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Over the past few decades, microscale duct flow has been the key element for many applications, such as drug delivery and microelectronics cooling. To enhance the performance of such systems and to save more energy, looking for new ways to control the hydrodynamic and thermal characteristics of the microchannel flow has been of great interest lately. The aim of this research is to gain a better understanding of the flow physics within microchannels with microtextured walls. Therefore, a set of numerical study has been conducted on the combined effect of flow and heat transfer for spanwise rectangular trenches. The surface microstructures increase the wetting surface area, which is supposed to increase friction (skin drag). Recirculation produced inside the grooves, on the other hand, aids in increasing main flow slippage and lowering pressure drop along the microchannel. It is also worth noting that recirculation creates a negative pressure difference in the opposite direction of the flow (pressure drag). The geometrical parameters of the trenches have a significant impact on the trade-off between the drag reducing and drag increasing factors in textured microchannel flow, which is addressed in this research. Furthermore, the textures disrupt the thermal boundary layer, which can boost thermal transport through recirculation mixing. However, the stagnant fluid trapped within the grooves has weak convective heat transfer. So far, the results have been promising and a drag reduction of about 25% has been reported for wide trenches at low Reynolds numbers. Thermal transport enhancement is also possible for some tested geometries when the flow has not achieved the thermally fully development.

Liquid-in-air generation of monodisperse, microscale droplets is an alternative to conventional liquid-in-liquid methods. Previous work has validated the use of a highly inertial gaseous continuous phase in the production of monodisperse droplets in the dripping regime using planar, flow-focusing, PDMS microchannels. The jetting flow regime, characteristic of small droplet size and high generation rates, is studied here in novel microfluidic geometries. The region associated with the jetting regime is characterized using the liquid Weber number (Wel) and the gas Reynolds number (Reg). We explore the effects of microchannel confinement on the development and subsequent breakup of the liquid jet as well as the physical interactions between the jet and continuous gaseous flow. Droplet breakup in the jetting regime is also studied numerically and the influence of different geometrical parameters is investigated. Numerical simulations of the jetting regime include axisymmetric cases where the jet diameter and length are studied. This work represents a vital investigation into the physics of droplet breakup in the jetting regime subject to a confined gaseous co-flow. By understanding the effects that different flow and geometry conditions have on the generation of droplets, the use of this system can be optimized for specific high-demand applications in the aerospace, material, and biological industries.