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The life of a bubble

They might be short-lived — but between the time a bubble is born (Fig 1 and Fig 2a) and pops (Fig 2d-f), the bubble can interact with surrounding particles and microorganisms. The consequence of this interaction not only influences the performance of bioreactors, but also can disseminate the particles, minerals, and microorganisms throughout the atmosphere. The interaction between microorganism and bubbles has been appreciated in our civilizations for millennia, most notably in fermentation. During some of these metabolic processes, microorganisms create gas bubbles as a byproduct. Indeed the interplay of bubbles and microorganisms is captured in the origin of the word fermentation, which is derived from the Latin word ‘fervere’, or to boil. More recently, the importance of bubbles on the transfer of microorganisms has been appreciated. In the 1940s, scientists linked red tide syndrome to toxins aerosolized by bursting bubbles in the ocean. Other more deadly illnesses, such as Legionnaires’ disease have been linked since.

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Figure 1: Bubble formation during wave breaking resulting in the white foam made of a myriad of bubbles of various sizes. (Walls, Bird, and Bourouiba, 2014, used with permission)

Bubbles are formed whenever gas is completely surrounded by an immiscible liquid. This encapsulation can occur when gas boils out of a liquid or when gas is injected or entrained from an external source, such as a breaking wave. The liquid molecules are attracted to each other more than they are to the gas molecules, and this difference in attraction leads to a surface tension at the gas-liquid interface. This surface tension minimizes surface area so that bubbles tend to be spherical when they rise and rapidly retract when they pop.

Figure 2: Schematic example of Bubble formation (a), rise (b), surfacing (c), rupture (d), film droplet formation (e), and finally jet droplet formation (f) illustrating the life of bubbles from birth to death. (Bird, 2014, used with permission)
Figure 2: Schematic example of Bubble formation (a), rise (b), surfacing (c), rupture (d), film droplet formation (e), and finally jet droplet formation (f) illustrating the life of bubbles from birth to death. (Walls, Bird, and Bourouiba, 2014, used with permission)

When microorganisms are near a bubble, they can interact in several ways. First, a rising bubble can create a flow that can move, mix, and stress the surrounding cells. Second, some of the gas inside the bubble can dissolve into the surrounding fluid, which can be important for respiration and gas exchange. Microorganisms can likewise influence a bubble by modifying its surface properties. Certain microorganisms secrete surfactant molecules, which like soap move to the liquid-gas interface and can locally lower the surface tension. Microorganisms can also adhere and stick on this interface. Thus, a submerged bubble travelling through the bulk can scavenge surrounding particulates during its journey, and lift them to the surface.

When a bubble reaches a surface (Figure 2c), such as the air-sea interface, it creates a thin, curved film that drains and eventually pops. In Figure 3, a sequence of images shows a bubble before (Fig 3a), during, and after rupture (Fig 3b). The schematic diagrams displayed in Fig 2c-f complement this sequence. Once a hole nucleates in the bubble film (Fig 2d), surface tension causes the film to rapidly retract and centripetal acceleration acts to destabilize the rim so that it forms ligaments and droplets. For the bubble shown, this retraction process occurs over a time of 150 microseconds, where each microsecond is a millionth of a second. The last image of the time series shows film drops launching into the surrounding air. Any particulates that became encapsulated into these film droplets, including all those encountered by the bubble on its journey through the water column, can be transported throughout the atmosphere by air currents.

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Figure 3: Photographs, before, during, and after bubble ruptures. The top panel illustrated the formation of small film droplets; the bottom panel illustrates the formation of larger jet drops. (Bird, 2014, used with permission)

Another source of droplets occurs after the bubble has ruptured (Fig 3b). The events occurring after the bubble ruptures is presented in the second time series of photographs. Here the time between photographs is one milliseconds, or 1/1000th of a second. After the film covering the bubble has popped, the resulting cavity rapidly closes to minimize surface area. The liquid filling the cavity overshoots, creating an upward jet that can break up into vertically propelled droplets. These jet drops can also transport any nearby particulates, also including those scavenged by the bubble on its journey to the surface. Although both film and jet drops can vary in size, jet drops tend to be bigger.

Whether it is for the best or the worst, bubbles are ubiquitous in our everyday life. They can expose us to diseases and harmful chemicals, or tickle our palate with fresh scents and yeast aromas, such as those distinctly characterizing a glass of champagne. Bubbles are the messenger that can connect the depth of the waters to the air we breathe and illustrate the inherent interdependence and connectivity that we have with our surrounding environment.

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