Soft porous beads vs micro-balloons under pressure

2023-04-21T00:00:00Z

Anechoic coatings for underwater acoustics typically consist of a soft polymer slab loaded with gas-filled micro-balloons whose resonances absorb sound. These coatings must also hold their performance under hydrostatic pressure: at 100 m a submarine already experiences ~10 bar.

This is a three-way coupling:

A resonance mechanics problem — the inclusions must resonate in the frequency band of interest;
A poroelasticity problem — open-porosity beads collapse under load;
A durability problem — repeated load cycles must not degrade the attenuation.

Micro-balloons vs soft porous beads

We compare two resonant inclusions:

Micro-balloons — a thin polymer shell with a sealed gas core (closed porosity), the reference technology in naval anechoism;
Soft porous beads — silicone beads obtained by emulsion templating whose open porosity (a silicone alkyl polyether surfactant) forms a cluster of connected air cavities resonating broadly around 200 kHz.¹

Both composites are compressed uniaxially at loads up to 10–20 bar, mimicking hydrostatic pressure at 100–200 m depth, and their attenuation compared.

Under uniaxial compression mimicking hydrostatic pressure: the micro-balloon keeps its sealed gas core, while the open porosity of the soft bead collapses irreversibly beyond a few bars.

What we found

At ambient pressure, soft porous beads attenuate substantially more than micro-balloons over a broad band around 200 kHz;
Under load, they remain more absorbing at low frequencies even at 10 bar, but their resonance weakens as the porosity collapses;
Beyond a few bars, the damage is irreversible: after compression/relaxation cycles the resonant attenuation is not recovered, whereas micro-balloons resist better because their shells deform.

The natural next step is a hybrid: reinforce the porous beads with a thin shell — the pressure resistance of micro-balloons with the clusterized air cavities of the beads — or stiffen the surrounding matrix.

T. Lacour, R. Poupart, O. Mondain-Monval, C. Aristégui, O. Poncelet and T. Brunet, "Pressure effects on the resonant attenuation of soft porous beads-based materials for underwater acoustics," J. Appl. Phys. 133, 155105 (2023). doi:10.1063/5.0144249 ↩

A model for acoustic vaporization of encapsulated droplets

2018-01-05T00:00:00Z

Acoustic droplet vaporization (ADV) is the phase change of a liquid droplet triggered by an acoustic energy supply. It is the enabling mechanism behind a promising cancer treatment: inject droplets of a liquid that vaporises into a bubble under ultrasound, use them as contrast agents or as drug carriers that release their payload locally and controllably.

Modelling it is a coupling nightmare:

A radial dynamics problem for the vapor bubble;
A heat transfer problem across the liquid layer;
A nonlinear elasticity problem for the encapsulating shell.

A bubble/droplet/shell system

We built a model where the bubble is nested inside the droplet, the whole system wrapped in a hyperelastic shell — a crude but faithful representation of surfactant-stabilised droplets. The resulting ODEs couple the vapor pressure (thermodynamics), the acoustic forcing (nonlinear propagation) and the visco- hyperelastic response of the shell.¹

Cross-section of the encapsulated droplet: a white vapor bubble at the centre surrounded by liquid, all enclosed in a hyperelastic shell. The acoustic field drives the structure from the outside.

What we found

The vaporization threshold depends strongly on geometrical and rheological parameters of the shell;
There is a regime where the droplet only partially vaporises and the bubble recondenses — a fate directly connected to the acoustic parameters.²

This was supported by the Plan Cancer 2014–2019 project AIDA (Acoustically Induced Droplet Vaporization for Anti-cancer targeted drug delivery), a research project in physics, mathematics and engineering sciences applied to cancer research.

T. Lacour, M. Guédra, T. Valier-Brasier and F. Coulouvrat, "A model for acoustic vaporization dynamics of a bubble/droplet system encapsulated within a hyperelastic shell," J. Acoust. Soc. Am. 143, 23–37 (2018). doi:10.1121/1.5019467 ↩
T. Lacour, T. Valier-Brasier and F. Coulouvrat, "Ultimate fate of a dynamical bubble/droplet system following acoustic vaporization," Physics of Fluids 32, 051702 (2020). pubs.aip.org ↩

Thomas Lacour — Notes

Soft porous beads vs micro-balloons under pressure

Micro-balloons vs soft porous beads

What we found

A model for acoustic vaporization of encapsulated droplets

A bubble/droplet/shell system

What we found