Invited speakers
- Isabelle Cantat (Univ. Rennes, France) - Flows in soap films
- Cécile Cottin-Bizonne (Univ. Lyon, France) - Active interfaces
- Philippe Coussot (Univ. Gustave Eiffel, France) - How “bound” water controls water transport in plant fibers
- Anne De Wit (Univ. Libre Bruxelles, Belgium) - Chemo-hydrodynamic self-organization around reaction fronts
- Philippe Gondret (Univ. Paris-Saclay, France) - Generation of tsunami waves by landslides
- Detlef Lohse (Univ. Twente, Netherlands) - Freezing of emulsions
- Dimos Poulikakos (ETH Zürich, Switzerland) - Roaming condensation and explosive freezing of droplets
- Olivier Pouliquen (Univ. Aix-Marseille, France) - Flow of sticky particles
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Abstracts
Isabelle Cantat IPR Lab. (Univ. Rennes, CNRS, France) - Flows in soap films
The thinning of the liquid films separating bubbles in a foam or in a bubbly liquid controls the coalescence process and the foam stability, and is highly relevant in many industrial processes. The spatiotemporal evolution of the film thickness is governed by nonlinear equations, the solutions of which are still mostly unknown.
In this talk we will discuss a few examples of flows in the plane of a horizontal foam film, driven by tiny capillary forces. These original flows will allow us to revisit the old problem of the 'marginal regeneration', a peculiar instability occuring between a flat film and a meniscus, which controls the drainage of the film.
Cécile Cottin-Bizonne, ILM Lab. (Univ. Lyon, CNRS, France) - Active interfaces
Active matter systems, comprised of self-propelled particles, exhibit rather intriguing dynamic properties, which have attracted considerable attention in recent years. In this study, we focus on active interfaces by considering a sediment of self-propelled Janus colloids. At low densities, they behave like a perfect hot gas, but at intermediate densities, we observe new collective phenomena, such as the formation of clusters. This leads us to question whether wetting-type effects occur in these active fluids. In this context, we investigate an analogy to the classical capillary rise effect in the realm of active matter. Specifically, we examine how a non-phase separated sediment of self-propelled Janus colloids behaves when in contact with a vertical wall.
Philippe Coussot, Navier Lab. (Ecoles des Ponts-ParisTech, CNRS, Université Gustave Eiffel, France) - How “bound” water controls water transport in plant fibers
How does water penetrate wood and make it swell? Why do we feel more comfortable in cotton, flax, or hemp clothes? Why is it so long to dry wood or paper? Why is it healthier to live in a house made of biobased walls?
At the origin of these phenomena is the remarkable ability of these materials to absorb liquid water or vapor from the environment thanks to hydrogen bonds, and thus to form nanoscale water inclusions between cellulose microfibrils. This so-called “bound water” can represent up to 30% of the dry mass of the material; it evaporates in a dry ambient air, thus regulating the ambient humidity; and it induces the swelling or shrinkage of these materials in proportion to its amount. Another remarkable property of bound water is that it can diffuse rather rapidly inside cellulose fibers.
The transport of bound water and its exchanges with the (standard) liquid and vapor phases lead to complex and original fluid transport properties in cellulosic materials or plant fibers. For example, the spontaneous imbibition of water in wood is not governed by capillary effects but by a bound water diffusion ahead of the liquid front, which slows down the dynamics by several orders of magnitude of time [1]. For its part, the drying of wood is controlled by the bound water diffusion which extracts free water in depth in the sample, again leading to a (slow) two-step diffusion process [2-3].
The observation and quantification of these phenomena require specific techniques or approaches, NMR and MRI being very helpful as they allow to detect molecular water amounts inside solids [4]. We will show how one can determine the transport diffusion coefficient of bound water in a wood piece, in a fiber network (the bound water jumping from one fiber to another in contact with it), and even along a single cellulosic fiber [5]. We will also show how further tests allow to determine specifically the vapor diffusion coefficient through the porosity, which finally makes it possible to build a general diffusion model of the transport of water through such systems under hygroscopic conditions.
[1] Zhou et al., Physical Review Research, 1, 033190 (2019)
[2] Penvern et al., Physical Review Applied, 14, 054051 (2020)
[3] Cocusse et al., Science Advances, 8, eabm7830 (2022)
[4] Maillet et al., Langmuir, 38, 15009−15025 (2022)
[5] Zou et al., Cellulose, 30, 7463–7478 (2023)
Anne De Wit (Univ. Libre Bruxelles, Belgium) - Chemo-hydrodynamic self-organization around reaction fronts
The interplay between reactions and diffusion can generate traveling reaction fronts in which the chemical reaction takes place in a localized zone, when the reactants are initially separated in space. Across such fronts, composition gradients car trigger density, viscosity or permeability gradients at the source of hydrodynamic motions. In the talk, we will review the properties of the reaction-diffusion-convection dynamics taking place around such traveling fronts, emphasing the new self-organizing patterns that can emerge due to combined specificities of reaction-diffusion and hydrodynamic instabilities.
Philippe Gondret, FAST Lab. (Univ. Paris-Saclay, CNRS, France) - Generation of tsunami waves by landslides
Tsunami waves can be generated by earthquakes but also by landslides such as by the recent partial flank collapse of Anak Krakatau (Indonesia) in 2018. To improve the prediction of these dangerous waves, we have considered the gravity driven collapse of a granular column into water in a quasi-two-dimensional setup and systematically investigated the influence of the initial geometry of the column and the water depth on the impulse wave generated. Our experiments reveal three nonlinear wave regimes: transient bores and solitary waves for shallow water, and ``Cauchy-Poisson" waves for deep water.
By coupling the dynamics of the column collapse to the wave generation in shallow water, we develop a model that allows us to estimate the amplitude of the generated wave either to the initial conditions or to the final immersed volume of grains. As a result, the tsunami wave generated by a landslide can be estimated from the knowledge of the pre-landslide geometry and bathymetry in a preventive approach, or from the final immersed deposit of a past geophysical event. The present modeling contributes to a better understanding of such events with complex granular and fluid flows.
Detlef Lohse Physics of Fluids Group (Univ. Twente, Netherlands) - Freezing of emulsions
An immersed soft particle or oil droplet is severely deformed when engulfed into an advancing ice front. This deformation strongly depends on the engulfment velocity, even forming pointy-tip shapes for low velocities. We found that such singular deformations are mediated by interfacial flows in nanometric thin liquid films separating the nonsolidifying dispersed soft particles or droplets and the solidifying bulk. The competing forces in the thin film originate from the disjoining pressure and the surface tension gradient (Marangoni forces). We analytically modelled the fluid flow in these intervening thin films, using a lubrication approximation in the boundary layers. In an exact analytical calculation and with a formal analogy to a nonlinear pendulum, we then related the fluid flow to the deformation sustained by the dispersed droplet. We find it astounding that the nanoscopic interaction (van der Waals forces, disjoining pressure) determines the shape of the macroscopic immersed soft particle or droplet.
We then extended this line of research to the interaction of several immersed soft particles or droplets over which a solidification front is passing. This time it is the relative thermal conductivity of the soft particles and the liquid which determines whether the two soft particles repel or attract. We call the effect the frozen Cheerios effect.
Finally, we identified a freezing-induced topological transition of a double-emulsion, i.e., an oil droplet with an immersed water droplet inside, and as a whole immersed in water, passing through a freezing front. Whether the water droplet inside the oil droplet survives or whether it literally bursts due to pressure forces emerging at solidification depends on the control parameters, in particular the freezing front velocity.
This is joint work with Jochem Meijer, Pallav Kant, Vincent Bertin, and Duco van Buuren, all Physic of Fluids group, University of Twente.
Dimos Poulikakos (ETH Zürich, Switzerland) - Roaming condensation and explosive freezing of droplets
In this lecture I will discuss the peculiar behavior of droplets on superhydrophobic surfaces, focusing on two very common phase change phenomena in nature and technology, namely, condensation and freezing. When condensate microdroplets coalesce, they can spontaneously propel themselves omnidirectionally on the surface independent of gravity, and grow by feeding from droplets they sweep along the way. I will explain the physics behind this roaming phenomenon of coalescing condensate microdroplets on solely nanostructured superhydrophobic surfaces, where the droplets are orders of magnitude larger than the underlaying surface nanotexture. This phenomenon is then utilized to prevent condensate flooding of the surface, impressively improving heat transfer. Going down in temperature, I will discussthe fascinating phenomen of explosive vaporization, manifesting itself during the freezing of supercooled droplets at low environmental pressures. I will then present surface topographydesigns harvesting the natural occurrence of this phenomenon to cause spontaneous ice expulsion from superhydrophobic surfaces in a fully passive manner, inherent to the surface design.
Olivier Pouliquen IUSTI Lab. (Aix-Marseille University, CNRS, Marseille, France) - Flow of sticky particles
Characterization and prediction of the `flowability' of powders are of paramount importance in many industries. However, our understanding of the flow of powders is sparse compared to the flow of coarse granular media. The main difficulty arises from the presence of adhesive forces between the grains, which prevent smooth and continuous flows. In this talk, we will present the results of both discrete simulations and experiments on model cohesive granular materials made of sticking grains, revealing new behaviors compared to simple dry granular materials.
