[PhD] Control of instabilities in flexible structures (LPENS Paris and/or LAUM Le Mans, France)

Application deadline: Sat 25 Jun 2022

Topics: Waves, mechanics and soft matter
Thesis to be started in Fall 2022.

Background. There is much to learn from nature. Animals [1] and plants [2] exploit mechanical instabilities to amplify and accelerate their motion. The unstable mechanical response of natural systems comes from the high flexibility of biological materials, specifically their low Young's modulus, as well as their ability to accommodate large deformations. Currently, scientists are drawing inspiration from these natural processes to develop new bio-inspired flexible robots [3].

Objectives. The objective of this thesis is to address the dynamic aspects of mechanical instabilities occurring in flexible structures. A soft beam (see picture) submitted to a parametric forcing is likely to develop an unstable response. The first challenge will consist in observing the instability and probing its threshold [4]. Next, we will attempt to displace the critical point in order to inhibit or, on the contrary, to trigger this instability. On possible solution is to introduce an external control parameter, such as a controllable temperature or magnetic field [5].
Finally, we will be interested in the propagation of an elastic wave near the instability threshold.

Motivations and perspectives. The understanding of the motion of soft structures is crucial for applications
ranging from energy recovery to the actuation of soft robots. The implementation of an unstable self-propelled robot prototype (e.g. an artificial swimmer) could be considered.

Application profile. The candidate should have a master degree, ideally in Physics, but also possibly in
Mechanics, Engineering or Applied Mathematics. He/she should be motivated by the prospect of conducting
experimental research in fundamental physics and have a great curiosity as well as a willingness to acquire new knowledge at the interface between several disciplines. Autonomy, initiative and fluency in English will be particularly useful. The PhD student will be expected to rapidly develop skills in modeling and experimentation.
Experiments will involve sample preparation (molding and realization of flexible structures), image acquisition, particle tracking and signal processing. The PhD student will also participate in the writing of scientific publications and travel to present his/her results at national and international conferences.

Environment. This project is part of a collaboration between the Acoustics Laboratory of the University of Le
Mans (LAUM) and the Physics Laboratory of the École Normale Supérieure (LPENS) in Paris. Therefore, the
student will have the possibility to work in both laboratories. Besides, this thesis may lead to a funded
international mobility opportunity in collaboration with Harvard University. The financing of the thesis is
ensured by a CNRS grant (80PRIME).

Submission of applications. Applications must be submitted via the CNRS employment platform at
New closing date: June 25, 2022.

Maxime Lanoy (LAUM) - maxime.lanoy@univ-lemans.fr
Francois Pétrelis (LPENS) - francois.petrelis@phys.ens.fr
Vincent Tournat (LAUM) – vincent.tournat@univ-lemans.fr

[1] Son, Menolascina and Stocker, PNAS 113 (2016).
[2] Forterre, Stokeim, Dumais and Mahadevan, Nature 433 (2005).
[3] Nagarkar, Lee, Preston, Nemitz, Deng, Whitesides and Mahadevan, PNAS 118 (2021).
[4] Raynal, Kumar and Fauve, EPJB 9 (1999).
[5] Wilson, Fuchs and Gordaninejad, JAPS 84 (2002).