Filling holes with liquid, or discharging gas from holes, is a fundamental process in both cleaning and painting. Discharging gas from small holes with closed end and high aspect ratio is extremely difficult due to surface tension. In this study, we developed a new gas discharge process from a hole by using acoustic wave irradiation. We irradiated two types of acoustic waves: waves with constant frequencies and those with variable frequencies in time i.e. sweep wave for test samples in a water pool. In addition, we observed gas discharge using high-speed video camera. As a result, we succeeded in completely discharging gas by using sweep frequency of the acoustic waves. From the observation results, we confirmed that gas discharge consists of three stages. The gas was discharged from the hole mainly at the first and third stage. In these stages, the natural frequency of the gas column in the hole was essential. In the second stage, the gas column in the hole breaks into multiple gas columns; then, the gas was hardly discharged even with the acoustic wave irradiation.

Wet cleaning methods using liquids are widely applied in many industrial fields. In such methods, it is first necessary to cover the object to be cleaned with the liquid. However, in structures with small holes, surface tension prevents the deformation of the gas-liquid interface, making it difficult to fill the object with the liquid. We have found that liquid infiltration into such small holes is promoted by the impingement of droplet trains, but the underlying mechanism has not yet been elucidated. In this study, we observed this liquid infiltration process through droplet train impingement into a closed-end hole, and compared the liquid column impact. The filling process was visualized with two high-speed video cameras. Our observations illustrate the importance of the oscillation and deformation of the gas-liquid interface inside the holes following droplet impingement. First, intermittent droplet impingement causes small droplets or large interface deformations to form, and then the gas column inside the hole becomes separated. This separated gas column is then gradually ejected. Therefore, the liquid infiltration can be increased by using a droplet train formed of a small-surface-tension liquid. Furthermore, we investigated the influence of the hole diameter and the uniformity of the droplet train frequency. The results show that droplet train impingement is effective for relatively large holes, although the uniformity of the droplet train frequency has little effect on the liquid infiltration.

Wet cleaning methods using fluid are widely applied in many industrial fields. For a cleaning inside closed-end holes, it is first necessary to fill the holes to be cleaned with the liquid. However, in structures with small holes, it is difficult to discharge inside the gas due to surface tension. In our early studies, we have found that the discharging a gas inside a closed-end hole was promoted by an impingement of droplet train. And the pressure fluctuation near the gas-liquid interface due to droplet impingement was important. In this study, we attempted the gas discharge from closed-end holes due to acoustic wave irradiation. First, we theoretically estimated the oscillation of the gas column inside the hole during acoustic wave irradiation. We modeled the natural frequency of the gas column using a spring-mass system. Then we experimentally measured the fluctuation of the gas-liquid interface for the evaluation of the model. In addition, we compared the gas discharge ratio with different frequency and pressure level. The fluctuation of gas-liquid interface and discharging the gas were observed with a high-speed video camera. As results, the natural frequencies of a gas column were depending on the length of the gas column and the diameter of the hole. From the experiments, we confirmed that the acoustic wave certainly propagated into the hole, and the frequency of the irradiated acoustic wave and the experimentally obtained natural frequency were in good agreement except for extremely low gas discharge ratio condition. Moreover, we observed gas discharge process and found that the high gas discharge ratio were achieved using the acoustic wave close to natural frequency. From these results, we concluded that the assumption based on a spring-mass system is valid.

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