PMMH Laboratory

"For outstanding contributions to elasto-capillary phenomena, elastic instabilities, and shape-morphing materials that combine creative experiments and elegant modeling."

https://www.aps.org/funding-recognition/aps-fellowship/dsnp-fellowship

We will show how this invariant may be used to quickly derive useful information on the equilibrium shapes of elastica in self-contact or in interaction with obstacles, sliding sleeves, force fields, and droplets.

This static-dynamic analogy allows us to write a quantity that is invariant along the elastic rod at equilibrium. For a pendulum, mechanical energy constant in time whereas, for a planar elastica, the sum of its curvature energy and its axial force will be uniform along the structure.

The static-dynamic analogy discovered by G. Kirchhoff shows that the statics of the planar elastica are equivalent to the dynamics of the pendulum. In this analogy, time and angular velocity are, for example, equivalent to arc length and curvatures.

To come to PMMH:
www.pmmh.espci.fr/Contact-357

Coming Seminars:

Oct 17: Marie Poulain-Zarcos (LMFA, Lyon)
Nov 7: Christina Kurzthaler (Max-Planck Dresden)
Nov 14: Romain Mari (LiPhy, Grenoble)
Nov 21: Christiana Mavroyiakoumou (Oxford, UK)

Full list at www.pmmh.espci.fr/Seminaires.

The PMMH seminar at 11am on Friday 10 Oct will be given by Florence Bertails-Descoubes (INRIA, Grenoble) and Sébastien Neukirch (Institut d'Alembert, Sorbonne Université). The talk will be in French.

Title: Elastic Rods made easy with a Noether invariant

Abstract follows. Zoom link upon request.

I will show that fracture toughness increases with the square root of the defect fraction.
This counterintuitive effect highlights how disorder, rather than weakening, can reinforce materials by arresting cracks.

fracture in heterogeneous materials, and the pinning of elastic lines.
In the second part, I shift from adhesive contacts to fracture in bulk disordered materials. Using numerical simulations of spring networks with randomly removed bonds,

—a phenomenon known as adhesion hysteresis.
In the first part of this talk, I will show that adhesion hysteresis originates from the pinning of the contact perimeter by surface roughness. This mechanism connects adhesion hysteresis to broader phenomena such as contact angle hysteresis,

Soft solids tend to stick: bringing two surfaces together lowers their surface energy, and separating them requires tensile forces. This simple energy balance, however, cannot explain why soft materials stick more strongly when we pull them apart than when they first come into contact

On Friday 3 Oct 11am, PMMH will host Antoine Sanner, ETH Zürich, who will talk about

How random roughness arrests cracks in the contact of soft solids and the fracture of spring networks

Abstract follows. Zoom link upon request.

Together, these results illustrate how active filamentous systems harness shape and flexibility to adapt, spread, and organize matter in complex environments.

Complementary experiments with robotic filaments and simulations reveal the same principles, providing a framework for locomotion in fixed landscapes and particle collection in mobile ones.

In the second part, we will discuss how the same worms, when interacting with freely moving particles, can act as autonomous collectors : repeated contact and bending drive aggregation into clusters whose size is controlled by filament length and flexibility.

We will first show how worms navigate quasi-2D arrays of obstacles: ordered pillar lattices confine and trap them, while disordered environments paradoxically enhance their spreading, with activity levels playing a key role in mobility.

Living worms such as Tubifex tubifex behave as active, flexible polymers that move and manipulate matter in confined environments. I present two recent studies that highlight how their polymerlike body and activity govern locomotion and transport.

Future seminars:
Oct 3: Antoine Sanner (ETH Zürich)
Oct 10: Florence Bertails-Descoubes (INRIA, Grenoble) & Sébastien Neukirch (∂Alembert, Paris)
Oct 17: Marie Poulain-Zarcos (LMFA, Lyon)
Nov 7th: Christina Kurzthaler (Max-Planck Dresden)

Full list at www.pmmh.espci.fr/Seminaires.

Our seminar will be given by Antoine Deblais of the University of Amsterdam, who will talk about the locomotion and particle collection by active, polymer-like worms on 26/9 at 11am.

Title:
Locomotion and Particle Collection by Active Polymerlike Worms

Abstract below. Zoom link upon request.

We show that at large distances (a few centimeters), the attraction results from film-scale deformations due to the weight the beads. At small distances (a few millimeters), the interaction force suddenly increases due to the capillary interaction between the menisci surrounding the particles.

The second system is a horizontal soap film. Friction is again very weak, while the attractive force now extends over a longer range. We study the origin of the friction, and measure the interaction force by analyzing particle dynamics, and by using the magnetic actuation of paramagnetic beads.

The particles spontaneously self-propel, moving in straight lines at a velocity of a few cm/s. We will explain the mechanism underlying this motion, and analyse the orbiting trajectories that appear when two particles interact.

We explore the dynamics of particles on two unusual interfaces, on which the friction is drastically reduced. The first system is an evaporating bath of liquid nitrogen. When deposited on the cryogenic bath, ambient temperature particles are levitated by the vapour escaping from the interface.

The motion of millimetre-sized objects trapped at liquid-air interfaces is familiar, seen in the clumping of cereal in a bowl or of bubbles at the surface of a sparkling drink. The interface deformation generates inter-particle attraction while friction with the underlying liquid damps the motion.

On Friday 19 Sept at 11am, PMMH will host Anaïs Gauthier (IPR Rennes & CNRS) who will talk about

Capillary choreographies at low-Friction interfaces

From Leidenfrost baths to soap films

Abstract below. Zoom link upon request

Our findings bring insights into how active inclusions alter the mechanics of fibrous networks, contributing to a better understanding of the role of cells in the mechanics of healthy but also diseased tissues.

a loss of the classic stress-stiffening behaviour of collagen. We decouple passive and active contributions by using cell-sized passive inclusions, blocking cell adhesion and contractility, and through rheo-confocal microscopy for simultaneous rheology measurements and in situ visualisation.

We find by bulk shear rheology that above a cell volume fraction of only a few percent, the mechanics of the collagen network is strongly affected. We observe a softening both of the mature network and over time during collagen polymerization, as well as

have yet to be disentangled. Here, we address this question by investigating how local cell-matrix interactions regulate the global mechanics of collagen networks in a biomimetic tissue model composed of fibrillar collagen type I with embedded invasive cancer cells (MDA-MB-231).

altering its structural and rheological properties. How these local matrix changes originated by the cells affect the overall global mechanics of tissues is not fully understood yet. Furthermore, the contributions to tissue mechanics of passive mechanical effects versus active cellular effects

Abstract

Tissues are composed of extracellular matrix (ECM) and cells in constant interplay. While ECM influences many cellular processes from division and migration to differentiation, cells interact with their environment by exerting pulling forces and remodelling or degrading the matrix,