When a tensile stress in the liquid, created by hydrodynamic or acoustic means, breaks the equilibrium between expanding and collapsing forces acting on a pre-existing gas pocket in favor of the expanding forces, then the water around the gas pocket will vaporize explosively. This leads to the formation of a large gas-vapor bubble which can be orders of magnitudes larger than the original gas pocket. This process is called heterogeneous cavitation. Cleaning, erosion and the production of light and sound caused by expanding, oscillating and collapsing cavitation bubbles form the main reasons why cavitation still attracts so much scientific and industrial interest today. See for example our related research pages on sonoluminescence, ultrasonic and megasonic cleaning, cells under shear stress, lithotripsy and our work on the snapping shrimp.
Controlled cavitation inception
We have demonstrated that cavitation inception studies - which are usually associated with irreproducibility and a poor nuclei characterization - can in principal be controlled and understood completely. The key to this achievement is to control the size and location of cavitation nuclei by patterning smooth hydrophobic subtrates with well-defined cylindrical pits. Upon emerging the substrate in water, the pits entrap air and nucleate bubbles after sufficient lowering of the liquid pressure. The resulting cavitation bubble dynamics is perfectly controlled in both space and time, and can be properly described by the Rayleigh-Plesset equation. The high spatial control of our nuclei down to nanometer length scales allowed us to quantitatively compare the minimum pressure required to nucleate the entrapped gas pockets with the theoretical predictions developed in the framework of the crevice model by Atchley & Prosperetti in 1989, yielding a perfect match between theory and experiment.
Cavitation on microparticles
In the more practical situation of sub-microscopic gas pockets present on freely floating microparticles suspended in a liquid, we have demonstrated that cavitation experiments can still be fully reproducible.
In addition, we have revealed both experimentally, theoretically and numerically the origin of the huge acceleration which microparticles can achieve due to cavitation inception originating on their surfaces (Figure 2). We found that particles acquire translational momentum (allowing them to reach velocities of > 10 m/s) as a result of a pressure gradient over the particle surface during the initial growth phase of the bubble (i.e. when the liquid pressure is lower than the vapor pressure).
These results are of potential great value in situations where absolute control on the presence, nucleation threshold, and activation of surface entrapped gas pockets is desired, as in naval engineering or sonochemistry studies.
Figure 1: Hexagonal array of cavitation bubbles. The distance between two bubble centers is 200 µm.
Example of a microparticle accelerated by a cavitation bubble which nucleated from a submicroscopic gas pocket originally present at the particle surface.
Info: Detlef Lohse
Researchers: Manish Arora, Bram Borkent, Nicolas Bremond, Stephan Dammer, Claus-Dieter Ohl, Detlef Lohse.
Collaborators: Han Gardeniers (Mesoscale Chemical Systems, U Twente), Albert van den Berg (BIOS group, U Twente)
Embedding: Spearhead program on Micro- and Nanofluidics, Mesa+, JMBC
Sponsors: STW Nanoned program