Gwennou Coupier
Grenoble, France

Focus story in

Elastic Spherical Shell Can Swim
by Philip Ball

An improved model for the spherical oscillations of shells

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.

J. Ac. Soc. Am. 149, 1240-1257 (2021).

Young Researcher Award of HTW Dresden for M. Mokbel

Congratulations to my colleague Marcel Mokbel for this achievement. With his PhD supervisor S. Aland, Marcel is working on numerical simulations to understand better shell buckling mechanisms and resulting swimming. See his video (in German):

Mechanics of beach ball deflation

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.

Eur. Phys. J. E. 42, 129 (2019) See also the highlight in Eur. Phys. J. E.

deflated balls

Quantification of the lateral migration of red blood cells

200 years ago, Poiseuille observed for the first time the presence of depleted layers in the vicinity of the walls of blood vessels. Through in-vitro experiments, we have characterised the migration mechanism that is at the origin of this phenomenon.

Microvasc. Res. 124, 30 (2019)


The dynamics of red blood cells is a signature of their discreet diversity

Red blood cells have a subtle mechanics which is not fully described yet by current models. We have carried out an extensive experimental study of their dynamics under shear flow, which is very sensitive to their intrinsic properties - that vary from one cell to another. This constitutes a reference work for the validation of new models.

J. Fluid Mech. 864, 408 (2019)

Scheme dyn

A spherical shell that swims thanks to sound waves

Size distribution in outlets

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.

Phys. Rev. Lett. 119, 224501 (2017). See also Focus "Elastic Spherical Shell Can Swim" in APS physics.

Video by Adel Djellouli

Phase separation of blood at bifurcations

Inversion of the usual separation law at low hematocrits

At the level of a bifurcation where the flows split unequally, red blood cells flow in such a way that the cell concentration often increases in the high flow rate branch. This splitting strongly depends on the upstream organization of the cell suspension. In some cases (high confinement and low concentration), a reverse effect is observed.

Microvasc. Research 105, 40 (2016)

Clusters in capillaries

Behaviour at bifurcation

At the level of the bifurcation, the trajectory of the flowing objects depends on the flow rate ratio but also on their lateral initial position, that we controlled thanks to a flow focusing device.

J. Fluid Mech. 674, 359 (2011)

Asymmetric bifurcation

Protein-induced clustering of red blood cells in microcapillaries

Plasma proteins cause red blood cells to form clusters called rouleaux which are usually assumed to be disaggregated in the circulation due to shear forces. However, despite the large shear rates present in microcapillaries, the presence of either fibrinogen or the synthetic polymer dextran leads to an enhanced formation of robust clusters.

Sci. Rep. 4, 4348 (2014)

These clusters are initiated by hydrodynamic interactions, which also contribute to their stabilization, in parallel to the adhesion-induced stabilization.

Soft Matter 12, 8235 (2016)

Clusters in capillaries

The structure of a red blood cells suspension

As observed by Poiseuille nearly two centuries ago, red blood cells organize themselves in layers of different concentrations when they flow in a blood vessel.

This is the result of two competing effects: hydrodynamic interactions with walls, that tend to push cells towards the center, and interactions between cells, that lead to diffusion towards depleted regions.

Pairwise hydrodynamic interactions between lipid vesicles induce a net lateral displacement. Such an interaction is the basic mechanism that leads to a non-Brownian diffusion in the suspension.

Phys. Fluids 26, 013304 (2014)

Size distribution in outlets

Size-dependent particle separation in a microfluidic chip

Pinched Flow Fractionation

PFF allows a precise and continuous sorting by sizes of particles in a suspension, with no use of external fields.

Microfluid. Nanofluid. 13, 697 (2012)

Size distribution in outlets

Lipid vesicles in confined flows

Lipid vesicles are closed membranes separating two fluids. Their dynamics under flow is rich, though controlled by only a few parameters.

This hydrodynamics problem with free boundary is met also when one considers red blood cells (RBCs), the main components of blood. Studying them allows to understand better how blood flows and how they are distributed in the capillary network.

Migration in Poiseuille flow

Vesicle in a channel of 90 microns width.

Lateral migration

Due to lift forces of viscous origin, a deformable object generally migrates towards the channel center line. In blood vessels, it is thus observed that there are less RBCs close to the walls than at the centre.

Phys. Fluids 20, 111702 (2008)

3D shape in Poiseuille flowVesicle in a channel of width between 45 and 120 microns.

3D shapes

Once at the center, the vesicle shape depends on several parameters (deflation, fluid velocity, confinement, channel symmetry). One can observe parachute, bullet or croissant shapes.

Phys. Rev. Lett. 108, 178106 (2012)

Croissant shape

Simulation d'A. Farutin

I use microfluidic devices and have also access to microgravity thanks to CNES and ESA: parabolic flights and sounding rockets (Maser 11 and Maser 12 campaigns). Data acquisition is facilitated by the use of digital holographic microscopy, that is developed in MRC in Bruxelles. It allows to get instantaneously the 3D position of the objects flowing in a suspension.

I belong to the Mecanics of Cells in Complex Medium (MC2) group in the Laboratory of Interdisciplinary Physics in Grenoble, France. We are interested in the physics and mechanics of cells in their environment, from the intracellular scale to tissues.

In the lab I work with:

  • Georges Chabouh (PhD)
  • Mourad Ismail  - for theory.
  • Catherine Quilliet, Benjamin Dollet, Philippe Marmottant - thin shells dynamics, artifical microswimmers.
  • Thomas Podgorski, LRP Grenoble - experiments on red blood cells
  • Adel Djellouli, Harvard - buckling of shells
  • Douglas Holmes, Boston University - buckling of shells
  • Sebastian Aland, Freiberg University, Germany - 3D simulations of shell dynamics
  • Christophe Minetti, Microgravity Research Center, Bruxelles, Belgium