# The Effects of Interface Mobility on Bubble and Drop Dynamics

Student thesis: Doctoral Thesis

### Abstract

The presence of bubbles within liquid pools is ubiquitous in many natural and industrial settings. Plants and other living systems can release gas bubbles which detach and rise up through lakes and the ocean. Degassing also forms gas bubbles on solid surfaces inside the liquids, like that from champagne or poured soda drinks. The bubbles eventually rise to the pool surface, where they can bounce or pop into the air. The detailed dynamical interaction of the bubble and the free surface can be greatly affected by any impurities on their surface, which can affect the mobility of the free surface. In this dissertation, we use both experiments and numerical simulations to study these hydrodynamics. First, we study the rise and bouncing of bubbles or water droplets from the free surface inside a perfluorocarbon liquid. From all four different configurations of mobile/immobile interface pairs we show that the mobile interface always induces stronger bouncing but faster coalescence. The bouncing enhancement ratio between mobile and immobile interface is $1.8\pm0.1$ for bubbles and $1.5\pm0.1$ for water droplets, with the size range from $250 \, \mu m - 550 \, \mu m$ for bubble and $600 \, \mu m - 1200 \, \mu m$ for droplet. Then the top phase is replaced with a glass plate to eliminate the influence from other internal properties besides surface mobility. Since our numerical simulations perfectly reproduce the experiments, we extend our simulations to the free frontal collision of two equivalent droplets. The results not only support our previous conclusions but also predict another peculiar second-collision phenomenon under certain conditions. Then we replace the surrounding liquid with more practical ones of water and ethanol. In extra-pure water, we find that a millimeter-sized bubble has a mobile interface. We add arachidic acid on the top surface to further investigate bouncing from an immobile interface without changing the interfacial tension. The bouncing enhancement by mobile vs immobile interfaces is once again verified for the water-air interface. For millimeter-sized bubbles, as we increase the bubble size from $780 \, \mu m - 1550 \, \mu m$ the bouncing enhancement ratio decreases from 1.8 to 1.2. Finally, we look into the bubble shape and evolution of the liquid film profile during the bouncing from a top glass substrate, using interferometry and numerical simulations. We use 640 nm laser interferometry with a maximum thickness resolution of 120 nm. The center-of-mass trajectory and film profiles are measured for the first bounce of bubbles between 0.8 mm to 1.2 mm. Then we compare the 1.48 mm bubble impact on a no-slip top wall with the SRYL model prediction, where they shared the same dimple diameter but have a non-trivial deviation in dimple depth. Lastly, we simulate the frontal collision between two identical 1.45 mm bubbles, which have complex multi-dimple formations during the bouncing process.
Date of Award Oct 2022 English (US) Physical Sciences and Engineering Sigurdur Thoroddsen (Supervisor)

• Bubble
• Droplet
• Interface
• Mobility
• Dynamics

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