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.