Lipid-coated microbubbles could swim (very) fast under ultrasound

Buckling is the key

Microbubbles can achieve significant net displacement through reproducible, non-destructive cycles of deflation and re-inflation, induced by an ultrasonic wave. Our numerical modeling supported by experiments showed that well-designed microbubbles could swim at speeds in the m/s range, enabling efficient motion within the bloodstream.
(Movie by M. Mokbel)

Comm. Eng. 2,  63 (2023)

How do lipidic shells buckle ?

A dialog between two views

Collapse of lipidic ultrasound contrast agents under high-frequency compressive loading has historically been interpreted by the vanishing of surface tension. In contrast, buckling of elastic shells is known to occur when the costly compressive stress is released by bending. Using quasi-static compression experiments on lipid shells, we analyzed the buckling events within the framework of classical elastic buckling theory and attempted to make these two views compatible.

Phil. Trans. R. Soc. A 381,  20220025 (2023)

Buckling dynamics of spherical shells

Joint studies with D. Holmes (Boston) and S.Aland (Freiberg)

We have modeled the buckling dynamics of elastic shells, paving the way for the controlled use of these objects to generate small-scale flows for applications such as mixing, or the propulsion and control of immersed microrobots.
We were interested in the initiation of buckling, modeling the process as the growth of the defect where the buckling was initiated.

Proc. Roy. Soc. A 477, 20210253 (2021)

We have also characterized how the shell oscillates in its buckled geometry (see video), and explained why it does so at a much lower frequency than in the spherical configuration.

Proc. Roy. Soc. A 477, 20210378 (2021)

Modeling spherical oscillations of shells

Going beyond the incompressible shell assumption

While existing models were focusing on shells made of isotropic incompressible material, we have developed a model that introduces compressible material, that are possibly anisotropic in the radial direction. The results explain part of the apparent dependency of material properties on shell radius, that is seen on most experiments on Ultrasound Contrast Agents.

J. Ac. Soc. Am. 149, 1240-1257 (2021)

Mechanics of beach ball deflation

Cheap and colourful experiments

The number of folds and the residual volume of deflated beach balls are functions of their mechanical properties. We have explored these relationships by means of simple experiments with commercial balls. The underlying laws apply to micrometric objects like cells, ultrasonic contrast agents, colloids, etc.

Eur. Phys. J. E 42, 129 (2019).
See also the highlight in Eur. Phys. J. E

A spherical shell that swims thanks to sound waves

Proof of concept macroscopic experiment

Powering microrobots inside a human organism would be useful for delivering small quantities of drugs at the right place, thus increasing their efficiency and reducing the possible side effects. To do so, we propose to use the simplest geometry ever: a hollow sphere. Under pressure, such a sphere becomes unstable and collapses. While this instability is generally seen as a mechanical failure, we use this property to propel the sphere.

Phys. Rev. Lett. 119, 224501 (2017)
See also Focus "Elastic Spherical Shell Can Swim" in APS physics.

Video by Adel Djellouli