As a bubble rises through liquid, along the way it collects contaminant particles such as bacteria, viruses, and microplastics on its surface and drags them with it. When the bubble surfaces and pops, it sends up a jet of liquid that often breaks into droplets, and the droplets can have contaminant concentrations that are more than 1000 times as high as those in the original liquid. That enrichment can have dramatic implications for the transfer of ocean pollutants to the atmosphere and for disease transmission. New work by Lena Dubitsky, Oliver McRae, and James Bird at Boston University offers insights into the details of the contaminant enhancement.
Much attention has focused on the jet’s top droplet, which is likely to persist longest and travel farthest. The conventional picture had been that the liquid in the droplet comes from a thin, uniform liquid microlayer surrounding the bubble; any scavenged particles that fit within the microlayer would end up in the top drop, and any particles that extend beyond the microlayer would end up elsewhere. Those expectations are consistent with past reports that for particles of a fixed size, the enrichment has a peak at a certain droplet size.
When the Boston University researchers released air bubbles with a diameter of 500–1500 μm into water containing polystyrene beads either 15 μm or 30 μm across, they found enrichments of up to 1400 in the top droplet. Yet the beads were always bigger than the calculated microlayer thickness—existing models predicted the droplet shouldn’t have contained any particles at all.
The team’s numerical simulations gave clues to the discrepancy. The figure, of a simulated bubble with a 700 μm diameter (left) just before it bursts and the resulting top droplet (right), shows the breakdown of the conventional picture. The panels’ left halves show where the liquid in the assumed uniform source microlayer (green) ends up: Not all of it is in the droplet. The right halves show where the fluid in the droplet (blue) does come from: a nonuniform layer whose thickness, exaggerated here for clarity, depends on the position along the bubble surface. And for smaller bubble sizes, that source layer covers a smaller surface region.
An additional effect is also at play: When the bubble reaches the surface and ruptures, the accumulated particles get swept to the base of the bubble, and the fluid surrounding the bubble gets compressed to a region roughly the size of the jetted droplet. That rearrangement causes the local thickness of the droplet-source layer—and particle sizes it can contain—to increase. Whether a particle gets transferred thus depends on the bubble radius, the ratio of the particle size to the droplet size, and the location of the particle on the bubble surface.
The team’s model shows good agreement with the experiments, including the enrichment peak as a function of droplet size, and it explains the observed variability in enrichment even under identical conditions. The new results differ from past models in two ways: Large bubbles can transfer into the top droplet a significant fraction of larger particles, even those approaching the droplet size, and fewer small particles will get transferred for smaller bubbles. (L. Dubitsky, O. McRae, J. C. Bird, Phys. Rev. Lett. 130, 054001, 2023.)
