Bubble entrapment during the recoil of an impacting droplet

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. Microsystems & Nanoengineering Microfluidics: Probing air bubbles trapped in dropletsResearchers in Japan have characterized bubbles of air that form within water droplets when they hit a hydrophobic surface. During impact, an air cavity can form through the center of a droplet, and this can become trapped as bubble when the droplet merges together again. Thanh-Vinh Nguyen and Masaaki Ichiki of the National Institute of Advanced Industrial Science and Technology used a MEMS-based sensor to measure acoustic vibrations during the impact. They found that the frequency of the oscillations is inversely proportional to the size of the entrapped bubble. Furthermore, the team showed that bubble formation can be prevented by changing the topology of the surface to allow the air to escape. These findings will help control the behavior of entrapped bubbles when used for mixing or prevent their formation in applications such as inkjet printing.