Abstract
The formation of a gas bubble on a solid surface submerged under liquid, either through an orifice or a nozzle, is a fundamental topic in multiphase flows. Understanding the bubble formation is crucial for many industrial and biomedical applications, for example, pool boiling heat transfer, froth floatation, surface cleaning, and drug delivery. However, previous studies mostly focused on the bubble formations from the hydrophobic and hydrophilic surfaces and from a nozzle. The formation of bubble on a superhydrophobic surface (SHS) has received very little attention. The SHS could significantly modify the bubble formation and the detached bubble size since it promotes a large equilibrium contact angle of more than 150° and entraps a gas layer between surface textures and liquid. This thesis aimed to understand the impact of surface super-hydrophobicity on the dynamics of bubble formation. To achieve this goal, we designed an experimental setup to study the bubble formation on SHS under constant gas injection flow rate. The shape of bubble during the growing process was firstly recorded by a high-speed camera, and subsequently analyzed by imaging processing. The SHS was fabricated by first sandblasting an aluminum surface and then coating the rough surface with hydrophobic nanoparticles. By performing a total of 19 experiments, we studied the impacts of the radius of SHS (Rₛₕₛ), the gas injection flow rate (Q), and the surface tension (𝜎) on bubble formation. First, we found that depending on the SHS size, bubble formation follows two different modes: A and B. For small Rₛₕₛ, bubble formation follows mode A where the contact line quickly pins at the rim of SHS after an initial expansion. For large Rₛₕₛ, bubble formation follows mode B where the contact line continuously expands as the bubble grows. During the mode A, the detached bubble volume increases by increasing Rₛₕₛ. Second, we found that bubble detached volume increases by increasing Q. The relationship between bubble detached volume and Q follows similar trends after proper normalizations, regardless of the types of surfaces and the values of equilibrium contact angle. The value of Q has a negligible impact on bubble shape but impacts the trend of volume during the necking: during the necking, the bubble volume is nearly constant for small Q but increases significantly for large Q. Third, we found that by reducing surface tension, the equilibrium contact angle and surface area covered by gas reduces, leading to a smaller bubble base radius and detached volume. Moreover, we performed a force balance analysis and found that the main forces acting on the bubble are one lifting force (pressure force) and two retaining forces (surface tension force and buoyancy force). The gas flow rate has a negligible impact on the magnitudes of these forces. We also investigated the necking process and found that the necking radius and time to pinch-off follow a power-law relation, which agrees well with that for the pinch-off of bubble on a nozzle. Last, we found that the Tate volume can be used to predict the detached bubble volume on SHS, which is a function of the maximum bubble base radius. Overall, our results provide better understanding of bubble formation on SHS. Results from this study can be applied for: (i) developing innovative methods to control bubble generation and bubble size by using complexed surfaces; and (ii) informing new approaches to restore the gas layer and extend the longevity of SHS for applications such as drag reduction, anti-icing, anti-biofouling, and anti-corrosion.