arXiv subjects

Théophile Gaichies

Publications and source records attributed to Théophile Gaichies.

6 recordsLinked to original sources

arXiv2026

A contaminant-concentration-dependent surface tension does not explain the absence of solutal Marangoni flow in evaporating droplets

Theoretical models of evaporating droplets predict Marangoni flows orders of magnitude faster than those observed experimentally. While this discrepancy is often attributed to surface contamination, the underlying mechanism by which contaminants weaken Marangoni stresses remains unclear. In this study, we compare particle image velocimetry (PIV) experiments with a coupled hydrodynamic and solute transport model to investigate the internal flow of evaporating aqueous droplets containing salt, glycerol, or ethanol. By analyzing both sessile and pendant droplets, we demonstrate that the flow is driven entirely by natural convection, in contrast to theoretical predictions that use surface-tension gradients. Remarkably, in some cases, the experimental surface velocity is found to be directed against the predicted surface-tension gradient. We further prove that commonly used contamination models---whether based on surfactants lowering the surface tension or on surface rheology---cannot account for this flow reversal, as they would require the surfactant to be transported against the flow when the surface velocity is opposed to the surface-tension gradient. Our results show that a reconsideration of these models is required to explain the absence of solutal Marangoni in evaporating droplets.

physics.flu-dyn↗

arXiv2026

Marangoni flow induced by crystallization in saline droplets

In this article, we study the flow induced by crystallization of sodium chloride in sessile droplets. We measure flow velocities on the order of 1 $\mathrm{cm.s^{-1}} during crystallization, two orders of magnitude higher than those reported in previous studies. We show that this discrepancy is due to surface contaminants in earlier experiments. Numerical simulations reveal that in the clean case, the flow is driven by Marangoni stresses, whereas it is density- driven in the contaminated case. Consequently, droplet cleanliness significantly influences the flow which, in turn, determines the growth rate of crystals.

physics.flu-dyn↗

arXiv2026

Air entrainment by an inclined smooth water jet

Air entrainment can occur when a water jet impacts a water/air interface, a process central in various real systems, ranging from dam spills to breaking waves. Despite its prevalence, a comprehensive description of the mechanism controlling bubble size distribution remains elusive. Here, we establish a link between the geometry and the dynamics of the cavity observed when an inclined impinging jet impacts a water interface and the resulting bubble cloud. We show that the bubbles result from the destabilization of the wavefield developing at the interface of the cavity. The origin of this wave field is the creation of a shear layer, due to the asymmetric detachment of the flow field from the interface.

physics.flu-dyn↗

arXiv2025

Capillary jet in a slab of foam

A capillary jet plunging into a quasi-2D slab of monodisperse foam of the same solution is studied experimentally. We show that the jet can have a drastic impact on the foam. At small speeds it inflates the channels separating the bubbles. At intermediate speeds, impact with channels creates very small bubbles. At higher speeds, the jet thins the foam films, which can result in bubble rupture. We study the mechanisms behind these processes using an elementary foam formed by three soap films in contact through a Plateau border. We obtain scaling laws, which transfer quantitatively from the elementary foam to the 2D foam.

cond-mat.soft↗

arXiv2023

Effective water/water contact angle at the base of an impinging jet

The base of a jet impinging on an ultrapure water bath is studied experimentally. At the impact point, a train of capillary waves develops along the jet. By performing Particle Tracking Velocity measurements, we show that there is a boundary layer separation between the jet and the meniscus. We thus describe the shape of this meniscus with a hydrostatic model. A striking observation is the existence of an effective non-zero water/water contact angle between the jet and the meniscus. The rationalization of this finite contact angle requires a full description of the shape of the interface. By doing an analytical matching between the meniscus and the jet, we show that the capillary waves can be considered as reflected waves present to ensure pressure continuity. It is finally shown that the value of the apparent contact angle is fixed by energy minimization, with an excellent agreement between prediction and experiment for small jets.

physics.flu-dyn↗

arXiv2023

Mechanical activation of reversible bonds by low amplitude high frequencies excitations

Reversible covalent or supramolecular bonds play an important role in materials science and in biological systems. The equilibrium between open and closed bonds and the association rate can be controlled thermally, chemically, by mechanical pulling, ultrasound or catalysts. In practice, these intrinsic equilibrium methods either suffer from a limited range of tunability or may damage the system. Here, we present a non-equilibrium strategy that exploits the dissipative properties of the system to control and change the dynamic properties of sacrificial and reversible networks. We show theoretically and numerically how high-frequency mechanical oscillations of very low amplitude can open or close bonds. This mechanism indicates how reversible bonds could alleviate mechanical fatigue of materials especially at low temperatures where they are fragile. In another area, it suggests that the system can be actively modified by the application of ultrasound to induce gel-fluid transitions and to activate or deactivate adhesion properties.

cond-mat.soft↗