Invited talks

• Sandra Lerouge - Laboratoire Matière et Systèmes Complexes, Université Paris-Diderot

Flow and stability of viscoelastic liquid curtains

Liquid curtains are sheets of liquid falling freely from an extrusion die. We investigate the flow of curtains made of polymer solutions and we show that gravity is initially balanced by the elastic stress arising from the stretching of polymer chains, after which the classical free-fall behaviour is recovered. We show how this is analogous to Newtonian curtains in which gravity is initially balanced by viscous dissipation. We also describe a varicose modulation of the sheet observed for the most shear-thinning solutions, which is linked to an unsteady three-dimensional flow instability at the planar contraction in the extrusion die.

• Elie Raphaël - Laboratoire Gulliver, ESPCI Paris

Rearrangement of two dimensional aggregates of droplets under compression: Signatures of the energy landscape from crystal to glass

We study signatures of the energy landscape's evolution through the crystal-to-glass transition by compressing two dimensional (2D) finite aggregates of oil droplets. Droplets of two distinct sizes are used to compose small aggregates in an aqueous environment. Aggregates range from perfectly ordered monodisperse single crystals to disordered bidisperse glasses. The aggregates are compressed between two parallel boundaries, with one acting as a force sensor. The compression force provides a signature of the aggregate composition and gives insight into the energy landscape. In particular, crystals dissipate all the stored energy through single catastrophic fracture events whereas the glassy aggregates break step by step. Remarkably, the yielding properties of the 2D aggregates are strongly impacted by even a small amount of disorder.

• Carlo Massimo Casciola - Dept of Mechanical and Aerospace Engineering, University of Rome La Sapienza

Bubble nucleation in flowing liquids

When the pressure falls below a critical level (cavitation) or the temperature raises above a threshold (boiling), the liquid-vapor transition takes place. The process starts with the nucleation phase, a rare event which is deeply routed in the atomistic nature of the fluid.
Successively, depending on the local thermodynamic conditions, the bubble may grow to macroscopic size and couple to the inertial dynamics of the surrounding fluid. In the classical approach each phase is treated separately. Classical Nucleation Theory (CNT) deals with the nucleation rate (number of bubbles formed per unit time and
volume). Once the bubble is formed, the celebrated Rayleigh-Plesset equation, or extensions therein, is classically used to describe the bubble dynamics. After reviewing the state of the art in the field, purpose of the talk is discussing a comprehensive model able to provide a unified description of the different phenomenologies described above. The model is based on the capillary Navier-Stokes equation where the liquid-vapor
interface is treated by a diffuse interface model accounting for the relevant thermodynamic properties of the fluid (e.g., equation of state, phase change and latent heat). In order to describe the nucleation phase, a noise term is included (fluctuating hydrodynamics) leading to a system of stochastic partial differential equations with the
unique capability of describing the nucleation of vapor cavities from the liquid in the context of continuum mechanics. Several examples of numerical solutions will be illustrated, including bubble collapse in free-space and near solid walls and their homogenous and heterogeneous nucleation in different geometries. New results
concerning nucleation and bubble dynamics in a flowing liquid will also be presented to finally touch upon the rare event techniques aimed at accurately extracting the cavitation pressure for actual water in a wide range of temperatures.

• Olivia Du Roure - Physique et Mécanique des Milieux Hétérogènes, ESPCI Paris

Dynamics of flexible filaments in microfluidic flows

The dynamics of individual flexible filaments in viscous flows is key to deciphering the rheological behavior of many complex fluids and soft materials. It also underlies a wealth of biophysical processes from flagellar propulsion to intracellular streaming. The interaction between an elongated object and a given flow depends strongly on the properties of the object - flexibility, aspect ratio and dimensions - and the flow geometry. This interaction is governed by the elasto-viscous number  comparing elastic  and  viscous  forces.  During  the  last  years,  we have  studied  different configurations  of  this  problem  by combining  microfluidics,  microfabrication  and  microscopy.  In this seminar, I will focus more on the dynamics of actin filament, used here as a model filament,in shear and pure straining flows and will describe the different morphologies the filament adopts when the elasto-viscous number varies and the flow geometry becomes more complex.

• Anniina Salonen - Laboratoire de Physique des Solides, Université Paris-Saclay

Foamed emulsion ageing: how a viscoelastic emulsion hinders foam coarsening

Foams are gas bubbles in a continuous phase. They are metastable and disappear in time. It is possible to change foam stability through the mechanical properties of the continuous phase, for example using viscoelastic fluids. Indeed foams from viscoelastic fluids can be much more stable, as the different ageing processes slow down or even stop. We want to understand how coarsening of foams is influenced by the viscoelasticity of the continuous phase, a problem important in many applications ranging from foamed food products to aerated construction materials. We use oil in water emulsions as the continuous phase, because this allows us to control the emulsion properties through changing oil volume fraction and drop interactions. We work on pseudo-2D foams, where individual bubble structure and dynamics can be followed. We show that the mechanical properties of the continuous phase strongly influence the ripening of foamed emulsions.  The bubble growth rate slows down and the foam cannot relax to its usual equilibrium structure. The complex interplay between the foam and the emulsion gives rise to unusual coarsening phenomena, which still await a complete description.

• Alessandro Siria - Micromégas, Ecole Normale Supérieure

Non linear ionic transport in angstrom-scale channel

Over the past decade, the ability to reduce the dimensions of fluidic devices to the nanometre scale (by using nanotubes or nanopores, for example) has led to the discovery of unexpected water- and ion-transport phenomena. More recently, van der Waals assembly of two-dimensional materials has allowed the creation of artificial channels with ångström-scale precision. Such channels push fluid confinement to the molecular scale, wherein the limits of continuum transport equations are challenged. Water films on this scale can rearrange into one or two layers with strongly suppressed dielectric permittivity or form a room-temperature ice phase. Ionic motion in such confined channels is affected by direct interactions between the channel walls and the hydration shells of the ions, and water transport becomes strongly dependent on the channel wall material. We explore how water and ionic transport are coupled in such confinement. The transport, driven by pressure and by an applied electric field, reveals a transistor-like electrohydrodynamic effect.
Further we will discuss how this novel fabrication techinque can lead to the creation of fluidic channel presenting highly non linear ionic transport leadice to memory like behaviours and paving the way for complex iontronic functionalities.

• Elie Hachem - CEMEF, MINES-ParisTech

Coupling Machine Learning and Fluid Mechanics: prediction and optimisation

The availability of accurate and efficient numerical simulation tools has become of utmost importance for the design and optimization phases of existing industrial processes. The latter requires the computation of multiple physical fields governed by coupled systems of partial differential equations and tends to require large computational resources. Recently, the coupling of machine learning techniques with numerical simulation tools has allowed lifting part of this computational problem, by mimicking the resolution process with pre-trained neural networks, which execution cost is far less than their traditional counterparts. In this talk, we focus on two particular approaches in this field, Deep Learning and Deep Reinforcement Learning, in which neural networks are used in the context of either prediction of a numerical solution related to a specific fluid mechanics problem or even more, of decision-making problems for optimization and control. Several illustrations and applications will be given to illustrate the utility of such coupling between Machine Learning and Computational Fluid Dynamics.

• Etienne Reyssat - Physique et Mécanique des Milieux Hétérogènes, ESPCI Paris

Thin sticky films

The bonding of two solid objects commonly involves the contact of opposing surfaces coated with a layer of adhesive. The dynamics of merging of liquid layers partly determines the quality of adhesion. As both layers of viscous fluid are brought in contact, liquid bridges form and grow, finally leading to complete adhesion. Upon propagation, the adhesive front develops an original fingering instability. I will describe the dynamics of such a front and the characteristics of the associated fingering pattern. I will discuss a series of minimal model experiments to explore some of the physical mechanisms involved in this complex process.