Research
Particles in a thin film
Adding solid particles to foams greatly enhances their stability. The grains become embedded in the liquid films that separate the bubbles, giving them remarkable properties. Grain-filled films refine more slowly (better resistance to drainage), are less permeable to gas (which limits foam ageing) and are much more resistant to mechanical stress. While our understanding of the macroscopic properties of grain-filled films is growing, we still need to better characterize the interactions taking place at the scale of a few particles.
The aim of this work is to gain a better understanding of how grains locally deform soap films, interact with each other and with the film (combining dynamic, wetting and capillary aspects) and modify bubble rheology. The originality of the approach lies in the use of magnetic solicitations to control displacement and forces between particles.
Interfacial instabilities
This work is inspired by the japanese art of suminagashi, which consists in painting at the water surface using soap and ink. We evidenced that when the right conditions are met, the patch of ink initially floating on the bath surface spontaneously destabilize from the center and spreads, forming in the process an intricate pattern an intricate pattern, almost fractal. We determined the conditions of its apparition and the spreading of the ink filaments. We are currently characterizing the flow that leads to this pattern, and model the hydrodynamic instability at its origin.
Non-wetting dynamics
Complex Fluid flows
With Annie Colin (from CBI lab, ESPCI), we are working on the characterization of complex fluids using a novel normal stress sensor, that we developped.
We noticed a lack of local measurements in the study of complex fluid flow, which hinders a better understanding and characterization of these fluids. To address this issue, I have developed a new type of sensor that can be placed on the plate of a conventional rheometer, enabling the mapping of normal forces (perpendicular to the flow direction) at 25 measurement points, each measuring 4.5×4.5 mm² (Figure a). I used the unique properties of a dielectric material previously developed in the laboratory, which forms the core of the sensor array, and developed an electronic data acquisition system.
After validating this new tool with well-known fluids such as polymer solutions, we studied the flow of concentrated suspensions of grains, such as cornstarch.
We evidence in cornstarch a unique, stable heterogeneous structure, which appears in the shear-thickening region of the flow curve (Figure b and c). This structure moves in the velocity direction and does not appear in calcium carbonate. We show that its nature changes from a stress wave to a rolling solid jammed aggregate, at high solid fraction and small gap width. The modeling of these heterogenities points to an adhesive force between cornstarch particles at high stress, also evidenced in microscopic measurements. Cornstarch being also attractive at low stress, it stands out of the classical shear-thickening frame, and might be part of a larger family of adhesive and attractive shear-thickening fluids.
This work is published in Journal of Rheology (description and characterization of the sensor on polymer flows) and in Journal of Colloid and Interface Science (shear-thickening of cornstarch)
Active particles at interfaces
At the University of Twente (Netherlands), I developed a novel experiment investigating the behavior of particles deposited on the surface of a liquid nitrogen bath. I demonstrated that these particles exhibit spontaneous self-propulsion, sliding in straight lines on the bath at speeds of a few centimeters per second, while forming remarkably regular trajectories (Figure a). Through numerical simulations and a theoretical model, I showed that this unexpected behavior arises from the development of a micrometer-scale instability at the surface of the bath. The particles are not only suspended by the vapor escaping from the bath but also propelled by this vapor in a preferred direction. The presence of a liquid interface has another consequence: when two sliders approach each other, they spontaneously orbit one another (Figure b). This motion is caused by capillary attraction between the particles, which occurs here (uniquely) in the almost complete absence of friction due to the evaporation of the bath. I directly measured the capillary interaction potential, a first since its theoretical prediction. As this potential differs significantly from the 1/r gravitational potential, the trajectories of these "millimeter-sized planets" fundamentally differ from classical conics, as I experimentally, theoretically, and through simulations demonstrated. These findings have been published in three papers: PNAS (self-propulsion mechanism), Nature Communications (capillary orbits), and Soft Matter (control of droplet motion by menisci).
Non-wetting dynamics (PhD thesis)
During my PhD, I focused on situations where the hydrophobicity of a surface is enhanced by leveraging motion - either using the droplet impact on a surface (hydrophobic dynamics) or the motion of the surface itself (dynamic hydrophobicity).
I investigated the dynamics of droplet rebound on hydrophobic (macro-)textured surfaces using a wire or a sphere with a size of around a hundred micrometers. I demonstrated that the macrotexture sculpts the liquid during spreading (Figure a), resulting in a stepwise reduction of the droplet's contact time with the surface, reaching a factor of 2. Based on the droplet's shape, we proposed a scaling law that precisely predicts this reduction in contact time: for example, for a wire, the contact time is reduced by a square root of two (for two lobes) or square root of four (= 2) (for four lobes). This work was published in Nature Communications (wire) and Soft Matter (spheres).
I also observed the effect of the motion of an horizontal surface on approaching liquids. I showed that droplets, small solid objects (Figure b), and rain (Figure c) can be repelled by the boundary layer of air induced by the motion of the solid surface. In particular, I studied the stability and motion of these objects, which result from a balance between lubrication pressure and inertial pressure. Moving surfaces are thus capable of repelling all liquids indiscriminately, even viscous or wetting ones, imparting them with omniphobic characteristics. This study resulted in two publications in Physical Review Fluids.
