Abstract
Filling microstructures in the air with liquid or removing trapped gases from a surface in a liquid are required in processes such as cleaning, bonding, and painting. However, it is difficult to deform the gas–liquid interface to fill a small hole with liquid when surface tension has closed one end. Therefore, it is necessary to have an efficient method of removing gas from closed-end holes in liquids. Here, we demonstrate the gas-removing method using acoustic waves from small holes. We observed gas column oscillation by changing the hole size, wettability, and liquid surface tension to clarify the mechanism. First, we found that combining two different frequencies enabled complete gas removal in water within 2 s. From high-speed observation, about half of the removal was dominated by droplet or film formation caused by oscillating the gas column. The other half was dominated by approaching and coalescing the divided gas column. We conclude that the natural frequency of both the air column and the bubbles inside the tube are important.
Highlights
IntroductionIt is difficult to deform the gas–liquid interface to fill a small hole with liquid when surface tension has closed one end
We showed that the natural frequency of both the air column and the divided bubbles inside the hole is important
We considered that bubble break-up and droplet generation were likely to occur in ethanol, a low surface tension liquid, which deformed the gas–liquid interface and caused pinching of droplets to occur frequently
Summary
It is difficult to deform the gas–liquid interface to fill a small hole with liquid when surface tension has closed one end. Cleaning, painting, and bonding using liquids are indispensable in product manufacturing To completely achieve these processes, the inside of surface holes must be filled with liquid [1,2,3]. It is difficult for liquid to enter small holes if one end is closed due to surface tension because it prevents gas–liquid interface deformation on a smaller, capillary-length scale [4,5,6]. Sanada et al [11] observed gas removal from a closed-end hole during a droplet train impact They saw that the pressure fluctuation during the droplet impact caused the gas column volume to oscillate inside the hole and that the droplet formation from the oscillating gas–liquid interface promoted gas removal.
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