Cavitation and bubble dynamics: from plants to soft matter. – CAVISOFT

The study of the rapid movement of fern sporangia could be carried out through the use of fast imaging up to 200,000 frames/s. For the study of the phenomenon of the fast propagation of cavitation nucleation, we could make movies up to 1,500,000 frames/s. These measurements were made possible by using microfluidic channels to block sporangia inside and flowing osmotic solutions. This helped to highlight a completely new phenomenon of cavitation propagation at time scale of the order of a microsecond. We have fully met our objectives by successfully reproducing this phenomenon in biomimetic systems made of hydrogels by soft lithography. We modified the method during the project to achieve geometries able to reproduce the natural phenomenon. At the end of the project, acoustical techniques complemented the imaging experiments. On the complex fluids aspect, we were able to determine the important role of rheology on the morphology of bubbles, and its weak influence on the bubble growth rate during the pressure runs.

The first major result of the project was to understand the two-stroke mechanism of the fern sporangia catapult. This was important to understand the limiting time scale of collective cavitation events. The second important result concerns the discovery of a new cavitation propagation mechanism in the sporangia of ferns and in biomimetic systems. In these devices, several parameters could be varied to better understand the propagation mechanism coupling oscillations of bubbles and sound wave. These experiments have interested a renowned theorists team from the University La Sapienza in Rome working on cavitation. A new collaboration was launched and a co-supervised thesis has just begun.

These experiments have interested a renowned theorists team from the University La Sapienza in Rome working on cavitation. A new collaboration was launched and a co-supervised thesis has just begun.

An article in the journal SCIENCE was published. This was followed by a popular movie of articles in the New York Times, Scientific American. Another was submitted in Journal of the Royal Society: Interface. Several articles are being submitted , one in a high-ranked journal. Other articles are also being prepared and should be submitted shortly.

During a first order phase transition, an energy barrier has to be overcome due to the cost to create an interface. The apparition of vapor by heating a liquid or decreasing its pressure is then not immediate, although the state is more stable. The metastable liquids are said to be respectively “superheated” and “stretched”. When tensile stresses can no longer be sustained, bubbles are then nucleated, for example behind propellers or when producing sonoluminesence. The utilization of this high energy density release at cavitation collapse is also used in ultrasonic cleaner. Many studies have been devoted to the case where transient and local negative pressures are applied to the liquid. We will focus here on a different fate for the bubbles: when the overall liquid is in static tension. Bubbles are expected to expand up to an equilibrium size depending on external boundary conditions. They can also interact with other ones or any kind of confinement. We will study such situations that happen in nature: for example trees transport water under tension at altitudes well higher than the 10 meters corresponding to atmospheric pressure. More precisely, we will study the fern sporangium, a catapult-like structure that ejects rapidly its spores. Elastic energy is stored with the help of evaporating water under tension and released at the time of cavitation. Plants can be a huge source of inspiration for both technological applications and new perspectives for fundamental physics. We propose here to draw our inspiration from them and study nucleation and bubble dynamics in microfluidics, complex fluids and porous media.
We propose to explore nucleation and growth of bubbles (or cavity in gel) in more complex situations than the classical cases of a pure liquid in the bulk or close to a solid wall. Even in pure liquids, such experiments in water under static tension have been in fact rarely developed compare to other methods (acoustic, laser, hydraulic); we will then use static depressions possibly with dissolved gas. First, we will study the case of cavitation in fern sporangia and study the threshold pressures for nucleation, and the correlations between nucleation events. Then we will build artificial devices inspired by ferns. The main point will be to look at confinement effects from 1D channel with rigid or flexible walls up to 3D porous media. We will also look at the interplay with various wetting conditions. Then, the case of a gel will be tackled. One reason for this is that it can be seen as a microscopic porous media with flexible skeleton. Another reason is that these kinds of materials are expected in trees to be the locus of initial bubble growth when an embolism propagates. In the end, the use of such viscoelastic / plastic materials will allow to extend our studies to the case of rupture in solid materials that arises also by cavitation events that are followed by fractures connecting together. We will then go from pure liquids to solids in passing by viscoelastic and / or plastic fluids.
This project will be strongly interdisciplinary and will involve collaborations with plant biologists. It will also be supported by multiple approaches as it combines experiments on natural plants, on model systems and microfluidics devices, but also different numerical simulations methods, used in symbiosis (at a microscopic and mesoscopic level) and theoretical analysis. Our group initiated recently new topics in the lab, this proposal is the natural extension that will allow structuring the team, on soft condensed matter and biophysics oriented aspects.
Cavitation and negative pressure have important applications in engineering, medicine but also agronomyl. Consequences of severe drought events due to climate change on plants are directly linked to the resistance to cavitation. Our work can help to understand its effects in plants, where microchannels, porous media and gels are all present.