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.

Droplet impingement phenomena are found in the wide variety of industrial processes, however the detail of liquid film structure formed by the continuous impact of droplets is not clarified. In this study, we experimentally investigated behavior of circular plate. The effects of nozzle diameter and liquid flow rate on the jump diameter were investigated. In addition, we compared the liquid film by the droplet train impact with that by a liquid column impact. As a result, the hydraulic jump was observed under the smaller water flow rate condition compare to the liquid column impact. And the jump diameters for the case of droplet train impact were greater than that of liquid column impact. In addition, the liquid film heights after hydraulic jump on a small circular plate were sensitive to either the droplet train impact or liquid column impact.

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