Do bubbles that can't be punctured by needles really exist?

Professor Zang Duyang from Northwestern Polytechnical University and his team have achieved this. He said: "Under normal gravity conditions, we have made the bubbles maintain long-term stability, achieving an effect comparable to the suppression of drainage in the microgravity environment of the space station, providing a ground simulation method suitable for bubbles and bubble films in a space environment."

Specifically, when pierced by a hot copper needle with a diameter of 0.8 millimeters, a suspended bubble can still maintain its integrity and set the Guinness World Record for "the longest-lived bubble on the ground."

This ultra-stable acoustic levitation bubble, which has no contact with solid surfaces and no chemical "contamination," has great application prospects in scientific research and industrial production.

For example, ultra-stable bubbles are conducive to measuring the surface tension and rheological properties of liquids, and can serve as ideal biological/chemical reactors for crystal growth, liquid templates for cell culture, and unique microenvironments, thereby being used in the fields of materials engineering, fluid physics, life sciences, etc.Small Bubbles, Big Impact

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Bubbles are ubiquitous in nature and daily life, playing a significant role in many industrial processes and fundamental research due to their unique interfacial mechanics and thermodynamic properties.

Particularly noteworthy is the bubble film, which can provide unique heat and mass transfer boundary conditions and two-dimensional flexible constraints for physical and chemical processes.

However, bubbles prepared by traditional methods often have short life spans and poor stability, greatly limiting their application in production and daily life.

To obtain bubbles with a longer lifespan, people usually need to use surfactants or micro/nano particles as stabilizers to suppress the drainage problem caused by gravity.This can indeed extend the lifespan of the bubble, but chemical stabilizers inevitably lead to "contamination" of the bubble.

In order to achieve long-lived bubbles without introducing chemical stabilizers, European scholars have directly used the microgravity conditions on the International Space Station to suppress drainage, achieving the stability and longer lifespan of pure water bubbles.

Although the space station experiment can obtain long-lived bubbles, its cost is extremely high, and it is not convenient to combine with other means for complex experimental design.

Therefore, an important question is: Can we find a bubble stabilization method without introducing chemical stabilizers under normal gravity conditions on the ground?

Driven by this question, the research group carried out research on bubble preparation using acoustic levitation based on the previous acoustic levitation control of droplets, and then adopted the physical method of the sound field to stabilize the bubbles.Experimental evidence has shown that even without the stabilizing effect of surfactants or micro/nano particles, bubbles can still be stabilized in suspension through the action of an acoustic field.

By applying a compression effect to suppress the liquid expulsion from the membrane, it is even possible to prevent membrane rupture caused by the penetration of copper needles. In addition, the acoustic field method can also affect the shape of the bubbles through its additional acoustic forces.

Bubbles that cannot be punctured by needles

As mentioned earlier, this study is based on the previous work of the research group in preparing bubbles using acoustic levitation [1].In their previous work, they discovered that under the right conditions, the sound field can induce the transformation of a suspended liquid film into a bubble through the mechanism of acoustic cavity resonance.

At the same time, the team also noticed that bubbles under acoustic levitation conditions seem to survive longer than conventional bubbles, which set the stage for this topic.

Later, they found that even when punctured with a needle, it could still remain intact. This very unusual phenomenon aroused their great interest.

At that time, Zang Duyang and the team members were all asking the same question: "Why are the bubbles in the sound field so tough and long-lived?"

Therefore, they decided to establish a research group to conduct in-depth research on this issue. After determining the topic, they conducted experiments on the above phenomenon.At the outset, the team first demonstrated the universality of the phenomenon, finding that different liquids such as pure water, silicone oil, glycerin solutions, and various surfactant solutions, all form bubbles under acoustic levitation that possess an extraordinary stability far beyond the conventional.

Analyzing the time scale of the ultra-stable levitated bubble's lifespan, the research group found that pure water bubbles without any surfactant stabilization could survive for more than 7 minutes.

In contrast, levitated sodium dodecyl sulfate (SDS) bubbles could remain intact for over 15 minutes, which is more than two orders of magnitude longer than the lifespan of bubbles without the presence of an acoustic field.

Once the acoustic levitator is turned off, the bubbles, due to the release of the acoustic radiation force applied to their surfaces, quickly release liquid and undergo noticeable shape changes, eventually bursting within a few seconds.

Subsequently, they began to delve into the main reasons for the bubbles' ultra-stability.The main reason why ordinary bubbles burst in an extremely short time is the liquid film drainage caused by gravity. The long-term stability of suspended bubbles is very likely due to some kind of inhibition of liquid film drainage.

To verify this conjecture, it is necessary to measure the change of the membrane thickness of the suspended bubble over time. Therefore, they cooperated with Associate Professor Zhao Wei from Northwestern University.

Relying on the latter's profound background in optical fluid measurement research, they built an optical measurement device for measuring the thickness of the liquid film by fluorescence profiling.

After measurement and analysis, it was found that in the absence of an acoustic field, the thickness of the liquid film at the bottom of the bubble increases rapidly to its maximum thickness within a few seconds according to the power law of two-thirds of time, which leads to the rapid rupture of the bubble.

However, under the condition of acoustic levitation, the thickness of the bottom film remains almost unchanged over a long time scale. This indicates that acoustic levitation almost completely inhibits the phenomenon of gravity drainage of the bubble membrane.The core concern of this topic is to reveal the physical mechanism by which acoustic levitation suppresses the drainage of bubble liquid films.

Considering that the drainage of the liquid film is suppressed, it may be due to a dynamic equilibrium caused by convection within the membrane, or it may be due to a static equilibrium caused by mechanical forces counteracting the effect of gravity. Therefore, they conducted the following verifications:

Firstly, verification of liquid film flow.

By using fluorescent polystyrene particles as tracer particles to track the flow within the levitated bubble liquid film, it was found that there was no significant flow present within the bubble membrane.

Secondly, verification of bubble rotation.Due to the acoustic streaming around the suspended bubble, which may stimulate the bubble to rotate around the vertical axis, they used a copper ring with a diameter almost the same as that of the bubble and placed it at the equator of the bubble. Due to the wetting between the ring and the liquid of the bubble, the rotation of the bubble is significantly inhibited.

At this time, the stability of the bubble still exists and can remain intact for several minutes.

Thirdly, ultrasonic field verification.

The shape of the bubble will change noticeably with the variation of the acoustic field intensity, indicating that the bubble membrane is affected by the acoustic field force. Moreover, after the acoustic field is removed, the liquid will gather at the bottom of the bubble in an extremely short time, leading to its rapid rupture.

These results indicate that the ultra-stability of the suspended bubble and the suppression of liquid drainage are not attributed to the liquid flow in the bubble membrane, or the bubble rotation caused by acoustic levitation.The mechanical effect of the ultrasonic field on bubbles is the main reason for the super stability of the bubbles.

Although it has been clearly identified that the super stability of suspended bubbles mainly comes from the contribution of the ultrasonic field, the specific physical and mechanical mechanism by which the acoustic field suppresses the drainage of the liquid film is still unclear.

Therefore, the research team simulated the acoustic field distribution and force conditions of the acoustically levitated bubble using the finite element software COMSOL.

The results show that: the sound wave can not only act on the outer surface of the bubble, but also pass through the bubble membrane and produce a considerable acoustic radiation pressure on its inner surface.

Under the stable levitation state, the gravity of the bubble can be balanced under the total acoustic radiation force. Moreover, the acoustic radiation force on the outer surface of the bubble is upward, while the acoustic radiation force on the inner surface is downward.This indicates that the direction of the acoustic radiation force acting on the inner and outer surfaces of the bubble is opposite, which can exert a squeezing effect on the liquid film at the bottom of the bubble, thereby significantly suppressing the drainage of the liquid.

Subsequently, based on the physical image revealed by the simulation results, in order to describe the mechanical mechanism of the stable bubble in the acoustic field from a theoretical perspective, they derived the theoretical formula for the force on the bubble membrane and established the theoretical model of the ultra-stability of acoustic levitation bubbles.

The research group stated: The integral of the acoustic radiation pressure acting on the inner and outer walls of the bubble not only balances the gravity of the bubble but also inevitably balances the integral of the hydrostatic pressure on the bubble membrane.

For hydrostatic pressure, it is the main driving force for the liquid in the bubble to be discharged. Therefore, the hydrostatic pressure can be suppressed by the acoustic radiation force, thereby completely suppressing the drainage of the liquid film, and thus greatly extending the life of the bubble.

In addition, they also analyzed the stability of the bubble from an energy perspective. In the stable suspension, the standing wave acoustic field provides a unique distribution of acoustic radiation pressure on the inner and outer surfaces, which results in the suspended bubble having the minimum total energy of acoustic potential, gravity, and surface energy.This type of energy not only enables levitation through coupling with the shape of the bubble but also maintains the contour of the bubble membrane. Therefore, the acoustic field can also manipulate the morphology and thickness distribution of the bubble.

As the acoustic intensity increases and the bubble gradually becomes flatter, the liquid in the liquid film will gradually gather at the equatorial position. With the weakening of the acoustic intensity, the shape of the bubble will change towards a spherical shape, and the liquid in the liquid film begins to gather at the bottom.

It can be seen that the acoustic levitation bubble has an astonishing ability to resist external disturbances.

The flat soap film placed horizontally or vertically in the standing wave acoustic field also shows similar acoustic stability phenomena, and it is also impossible to penetrate with a needle. This is in sharp contrast to the natural liquid film that will quickly rupture after being disturbed slightly.

This is mainly because: the acoustic radiation pressure applied on both sides of the liquid film can greatly suppress the thinning process caused by needle-like deformation, so the formation of holes in the liquid film will be completely suppressed.Here, the research has essentially concluded, and they then began to write the paper. Recently, the related paper was published in the journal Droplet under the title "Extraordinary stability of surfactant-free bubbles suspended in ultrasound" [2].

Ji Xiaoliang, a doctoral student at Northwestern Polytechnical University, is the first author of the paper, and undergraduate student Jiang Yichen also participated in this work, with Zang Duyang serving as the corresponding author.

In general, bubbles suspended in an ultrasonic standing wave field have a significantly extended lifespan. The team's acoustic levitation device provides a new method for studying the surface properties of bubbles and the heat and mass transfer processes.

Based on this achievement, they plan to carry out the following follow-up research:

In the acoustic field, the internal and external acoustic flows of the bubbles will significantly affect the heat and mass transfer processes of the bubbles. This influence may lead to peculiar phenomena in the crystallization process of bubbles in the acoustic field.At the same time, the flow within the levitated bubble liquid film is suppressed by the acoustic field, so its nucleation and growth may undergo new changes.

Therefore, they will explore the dynamics and thermodynamics of levitated bubbles, thereby providing new ideas for understanding the essence of crystallization.

In addition, both the inner and outer films of the levitated bubble are affected by the acoustic radiation force, which makes the total force on the outer film of the bubble upward and the total force on the inner film downward.

If the bubble is regarded as a mass block with an outer surface composed of a liquid film and a spring composed of an air core in the center, then the entire bubble can be regarded as a spring mass oscillator.

Based on this, the research group plans to use this virtual mass model to solve the energy absorption problem of the bubble in the acoustic field, providing a new theoretical model for the manufacture and control of bubble materials.