Microscale Fluids Engineering For Droplet Microfluidics And Drag Reduction

"Understanding the dynamics of fluids has enabled humans to devise strategies for a myriad of global challenges on a wide range of scales, from nanotechnology to space exploration. By designing and developing systems for manipulating fluid flows, researchers not only constantly optimize many of the conventional approaches to older engineering tasks, but also establish novel methods to tackle new and unsolved problems in science and technology. The advent of microfabrication and microfluidics along with improvements in numerical methodologies and computational power have equipped researchers with unprecedented resources to address these problems. In this work, I aim to investigate two areas where fluids engineering plays a critical role: droplet generation in microfluidic systems and fluid drag reduction of microtextured surfaces. In the first part, I study a relatively new area - droplet generation in microchannels using a high-speed gas phase flow. The use of microdroplets has become ubiquitous in many Lab-on-a-Chip applications ranging from material synthesis to biochemical sensing and testing. Droplet-microfluidics offers a promising approach for controlled generation and manipulation of uniform fluid entities. Despite the versatility and unparalleled control of the droplet formation and transport, the range of droplet microfluidic applications could be greatly expanded by enhancing the per-channel throughput and improving the interaction of droplets and jets with the continuous flow and the microchannel. I investigate a new class of droplet-based systems that uses a high-speed gaseous flow to generate uniform liquid droplets in a flow-focusing geometry. The interaction of the liquid and gas inside the microchannel creates distinct flow patterns. By characterizing the operation extent of resulting flow maps, I then focus on the Dripping and Jetting modes of droplet formation. To improve the performance of the droplet generation, I design and fabricate a three-dimensional microchannel architecture to facilitate formation of a contact-less liquid jet within the air flow inside the microchannel. Droplet generation frequency in this non-planar microchannel increase by at least one order of magnitude in comparison to conventional liquid-liquid droplet systems. The high-speed nature of the flows in this system, however, poses new challenges, namely inertial and compressibility effects on precise control of the jets and droplets. I use numerical simulations to further investigate the flow dynamics and determine the local pressure and velocity of the air inside the microchannel. The knowledge gained from this discussion of the fundamentals and physics is subsequently leveraged in applications that can benefit from this system. I propose a novel technique to use droplets as isolated micro-reactors for absorbing and detecting airborne targets and provide a proof-of-concept for sensing gaseous ammonia using sample digitization with microdroplets. In addition, I present the potential of using gas-liquid droplets as a template for polymer particle fabrication by showing experimental results that demonstrate the generation of calcium alginate microgels purely in air. At the end of this part, I discuss the current challenges in each application and suggest possible solutions towards effective implementation of this system for next generation microfluidic systems in particle synthesis and biochemical diagnostics. In the second part, I revisit a relatively old problem - understanding the dynamics of the laminar flow over textured surfaces for drag reduction applications. Inspired by the natural skins of certain plants and animals, introducing surface corrugations is a proven strategy to reduce the fluid drag and create a superhydrophobic effect, which has potential environmental and economic benefits. I examine the effects of periodic transverse grooves on the laminar boundary layer flow for mid- to high-Reynolds numbers (1000-25000). The width-to-depth aspect ratio of the grooves (AR) is a critical parameter that determines flow behavior near the grooves. Using finite element simulations, I study the local and temporal pressure and velocity fields in rectangular grooves for a wide range of aspect ratios (0.2