Science and Engineering

Cavitation Bubble Dynamics inside Liquid Drops in Microgravity


Overview

We studied single cavitation bubbles inside spherical water drops produced in microgravity. Via high-speed visualizations, the bubble collapse and subsequent phenomena revealed important features related to the isolated, spherical drop volume. Bubble lifetimes in drops are shorter than in extended volumes in remarkable agreement with herein derived corrective terms for the Rayleigh-Plesset equation. Further, spatially resolved images uncover strong secondary cavitation induced by reflected shockwaves, offering a novel way to estimate integral shockwave energies. For eccentrically placed bubbles, the toroidal collapse generates two liquid jets, consistent with theoretical predictions. Due to the closed spherical surface both jets visibly escape from the drop and disclose an important jet broadening.

All results will be presented in a scientific journal. On this page you find:

     


1. Scientific Background Information

The hydrodynamic cavitation phenomenon is a major source of erosion damage in many industrial systems, such as fast ship propellers, cryogenic pumps, pipelines, and turbines. Yet, controlled cavitation-erosion proves a powerful tool for modern technologies like ultrasonic cleaning, effective salmonella destruction, and treatment for kidney stones. Erosive processes are associated with liquid jets and shockwaves emitted by collapsing cavitation bubbles, but the relative importance of these two processes remains a topic of debate. Single bubble dynamics strongly depends on nearby surfaces by means of boundary conditions imposed on the surrounding pressure field. Recent investigations revealed interesting characteristics of bubbles collapsing next to flat and curved rigid surfaces or flat free surfaces.

 

It would be a fruitful idea to study bubbles inside spherical drops and probe their interaction with closed spherical free surfaces. Yet, for larger volumes, such geometries are inaccessible in the presence of gravity and require a microgravity environment, even though gravity plays a negligible direct role for most single bubble phenomena.

Stimulated by this situation, we decided to carry out the first experimental study of single bubbles inside centimetric spherical water drops produced in microgravity. The experimental result disclose a wide range of surprising and interesting phenomena, including (1) jet dynamics, (2) shockwave effects and (3) shortened spherical collapse.


2. Experimental Techniques

A detailed technical description is available here as pdf. Contains:

  • Flight Manoeuver

  • G-level Fluctuation

  • Setup, Electrodes, Injector tube

  • Study of drop stability

  • Optical refraction by water drop


3. Images of Experimental Installation

30 days to go: preliminary tests in the laboratory

 

5 days to go: installing experiment inside the aircraft

 

3 days to go: Last mental training

 

2 days to go: proper installation completed

 

1 day to go: aircraft closing

 

"3, 2, 1, injection to zero-g"

 

Schematic overview of all functional units

 

 

Transparent test chamber, which contains the studied drops

 

Automated micropump for drop generation

 

 

4. Images and Movies of Results

Double Jet Formation

Cavitation bubbles, which are eccentrically placed inside a spherical water drop, collapse with toroidal symmetry. Thereby two liquid jets are ejected in antipodal directions. Both jets visibly escape from the drop. This double-jet picture aligns with established studies of bubbles in the vicinity of free surfaces, and provides the first direct visualization of both bubble-induced jets escaping from a steady liquid volume.

 

Double jet formation 1
(wmf, 1.2 MB)

Double jet formation 1
(wmf, 1.9 MB)

 

 

Official Movie for Media
(mpeg, 3 MB)

Shockwave-Nuclei Interaction

Many high-speed visualizations show isolated simultaneous flash ups of a brilliant mist of microbubbles, lasting for roughly 50 microseconds. Strikingly, these flash ups exactly coincide with instants of predicted shockwave radiation. We could show  that this mist of microbubbles is a strong form of shockwave induced secondary cavitation. In other words, microbubbles are small cavitation bubbles inside the drop volume, which arise from nuclei, i.e. microscopic impurities and dissolved gas, at the passage of shockwaves.

   

Official Movie for Media
(mpeg, 4MB)

Detailed Bubble Collapse

Cavitation bubbles that are centered inside spherical water drops, conserve spherical symmetry along the collapse and don't show liquid jet formation. We found that such bubbles collapse faster than bubbles in "infinitely" extended liquid volumes, which are accuratelly described by the Rayleigh-Plesset theory. We could derive three correction terms for this theory, which generalize it to bubbles in finite drops. This novel theory is in excellent agreement with the experimental data.

  

Selected Images

Bubble inside drop with no eccentricity

 

Bubble inside drop with medium eccentricity

 

Bubble inside drop with high eccentricity

 

"Hairjets" appearing if microbubbles implode close to the free surface

 

Bubble collapse in a drop of champagne

 

Harmonic mode due to g-jitter

 

5. References

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