Interaction between a solid body and a liquid-air interface plays an important role in multiphase flows, which includes numerous engineering applications such as mineral flotation, dip coating operations, and air-to-sea and sea-to-air projectiles. It is also crucial in animal behaviors like the locomotion of water-walking animals, the plunge-diving of birds, and the jumping out of water of marine creatures. Depending on the moving direction of a solid, such diverse phenomena can be classified into two categories; liquid-entry and liquid-exit. Liquid-entry, or more widely called water-entry, is the behavior of a solid object entering liquid from air. The opposite case is referred to as liquid-exit. Liquid-entry has been extensively studied, especially focusing on cavity formation and the estimation on capillary and hydrodynamic forces on a solid object. However, as the behavior of a triple contact line has not been understood on a sinking object, previous studies were limited to the special case of hydrophobic object to fix the contact line. Moreover, a more recent study pointed out the important role of contact line behavior to characterize the performance of film flotation, which is one of the direct applications of liquid-entry. However, there are no existing previous studies on the dynamics of the contact line on a sinking object. This subject will be first discussed in Chapter 2. In Chapters 3 and 4, the topics related to liquid-exit will be discussed, where a solid sphere exits out of a liquid toward air with constant velocity, acceleration, or deceleration. Chapter 3 will focus on the penetration and bouncing behaviors of a solid sphere while impacting a liquid-air interface. The solid sphere experiences the resistance of surface tension and gravity while impacting the interface. Thus, liquid-exit spheres should have enough momentum to penetrate the interface to overcome these resistances, which indicates that the critical momentum exits. This understanding would give a mechanistic explanation as to why some aquatic species, especially plankton, are able to jump out of water while the others cannot despite their similar size. This study can help to understand the particle-bubble interaction for froth flotation applications, in which the particle tends to attach to the bubble. In the last Chapter, the formation of a liquid column during the liquid-exit will be discussed. It has been observed that the evolution of a liquid column strongly depends on experimental conditions, especially the acceleration of a solid sphere. The pinch-off dynamics of a liquid column is categorized as two branches: upper and lower pinch-off's. The pinch-off location affects the entrained liquid volume adhered to the solid object, which is directly related to the uniform quality of a dip-coating operation. In addition to the pinch-off location and time in relation to the aforementioned experimental conditions will be discussed. In summary, studies in the present dissertation are designed and performed to provide mechanistic insight to the problems in the liquid-entry and liquid-exit, which are all closely related to animal's daily life as well as engineering applications. / PHD / Interactions between a solid body and a liquid-air interface play an important role in multiphase flows, which is also crucial in animal behaviors [16, 19, 77] and numerous engineering applications [92, 93, 129, 143]. Depending on the moving direction of a solid, such diverse phenomena can be classified into two categories; liquid-entry and liquid-exit. Liquid-entry is when a solid object enters a liquid from air. The opposite case is referred to as liquid-exit. In the Chapter 2, contact-line spreading dynamics on a sinking solid sphere will be discussed, where the contact-line indicates the line meeting all three phases of liquid/air/solid. The contact line motion is important as this local motion significantly affects macroscopic liquid flow. Experiments performed in high temporal and spatial resolutions by x-ray illumination show the characteristics of capillarity-driven (in other words, driven by the surface tension) spreading up to the sinking speed ≈ 1 m/s. Scaling dynamics based on capillary-viscous and capillary-inertial balances are observed to rationalize the contact-line motion. In the last two Chapters, the liquid-exit behaviors will be presented with focusing on a liquid-exit solid (Chapter 3) and a stretched liquid column (Chapter 4). The penetration & bouncing behaviors while a solid sphere exits out of a liquid bath are viewed in analogy with bio-example of jumping and non-jumping plankton. The dynamics on the liquid-exit sphere are described by the exit-momentum of the sphere and the resistance of surface tension and gravity. Lastly, the pinch-off dynamics on the stretched liquid column are investigated with noticing that the column evolution shifts from capillarity-driven to inertial-driven as the sphere exits out with higher acceleration.

The forced water entry and exit of two-dimensional bodies through a free surface is computed for various 2D bodies (symmetric wedges, asymmetric wedges, truncated wedges and boxes). These bodies enter or exit water with constant velocity or constant acceleration. The calculations are based on the fully non-linear timestepping complex-variable method of Vinje and Brevig. The model was formulated as an initial boundary-value problem with boundary conditions specified on the boundaries (dynamic and kinematic free-surface boundary conditions) and initial conditions at time zero (initial velocity and position of the body and free-surface particles). The formulated problem was solved by means of a boundary-element method using collocation points on the boundary of the domain and solutions at each time were calculated using time stepping (Runge-Kutta and Hamming predictor corrector) methods. Numerical results for the deformed free-surface profile, the speed of the point at the intersection of the body and free surface, the pressure along the wetted region of the bodies and force experienced by the bodies, are given for the entry and exit. To verify the results, various tests such as convergence checks, self-similarity for entry (gravity-free solutions) and Froude number effect for constant velocity entry and exit (half-wedge angles 5 up to 55 degrees) are investigated. The numerical results are compared with Mackie's analytical theory for water entry and exit with constant velocities, and the analytical added mass force computed for water entry and exit of symmetric wedges and boxes with constant acceleration and velocity using conformal mapping. Finally, numerical results showing the effect of finite depth are investigated for entry and exit.

This dissertation aims to contribute toward the understanding of water-entry and -exit behaviors in nature. Since water is nearly a thousand times denser than air, transitioning between the two mediums is often associated with significant changes in force. Three topics with implications in water-entry are discussed, along with a fourth topic on water-exit. For a plunge-diving seabird, the first two stages of water-entry (initial impact and air-cavity formation) create large stresses on the bird's neck. Linear stability analysis of a cone-beam system impacting water shows buckling and non-buckling behaviors on the beam, which is extended to the diving birds. The next topic is related to the third stage of water-entry (air-cavity pinch-off), in which the chest feathers come in contact with the water. Here, the elasticity of Northern Gannet contour feathers is calculated using the nonlinear bending equation. The third topic will explore the formation of ripples along air cavity walls and their resulting force after pinch-off. An acoustic model predicts the observed wavelengths of the ripples. The final topic will delve into the mechanics of how animals leap out of water. A scaling law that balances the power of thrust and drag will predict the height of the jump. Finally, a bio-inspired robot was built to help identify physical conditions required to jump out of water. / Ph. D. / In nature, animals use enter and exit water (water-entry and water-exit, respectively) as a strategy for hunting prey and/or escaping predators. In this dissertation, we focus on the fluid mechanics of water-entry and water-exit phenomena as it pertains to animals. First, we study how seabirds plunge-dive into water at high speeds without neck injuries. Second, we discuss calculating the elasticity of bird feathers. Next, the rippling behavior of air-cavities is studied in the context of force production. Finally, we study the water-exit phenomenon of animals leaping out of water. The topics of this dissertation have implications in the water-entry and -exit of vehicles and autonomous robotics.