Detection and Control of Bubble Entrapment During the Recoil Phase of Droplet Impact

In this paper, we report a method to detect the formation of bubble during the recoil phase of droplet impact on a (super-)hydrophobic surface using a MEMS-based acoustic sensor. We show that by this method, not only the occurrence of bubble entrapment can be detected but also the size of the bubble can be measured from the frequency of the oscillation recorded by the sensor. Moreover, we found that substrate texture is an important factor determining the occurrence of bubble entrapment. It is possible to prevent the bubble entrapment by allowing the air to escape through the contact area, for example in the case of substrate with relatively large textures. The findings in this study are useful in many applications including bubble prevention in ink-jet printing, heat transfer and droplet mixing using bubble oscillation induced microstreaming.