic technologies to address energy and environmental problems in a plethora of areas such as environmental monitoring and detection, new nanofluid application in traditional mechanical manufacturing processes, development of novel biosensors, and thermal management. This book will provide a new perspective to the understanding of the ever-growing importance of microfluidics.

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.

Biomedical Applications of Microfluidic Devices introduces the subject of microfluidics and covers the basic principles of design and synthesis of actual microchannels. The book then explores how the devices are coupled to signal read-outs and calibrated, including applications of microfluidics in areas such as tissue engineering, organ-on-a-chip devices, pathogen identification, and drug/gene delivery. This book covers high-impact fields (microarrays, organ-on-a-chip, pathogen detection, cancer research, drug delivery systems, gene delivery, and tissue engineering) and shows how microfluidics is playing a key role in these areas, which are big drivers in biomedical engineering research. This book addresses the fundamental concepts and fabrication methods of microfluidic systems for those who want to start working in the area or who want to learn about the latest advances being made. The subjects covered are also an asset to companies working in this field that need to understand the current state-of-the-art. The book is ideal for courses on microfluidics, biosensors, drug targeting, and BioMEMs, and as a reference for PhD students. The book covers the emerging and most promising areas of biomedical applications of microfluidic devices in a single place and offers a vision of the future. - Covers basic principles and design of microfluidics devices - Explores biomedical applications to areas such as tissue engineering, organ-on-a-chip, pathogen identification, and drug and gene delivery - Includes chemical applications in organic and inorganic chemistry - Serves as an ideal text for courses on microfluidics, biosensors, drug targeting, and BioMEMs, as well as a reference for PhD students