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
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
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.
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.
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.