Air bubbles in a glass of water float freely up to the surface, and the mechanisms behind this are easily explained by the basic laws of science. However, the same laws of science cannot explain why air bubbles in a tube a few millimeters thick don’t rise the same way.
Physicists first observed this phenomenon nearly a century ago, but couldn’t come up with an explanation – in theory, the bubbles shouldn’t encounter any resistance unless the fluid is in motion; thus a stuck bubble should encounter no resistance.
Back in the 1960s, a scientist named Bretherton developed a formula based on the bubbles’ shape to explain this phenomenon. Other researchers have since postulated that the bubble doesn’t rise due to a thin film of liquid that forms between the bubbles and the tube wall. But these theories cannot fully explain why the bubbles don’t rise upwards.
While a Bachelor’s student at the Engineering Mechanics of Soft Interfaces laboratory (EMSI) within EPFL’s School of Engineering, Wassim Dhaouadi was able to not only view the thin film of liquid but also measure it and describe its properties – something that had never been done before. His findings showed that the bubbles weren’t stuck, as scientists previously thought, but actually moving upwards extremely slowly. Dhaouadi’s research, which was published recently in Physical Review Fluids, marked the first time that experimental evidence was provided to test earlier theories.
Dhaouadi`s measurements also show that the bubbles are actually moving, albeit too slowly to be seen by the human eye. Because the film between the bubble and the tube is so thin, it creates a strong resistance to flow, drastically slowing the bubbles’ rise.
These findings relate to fundamental research but could be used to study fluid mechanics on a nanometric scale, especially for biological systems.
To read the full article go to Phys.org
