TITLE: UNSTEADY HYDRODYNAMICS OF PARTICLES AND BUBBLES NEAR FLUID INTERFACES

ABSTRACT: Unsteady hydrodynamic forces near fluid interfaces govern how particles move, interact, and exchange momentum with their surroundings, influencing systems from marine larval transport to industrial cleaning. These forces emerge from the tran-sient coupling of inertia, viscosity, and capillarity, yet their role across biological and engineered contexts remains underexplored. This dissertation examines specific case studies where such forces arise, using high-speed experiments, scaling analysis, and direct numerical simulations to quantify their role in setting the efficiency, timescales, and mechanical outcomes of particle motion. Firstly, we quantify the energetic cost of swimming in barnacle cyprid larvae (Am-phibalanus amphitrite) near the air–water interface. Using high-speed imaging and an unsteady Basset–Boussinesq–Oseen framework, we resolve the relative contributions of drag, added mass, inertia, and history forces during power strokes, and assess how locomotion energetics vary with fluid temperature. The waiting time between succes-sive strokes sets the average power expenditure and shows that sinking may not be a separate locomotion mode, but rather a limiting case within the broader swimming strategy. We then examine the interaction dynamics of two granular rafts at the air–water interface, commonly known as the ”Cheerios Effect”. Current literature predicts differing responses between rafts above and below the capillary length, so we use laboratory experiments and scaling analysis to more fully examine these unsteady forces. By balancing capillarity and hydrodynamic drag, we develop a predictive framework for the crossover between these regimes and quantify the time required for two rafts to merge. Lastly, we employ direct numerical simulations to probe the behavior of large bubbles rising and impacting inclined solid surfaces. Using the open source solver Basilisk, we resolve the spatiotemporal evolution of shear stress over the impact event, and examine the interplay between maximum stress and wetted area. The results reveal trade-offs between bubble size, surface inclination, and cleaning effectiveness, bridging the gap between semi-analytical small-bubble theory and realistic large-bubble dynamics.

COMMITTEE: ADVISOR/CHAIR Professor James Bird, ME/MSE; Professor Douglas Holmes, ME/MSE; Professor Valeri Frumkin, ME