When air bubble in water collide with a tilted surface, some bounce away and then reverse course, hitting the surface again in a 鈥渂ackflipping鈥� motion. These acrobatics are caused by the flow of fluid around a bubble and could potentially be harnessed for cleaning.
at Brown University in Rhode Island and his colleagues shot differently sized air bubbles in a tank of water towards a glass slide. The slide was held flat against the top of the water or tilted downwards at various angles up to 20 degrees. They generated the bubbles by pushing air through a special needle at the tank鈥檚 bottom and filmed them with a high-speed camera.
The bubbles with a radius around 0.7 millimetres experienced the most prominent backflip when the glass was at 3 degrees. The team developed a computer model which found that after a bubble bounced off the glass, it got caught in its own wake and spun back towards the glass.
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鈥淭he bubble can affect its own destiny. This is unique,鈥� says at Brown University, who was not involved with the experiment. He says that the backflipping phenomenon hasn鈥檛 been seen and analysed in experiments before.
Hooshanginejad says that backflipping bubbles could be used for cleaning in the future because a tumbling bubble has two collisions with a surface, each of which can remove dirt from it. The researchers tested this idea by shooting bubbles at a surface covered with a protein solution or a biofilm. Based on images taken afterwards, Hooshanginejad says, backflipping bubble collisions scraped clean roughly 20 per cent more of the surface than bubbles that only hit the surface once in the same amount of time.
However, at Otto von Guericke University Magdeburg in Germany says that to be certain that backflipping provides significant cleaning advantages, future studies should measure the shearing forces that move dirt off the surface when a bubble repeatedly collides with it.
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