The longest-lived bubble on the ground! It can last for nearly half an hour without bursting

Produced by: Science Popularization China

Author: Shi Chang (PhD in Physical Chemistry)

Producer: China Science Expo

Bubbles, as a ubiquitous substance in nature, frequently appear in our lives. In the park, children insert a plastic stick with holes into the solution and blow gently, and a series of bubbles rise up leisurely, blooming with a brief splendor. In the kitchen, under the flushing of detergent, countless tiny bubbles jump like elves. It is such a series of crystal clear bubbles that not only add color to our lives, but also play an important role.

On March 20, 2024, Chinese scientists published a study on the use of ultrasonic levitation technology to extend the life of bubbles in the journal Droplet. With this technology, the team prepared the "longest-lived bubble on the ground", which lasted for 23 minutes and 36 seconds. This makes us curious about how bubbles are formed? Why do the bubbles we blow burst so quickly?

Bubbles blown from a plastic stick

(Photo source: veer photo gallery)

Bubbles - coexistence of gas and liquid phases

Bubbles are composed of two parts: gas and liquid film. When acted upon by external force, the gas enters the liquid through small holes. Under the action of surface tension, the liquid wraps around the gas, thus forming bubbles.

The contact between gas and liquid is the primary condition for the generation of bubbles. When children blow bubbles, they need to dip a plastic stick with holes in the solution and blow gently with their mouths. The gas is then "loaded" into the liquid film to form bubbles. We can also observe the generation of bubbles when boiling water. Where does the gas come from in this process and who is "blowing" the bubbles?

Bubbles when water boils

(Photo source: veer photo gallery)

In fact, during the heating process, the temperature at the bottom of the kettle is higher, and the water molecules near the bottom will first break free from the constraints of the intermolecular interaction force and vaporize into water vapor. At this time, the "tiny irregularities" or impurities at the bottom of the kettle provide sites for the formation of bubbles, where water molecules gather to form bubbles. Therefore, from this process, the bubbles are "blown" by the vaporized water vapor.

Bubbles are a phenomenon when water boils, but they also reflect the boiling point of water. When boiling water at high altitudes, even if the water has begun to boil, the temperature is below 100 degrees Celsius. Why is this?

The sign of water boiling is that the saturated vapor pressure is the same as the external atmospheric pressure, that is, the air pressure in the bubbles produced is the same as the external atmospheric pressure. When the altitude increases, the atmosphere becomes thinner, resulting in a decrease in atmospheric pressure, and the boiling of water will also be advanced.

Outdoor kettle

(Photo source: veer photo gallery)

Related research shows that the boiling point of water drops by about 1 degree Celsius for every 300 meters of altitude increase. Mount Everest is about 8,848 meters above sea level, where the boiling point of water is only about 70 degrees Celsius. If you climb Mount Everest, you may need to add more water and cook noodles for a longer time.

Can water be boiled in the space station?

On the afternoon of March 23, 2022, Shenzhou 13 crew astronauts Zhai Zhigang, Wang Yaping and Ye Guangfu successfully taught the second lesson of the "Tiangong Classroom" at the Chinese space station, attracting widespread attention from audiences at home and abroad.

During the space-ground interaction session, students from the ground branch classroom in Lhasa, Tibet wanted to know whether water could be boiled on the space station. Astronaut teacher Wang Yaping answered no, drinking water on the space station is generally heated by a specific heating device before drinking, and cannot be boiled directly like on the ground.

space station

(Photo source: veer photo gallery)

This is because in the weightless environment of the space station, the bubbles generated when water is heated are difficult to float up, and the convection and bubble disturbance caused by the floating bubbles are suppressed. The increase in water temperature can only rely on heat conduction. Water far away from the heating surface is difficult to be heated to 100°C and cannot reach a uniform boiling state.

What is the longest time a bubble can exist?

Whether it is the bubbles produced by boiling water or the bubbles produced by stirring detergent, they basically exist for a few seconds. Even the bubbles produced by children blowing bubbles only exist for a few tens of seconds. So what is the longest time that bubbles can exist?

Under normal circumstances, due to the effect of gravity, the liquid in the liquid film will flow downward, which is the process of drainage. This causes the thickness of the liquid film to become uneven, and the bubbles cannot exist stably for a long time. In order to extend the life of the bubbles, researchers turned their attention to the microgravity environment of the space station. In 2014, European researchers conducted an experiment on the retention of bubbles in water samples in the space station. After shaking the water samples in the space station, the bubbles produced could exist for 18 minutes, while after the water samples on Earth were shaken, the bubbles produced disappeared after 1 second.

Comparison of the existence time of bubbles in the space station and bubbles on Earth

(Image source: Reference 1)

Although the microgravity environment can suppress the drainage of bubbles and extend the life of bubbles, it is extremely expensive and difficult to combine bubbles with other research in the space station. The research work on extending the life of bubbles using ultrasonic levitation technology published by Chinese scientists in the journal Droplet revealed the super stability of acoustic levitation bubbles.

The researchers found that in a stable acoustic levitation state, the gravity of the bubble liquid film is balanced by the acoustic radiation force, and the acoustic radiation force also offsets the static pressure of the liquid on the bubble film, thereby inhibiting the drainage and producing an effect similar to extending the life of the bubble under microgravity conditions.

Comparison of the existence time of sodium dodecyl sulfate (SDS) bubbles (a) and pure water bubbles (b) under acoustic levitation conditions

(Image source: Reference 2)

The results show that the lifespan of bubbles produced using sodium dodecyl sulfate (SDS) solution exceeds 15 minutes, and even pure water bubbles without added surfactants have a lifespan of 7 minutes. In addition, acoustic radiation forces can also stabilize other external disturbances. A suspended bubble can remain unbroken when penetrated by a hot copper needle with a diameter of 0.8 mm (about 50 degrees Celsius). Using this technology, the team prepared the "longest-lived bubble on the ground", which lasted for 23 minutes and 36 seconds.

What is the application value of extending bubble life?

First, as a microreactor and catalyst carrier, in chemical synthesis and materials science, stable bubbles as microreactors can provide a large gas-liquid contact area, thereby improving mass transfer efficiency and reaction rate. In addition, the bubble surface can adsorb catalysts for environmental remediation and green chemical synthesis.

Bubbles in chemical reactions

(Image source: Reference 2)

Second, it is used in heat exchange and energy conversion systems. In heat pipes and heat dissipation systems, stable bubbles can reduce thermal resistance and improve the thermal management capabilities of equipment. In addition, the oscillation and resonance characteristics of bubbles can also be used in energy collection and conversion devices, such as acoustic energy conversion systems.

Third, it is used in basic physics research. Stable bubbles provide an ideal experimental model for studying the surface tension, viscosity and fluidity of liquids. By observing the behavior of bubbles in the sound field, we can have a deeper understanding of bubbles.

Conclusion

Seemingly ordinary bubbles bring a lot of fun and convenience to our lives. In the eyes of scientists, tiny things often contain important principles and application potential. Look at the world with scientific eyes, and you will find that there is interesting science everywhere in life!

References:

1.Langevin, Dominique. Aqueous foams and foam films stabilized by surfactants. Gravity-free studies[J]. Comptes Rendus Mecanique, 2017.

2.Xiaoliang Ji, Wenxuan Zhong, Kangqi Liu, et al. Extraordinary stability of surfactant-free bubbles suspended in ultrasound[J]. Droplet, 2024.

3. Du Wangfang, Liu Peng, Zhao Jianfu, et al. "Boiling water" on the Chinese space station to reveal the mechanism of gravity in boiling phenomena[J]. Mechanics and Practice, 2022.

4. Guo Lei, Zhang Shusheng, Chen Yaqun, et al. Study on bubble dynamics of water boiling heat transfer in vertical narrow channel[J]. Journal of Xi'an Jiaotong University, 2010.