Abstract
When a droplet impacts a (super-)hydrophobic surface, there is a range of Weber numbers within which bubble entrapment will occur during droplet recoil due to closure of the air cavity developed when the droplet spreads out during the impact. In this study, we studied bubble entrapment using a microelectromechanical system (MEMS)-based acoustic sensor fabricated on a substrate. We found that bubble entrapment is followed by an acoustic vibration that can be detected by the sensor. Moreover, the frequency of the vibration is inversely proportional to the radius of the droplet, which indicates that this vibration is the resonant oscillation of the bubble. Therefore, the MEMS-based acoustic sensor can be used not only to detect but also to measure the size of the entrapped bubble. Finally, we demonstrated that it is possible to prevent bubble formation by allowing the air to escape to the underside of the droplet contact area. This can be done by creating through-holes on the substrate or decorating the substrate with sufficiently large textures.
Highlights
Droplet impact on a rigid substrate has been the topic of various studies because of its fascinating dynamics as well as its important role in a wide range of practical applications[1,2]
The cantilever’s resistance changed immediately after the droplet hit the sensor chip owing to the pressure caused by the impact. This resistance change was larger for higher impact velocities, as the pressure increases with increasing impact velocity
In this study, we experimentally examined the bubble entrapment induced by air cavity closure during the recoil phase of droplet impact
Summary
Droplet impact on a rigid substrate has been the topic of various studies because of its fascinating dynamics as well as its important role in a wide range of practical applications[1,2]. It was shown that prior to the contact of the liquid with the substrate, the droplet slides on a very thin air film sandwiched between the droplet and the substrate[3,4,5,6,7,8,9], and the liquid/solid contact causes the air film to be trapped, resulting in the formation of a bubble inside the droplet[3,4,5,10,11,12] This process is not the only mechanism of impactinduced bubble entrapment. To achieve the best efficiency of bubble oscillation-based droplet mixing, it is necessary to know the size of the bubble to determine the frequency of the external acoustic vibration
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