Swimming through shell buckling

Under pressure, a hollow elastic sphere becomes unstable and
collapses. It reinflates back when the pressure is decreased. The
hysteresis in shape sequence and in deformation velocity associated
to this cycle makes this simple object a good candidate for becoming a
swimmer able to move at low or high Reynolds number.

We explore this possibility through a macroscopic experiment in fluids
of varying viscosities so as to explore different flow regimes. We
show that not only the shape hysteresis leads to swimming but the fast
buckling phase is an efficient mechanism for propulsion that implies
inertial effects and subtle coupling between shape post-buckling
oscillations and fluid flow patterns. Our modeling shows that such an
inertial regime could even be reached at microscopic scale.

We anticipate that a conveyor made of a few of microbubbles with
different shell thicknesses, would constitute a microrobot whose 3D
displacement can be remotely controlled by ultasounds.

Ref : A. Djellouli, P. Marmottant, H. Djeridi, C. Quilliet and G.
Coupier, PRL 119, 224501 (2017).