UC San Diego

Interfaces, separating bulk fluids of the same or multiple phases, play an important role in controlling the transport processes, which is critical in natural and industrial systems. A delicate balance between various forces acting on the interface dictates its shape, geometry, and stability. In many systems, disturbances in bulk fluids often lead to dynamic changes in the interface, affecting the transport processes across it. This thesis is focused on two types, non-reactive and reactive interfaces, and investigates their dynamics under unsteady forcing.

The first part of the thesis focuses on investigations of droplets impacting vibrating substrates. Here, the interface between liquid droplets and their surroundings undergoes significant changes influenced by surface tension, inertia, and viscosity forces. The study explores the spreading of impacting droplets on both low-frequency and ultrasonically vibrating substrates, providing insights into the distinct mechanisms at play. The examination extends to delineating the influence of substrate oscillation on post-impact jet formation and droplet ejection behavior, offering a detailed understanding of the dynamics involved.

Shifting to the second part of the thesis, our investigations delve into the response of premixed counterflow flames to unsteady forcing, particularly variations in the upstream reactant flow rate. Using experiments and numerical simulations, the study explores the effect of mono-frequency forcing on flame extinction in non-equidiffusive flames. This reveals insights into the relationship between the extinction strain rate under oscillating incoming flow as a function of the mean flow rate for different Lewis numbers of incoming reactants.

Impacting droplets and premixed flames under unsteady forcing is important for various applications. For impacting droplets, the findings find applicability in coating processes, lab-on-chip analyzers, spray cooling systems, and advanced printing technologies. In the realm of premixed flames, understanding their extinction response to unsteady conditions is critical for optimizing combustion processes in gas turbines, rocket engines, and industrial burners. Furthermore, it provides fundamental insights into the behavior of turbulent flames. This research not only advances specific technologies but also enhances our fundamental understanding of complex fluid dynamics, offering potential avenues for innovation in diverse fields.