Researchers solve the mystery of the formation of gas bubbles in a liquid

The formation of air bubbles in a liquid seems very similar to its reverse process, the formation of liquid droplets from, for example, a dripping water faucet. But the physics involved is actually quite different and, although the size and spacing of the water droplets are uniform, the formation of bubbles is usually a much more random process.

Now, a study by researchers at MIT and Princeton University shows that, under certain conditions, bubbles can also be tamed to form spheres as perfectly matched as droplets.

The new findings may have implications for the development of microfluidic devices for biomedical research and for understanding the interaction of natural gas with oil in the tiny porous spaces of underground rock formations, the researchers say. The results are published today in the journal PNAS, in an article from the MIT graduate, Amir Pahlavan, Ph.D. 18, Howard Stone of Princeton, Gareth McKinley, professor of innovation teaching at the MIT School of Engineering, and Ruben Juanes, professor at MIT.

The key to producing bubbles of uniform size and spacing lies in their confinement in a narrow space, explains Juanes. When air or gases are released into a large reservoir of liquid, the dispersion of the bubbles is dispersed. Once released into a confined liquid in a relatively narrow tube, the gas will produce a perfectly matched bubble flow and forming at regular intervals. This uniform and predictable behavior, independent of specific starting conditions, is called universality.

The process of forming droplets or bubbles is very similar, starting with an elongation of the flowing material (be it air or water) and possibly thinning and pinching of the "neck". "connecting the droplet or bubble to the flowing material. . This nip then allows the droplet or bubble to fold to take a spherical shape. Image blowing soap bubbles: When you blow into the ring, a tube of soap film gradually spreads outward in a long pocket before pinching to form a round bubble that floats.

"The process of droplets drip from a tap is known to be universal," says Juanes, who holds a joint position in the civil and environmental engineering departments and Earth Sciences, Atmosphere and planets. If the dripping liquid has a different viscosity or surface tension, or if the opening of the faucet is of a different size, "it does not matter.You can find relationships that you determine a master curve or master behavior to describe this process, "he says.

But in terms of what is, in a sense, the opposite process to a dripping faucet – the injection of air through an opening in a large reservoir of liquid such as a whirlpool, the process is not universal. "So, if you have irregularities in the orifice, or if the orifice is bigger or smaller, or if you inject with a pulsation, all this will lead to a different pinching of the bubbles," says Juanes .

The new experiments involved the percolation of gases on viscous liquids such as oil. In an unconfined space, the size of the bubbles is unpredictable, but the situation changes as they turn into liquid in a tube. Up to a point, the size and shape of the tube matter little, nor does the characteristics of the orifice through which the gas passes. Instead, the bubbles, like the droplets of a faucet, are uniform in size and spaced apart.

Pahlavan said, "Our work is actually a story of two startling observations.The first surprising observation was made about 15 years ago, when another group investigating the formation of bubbles in large fluid reservoirs observed that the pinch process was not universal "and depended on the details of the experimental setup. "The second surprise now comes in our work, which shows that confining the bubble inside a capillary tube makes the pinch insensitive to the details of the experiment and therefore universal."

This observation is "surprising", he says, because intuitively, it could seem that bubbles capable of moving freely in the liquid would be less affected by their initial conditions than those that are surrounded. But the opposite has turned out to be true. It turns out that the interactions between the tube and the bubble in formation, when a line of contact between the air and the liquid advance along the inside of the tube, play an important role. This "effectively erases system memory, details of initial conditions, and thus restores universality to the pinch of a bubble," he says.

While this research may seem esoteric, its results have potential applications in a variety of practical contexts, says Pahlavan. "The controlled generation of drops and bubbles is very desirable in microfluidics, with many applications in mind.Some examples are inkjet printing, medical imaging and the manufacture of particulate materials. . "

The new understanding is also important for some natural processes. "In geophysical applications, we often see fluid flows in very narrow and confined spaces," he explains. These interactions between fluids and surrounding grains are often overlooked when analyzing such processes. But the behavior of such geological systems is often determined by grain-scale processes, which means that the type of small-scale analysis performed in this work could be useful for understanding even very large situations. scale.

According to Juanes, the formation of bubbles in such geological formations can be a blessing or a curse, but anyway, it is important to understand. For carbon sequestration, for example, the hope is to pump carbon dioxide, separated from power station emissions, into deep formations to prevent gas from escaping into the atmosphere. In this case, the formation of bubbles in minute porous spaces of the rock is an advantage because the bubbles tend to block the flow and keep the gas anchored in position, preventing it from escaping.

But for the same reason, the formation of bubbles in a natural gas well can be a problem because it can also block the flow, inhibiting the ability to extract the desired natural gas. "He can be immobilized in the pores," he says. "It would take a lot more pressure to move that bubble."