Queue of flat nanobubbles dissolves with difficulty
How quickly an air bubble dissolves in a fluid strongly depends on the size of the bubble. Yet minuscule bubbles on a water surface can exist for hours to days. Researchers from the FOM Foundation and the University of Twente have developed a new model that explains this: the nanobubbles become flatter and accumulate in a sort of queue. By allowing for this the researchers can correctly predict the lifespan of nanobubbles. A new theory about this is published today in the journal Physical Review Letters.
Surface tension
Surface tension ensures that the smaller a bubble is, the faster the gas inside the bubble dissolves in the surrounding fluid. Minuscule bubbles (smaller than ~100 nanometres) should therefore dissolve within several microseconds. The lifespan of nanobubbles on a surface in contact with water is, however, a magnitude of ten times bigger. FOM PhD researcher Joost Weijs MSc and FOM workgroup leader professor Detlef Lohse can now explain why this is the case. "As nanobubbles become smaller, the outside edge, the contact line with the surface, does not move with the bubble. The bubble is stuck in place so to speak," says Weijs. "Consequently the bubble becomes flatter as its volume decreases, and in flatter bubbles the surface tension squeezes the gas out of the bubble less quickly. Therefore the flatter the bubble becomes, the slower it empties."
Queue of bubbles
"In addition to this the surrounding fluid is saturated with dissolved gas," continues Weijs. "This gives rise to a sort of gas queue: gas must first of all discharge to the atmosphere before new gas from the bubbles can be dissolved in the liquid. The rate at which gas escapes from a bubble strongly decreases as a result of this, dependent on the thickness of the fluid layer above the bubbles."
Calculations with the new model give a predicted lifespan that agrees with that measured in experiments. In follow-up experiments the researchers want to validate the model so that it can be used in practical applications. Weijs: "One highly promising application of nanobubbles is using these to cover the walls of channels in microfluidic devices. The liquid that flows through such channels then experiences less resistance."
Reference
- Joost H. Weijs, Detlef Lohse, 'Why surface nanobubbles live for hours'. Physical Review Letters (2012).
- Synopsis in Physics
Contact
Detlef Lohse
+31 (0)53 489 80 76
Joost Weijs
+31 (0)53 489 25 68