Laboratoire Matière et Systèmes Complexes

The superpropulsion effect with soft elastic projectiles and droplets

Christophe Raufaste Associate Professor Institut de Physique de Nice Université Côte d’Azur

The mechanical problem of the ejection of a projectile with a catapult-like engine is not as obvious as we could initially imagine. While rigid projectiles exhibit a regular acceleration phase, the propulsion of soft elastic projectiles requires to solve the whole elastodynamics problem to account for the balance between inertia and elasticity.

Experiments are performed with both hydrogels and droplets, which are propelled with a catapult. For the droplets, surface tension plays the role of the elastic restoring force.

Data show that the ejection velocity depends on both the projectile deformation and the catapult acceleration dynamics. With a matching given by a specific value of the projectile/catapult characteristic times ratio, a 250% kinetic energy gain is obtained as compared to the propulsion of a rigid projectile. A model based on solving the wave equation for the mechanical deformation gives a very good agreement.

We will discuss several applications : sorting of objects according to their elastic properties, impedance-matching for a better ejection of rigid projectiles, actuation of droplets

Reference : Superpropulsion of droplets and soft elastic solids, C. Raufaste, G. Ramos Chagas, T. Darmanin, C. Claudet, F. Guittard, F. Celestini, Phys. Rev. Lett. 119, 108001 (2017).