Séminaires

Articles

• Séminaire PMMH - Etienne Jambon-Puillet, LadhyX

  7 mai
  Vendredi 24 mai de 11h00 à 12h00 - Salle réunion PMMH 1

  Pendant drops on the underside of wet surfaces : growth, motion and solidification

  Pendant drops spontaneously appear on the underside of wet surfaces through the Rayleigh-Taylor instability. Due to their detrimental effect on coatings and their tendency to drip, several strategies have been developed to avoid their formation and rationalized with linear stability analysis. I will first briefly show how the Rayleigh-Taylor instability can be harnessed to make patterned elastic surfaces with a tunable spacing [1] and aspect ratio [2]. However, having a fine control over the drops in the non-linear regime of the instability is hard. They are connected to a thin film with which they continuously exchange liquid. They thus have no contact line and move around easily which disrupt the pattern. I investigate these drops dynamic experimentally, numerically and theoretically with a model system that consider a single pendant drop surrounded by a uniform thin film. I will show that under smooth surfaces, a transition from growth to decay occurs as the substrate inclination angle is increased by a few degrees [3]. I will then consider uneven surfaces, and show that the underlying topography can control the drop motion via gravito-capillary pinning-like forces [4].

  [1] Marthelot, J., Strong, E. F., Reis, P. M., & Brun, P. T. (2018). Designing soft materials with interfacial instabilities in liquid films. Nature communications, 9(1), 4477.
  [2] Jambon-Puillet, E., Piéchaud, M. R., & Brun, P. T. (2021). Elastic amplification of the Rayleigh–Taylor instability in solidifying melts. Proceedings of the National Academy of Sciences, 118(10), e2020701118.

  [3] Jambon-Puillet, E., Ledda, P. G., Gallaire, F., & Brun, P. T. (2021). Drops on the underside of a slightly inclined wet substrate move too fast to grow. Physical review letters, 127(4), 044503.

  [4] Jambon-Puillet, E. (2024). Gravito-capillary pinning of pendant droplets under wet uneven surfaces. arXiv, 2403.17584.

• Séminaire PMMH - Louis-Alexandre Couston (ENS Lyon)

  2 mai
  Vendredi 17 mai de 11h00 à 12h00 - Salle réunion PMMH 1

  What we know and don't know about the ocean circulation and ice melting around Antarctica

  The Antarctic ice sheet is losing mass increasingly rapidly and could
  contribute several tens of centimetres of sea-level rise by 2100. In
  this presentation I will review our understanding of the southern ocean
  circulation, which (we know) is controlling ice melting around
  Antarctica and driving the retreat of the ice sheet. I will describe the
  ocean dynamics at both large and small scales. Notably, I will explain
  why the ocean circulation below ice shelves (floating extensions of the
  ice sheet, hundreds of kilometres long) may tip from cold to warm
  conditions abruptly. Then, I will review the different (turbulent)
  regimes of ice-ocean boundary layers (only few meters thick), which
  control heat exchanges between warm deep-water masses and the overlying
  ice shelves and icebergs. Throughout the presentation I will highlight
  knowledge gaps and discuss recent works from our group, including
  laboratory experiments and turbulence-resolving phase change
  simulations, aiming to improve our understanding and modelling
  capabilities of ocean flows around Antarctica.

  
  

• Séminaire PMMH - Avin Babataheri (Ladhyx)

  27 avril
  Vendredi 20 septembre de 11h00 à 12h00 - Salle réunion PMMH 1

  1D confinement controls cell shape and migration

  Pericytes are mural cells of the microvasculature, they wrap around small vessels, support the vessels mechanically and participate in blood flow regulation.

  Pericytes are distinguished by two main characteristics ; first their distinct morphology, which has been likened to a “bump on a log”, as they present long processes spanning along the axis of the vessels they adhere to, and a protruding soma that houses the nucleus. The second characteristic is their distribution along the microvascular tree in a cellular chain where each cell occupies its own territory. This spatial distribution is established by cell migration during the embryonic stage and maintained through controlled motility in the adult.

  Because of their position in the microvascular tree, pericytes experience extreme lateral and longitudinal confinement, we hypothesize that the confinement impacts their morphology. Although pericyte shape is key for their function during vascular regulation, the manner in which pericyte morphology is associated with migration and function remains unknown.

  In this project, we use micropatterns to mimic pericyte adhesion to microvessels, we show that lateral confinement controls cell shape and produces in vivo-like phenotypes. We study pericyte migration on both laterally confining lanes, and longitudinally constraining motifs and propose a model that describes their kinetics as a stochastic motion with dry friction.

  In vivo pericytes can bridge nearby capillaries, we show that in vitro pericytes are also capable of crossing gaps of different sizes. The percentage of crossings is correctly predicted by the likelihood of a fluctuating system to overcome an energy barrier. Our joint experimental and theoretical approach demonstrates the effect of in vivo-like geometrical confinement on pericyte morphology and migration.

• Séminaire PMMH - Sara Puijalon (LEHNA – CNRS, Université Claude Bernard Lyon 1)

  26 avril
  Vendredi 26 avril de 11h00 à 12h00 - Salle réunion PMMH 1

  Interactions between submerged aquatic vegetation and flow : applications of ecohydraulics and biomechanics to ecological questions

  Interactions between submerged aquatic vegetation and flow in rivers and streams are complex. Flowing water induces hydrodynamic stress on aquatic plants colonizing these habitats, which can, in some cases, cause mechanical failures such as breakage and uprooting. Additionally, aquatic plant beds have a significant impact on their physical habitat by deflecting flow and modifying water velocity and turbulence. This talk will present research on the interactions between aquatic vegetation and flow. It will cover hydrodynamic and biomechanical responses of plants to flow, at both the individual plant and patch scale. It will also present how aquatic vegetation modifies flow conditions and contributes to the functioning of flowing ecosystems.

• Séminaire PMMH - Karen Mulleners (EPFL)

  21 avril
  Vendredi 3 mai de 11h00 à 12h00 - Salle réunion PMMH 1

  Getting smarter overnight : how automated experiments help unwind unsteady vortex-dominated flows

  Abstract : Typical unsteady vortex-dominated flows like those involved in bio-inspired propulsion, unsteady airfoil separation, and vortex-induced vibrations can be prohibitively expensive to simulate and impossible to measure comprehensively. They are inherently non-linear, often involve moving boundaries, high-dimensional parameter spaces, and multiscale flow structures. The classical way to get around these challenges has been to reduce the experimental complexity by using canonical motions (e.g. ramp-up or sinusoidally pitching motions) or simplified unsteady inflow conditions (e.g. one-minus-cosine or trapezoidal gust shapes). In our lab, we design automated experiments that can run continuously and autonomously such that we can explore and exploit higher-dimensional parameter spaces that cover more realistic and technically relevant unsteady conditions compared to what is traditionally feasible when conducting supervised canonical motion experiments. This approach give us the ability to derive more robust and generalizable models and control solutions while still discovering rare and extreme events. Our recent experiments allowed us to uncover flapping wing kinematics that maximize lift and efficiency, to optimize blade pitching kinematics that improve the power production of vertical axis wind turbines, and to gain insight into the influence of morphology on the forces of thin flexible objects. Once the experiments are started, the experimentalists' input is minimized while the potential for scientific discovery is maximized.