Fluid Dynamics in Liquid Entry and Exit

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.