Acoustically levitated complex droplets

Droplets can exhibit complexity in terms of composition, structure, and unique properties. However, studying these intricate droplets in a sessile droplet configuration becomes exceedingly complex due to interactions between the droplet and substrate. These interactions significantly influence droplet behaviors, such as evaporation and oscillation. To investigate droplets in their purest form, a free droplet configuration-where the droplet is entirely detached from any surface-should be adopted. To achieve this, acoustic levitation is employed to suspend droplets midair. In this thesis work, behaviors of four different types of complex droplets under levitated conditions have been investigated: ouzo droplets, air-in-liquid compound droplets, highly viscous droplets, and ferrofluid droplets. Various aspects, ranging from evaporation kinetics to oscillation dynamics have been studied.Ouzo droplets are complex due to their multicomponent composition. The evaporation process of ouzo droplets is a physicochemical phenomenon involving spontaneous emulsification. A comparative study reveals that levitated ouzo droplets have significantly longer lifetimes than sessile ouzo droplets. The evaporation process of levitated ouzo droplets can be segmented into five distinct stages: 1) preferential evaporation of ethanol, 2) ethanol depletion, 3) emulsion droplet evaporation, 4) water depletion, 5) oil remnants. Realistic numerical modeling provides insights into compositional changes during the evaporation process. Self-similar evaporation patterns are observed for ouzo droplets with varying initial volumes. These findings enable precise control of the emulsification process and easy prediction of ouzo droplet composition. Air-in-liquid compound droplets exhibit complexity due to their intricate structure. Oscillation of air-in-liquid compound droplets demonstrates unique sloshing resonant modes where the inner bubble oscillates out of phase with the outer liquid shell. To trigger sloshing resonance, core-shell ratios are found to be within 0.65 and 0.80. Smaller bubbles no longer remain at the center but instead shift to skewed positions. Introducing additional bubbles to form a multicompartment droplet further complicates the structure, though sloshing resonance no longer occurs. Highly viscous droplets are distinct due to their non-volatile nature and resistance to deformation, as seen in substances like honey or glycerol. While droplet oscillation studies have historically focused on low-viscosity droplets, our experiments reveal a novel oscillation pattern-flexural bending oscillation-for highly viscous droplets. Three oscillation modes have been observed: the see-saw mode, the saddleback mode, and the monkey saddle mode. Ferrofluids exhibit complexity through their interactions with external magnetic fields. Acoustically levitated ferrofluid droplets elongate when exposed to external magnetic fields. Varying the acoustic levitation and magnetic fields enables the production of magnetic supraparticles in different shapes. Contactless surface tension measurement for ferrofluids has been verified. Furthermore, upon the application of the acoustic radiation force on the surface of a substantial liquid pool, it functions as a propulsive force on the liquid's surface. This effect results in the establishment of a stable depression on the liquid surface, which proves useful as a method for contactless surface tension measurement. Once the acoustic radiation force surpasses a certain threshold level, instability arises, leading to the formation of bubbles. In this study, it has been demonstrated that the size of these bubbles correlates with the liquid's surface tension and the intensity of the acoustic radiation force. Notably consistent bubbles can be generated in a continuous manner. Through the manipulation of the acoustic radiation source, which allows us to envision the concept of bubble lithography. In summary, these studies collectively advance our knowledge of complex droplets in the context of acoustic levitation, many of which present novel insights. These include the distinctive observations of the five-stage evaporation process in ouzo droplets, the resonance-driven sloshing in air-in-liquid compound droplets, the oscillatory flexural bending in highly viscous droplets, and the impact of magnetic fields on ferrofluid droplets. These discoveries hold relevance across a spectrum of disciplines, spanning from pharmaceutical manufacturing to the precise measurement of liquid surface tension. The integration of experimental investigations, theoretical analyses, and numerical modeling offers a comprehensive grasp of the fundamental physics governing the behavior of various complex droplets when subjected to acoustic levitation conditions. This groundwork paves the path for future advancements in the manipulation and application of droplets. --Author's abstract