Capillary imbibition of aqueous foams by miscible and nonmiscible liquids

International audienceWhen put in contact with a large liquid drop, dry foams wick owing to surface-tension-driven flows until reaching equilibrium. This work is devoted to the dynamics of this imbibition process. We both consider imbibition of wetting or non-wetting liquid, either by putting the dry foam into contact with the foaming solution that constitutes the foam or with organic oils. Indeed, with the appropriate choice of surfactants, oil spontaneously invades the liquid network of the foam without damaging it. Our experiments show an early-time dynamics in t 1/2 followed by a late-time dynamics in t 1/4. These features, which differ from theoretical works predicting a late-time t 1/3 dynamics, are rationalized considering the influence of the initial liquid fraction of the foam in the driving capillary force and the impact of gravity through the capillary-gravity equilibrium

Coalescence of dry foam under water injection

International audienceWhen a small volume of pure water – typically a drop – is injected within an aqueous foam, it locally triggersthe rupture of foam films, thus opening an empty cavity in the foam's bulk. We consider the final shape ofthis cavity and we quantify its volume as a function of the volume of injected water, the diameter of thebubbles and the liquid fraction of the foam. We provide quantitative understanding to explain how andwhen this cavity appears. We epitomize the dilution of surfactants at the water–air interfaces as the maincause lying behind the coalescence process. We identify a new coalescence regime for which a criticalsurfactant concentration rules the stability of the foam

Magnetic control of Leidenfrost drops

International audienceWe show how a magnetic field can influence the motion of a paramagnetic drop made of liquid oxygen in a Leidenfrost state on solids at room temperature. It is demonstrated that the trajectory can be modified in both direction and velocity and that the results can be interpreted in terms of classical mechanics as long as the drop does not get too close to the magnet. We study the deviation and report that it can easily overcome 180 degrees and even diverge under certain conditions, leading to situations where a drop gets captured. In the vicinity of the magnet, another type of trapping is observed, due to the deformation of the drop in this region, which leads to a strong energy dissipation. Conversely, drops can be accelerated by moving magnets (slingshot effect)

Role of evaporation rate on the particle organization and crack patterns obtained by drying a colloidal layer

International audience– A scientific hurdle in manufacturing solid films by drying colloidal layers is preventing them from fracturing. This paper examines how the drying rate of colloidal liquids influences the particle packing at the nanoscale in correlation with the crack patterns observed at the macroscale. Increasing the drying rate results in more ordered, denser solid structures, and the dried samples have more cracks.Yet, introducing a holding period (at a prescribed point) during the drying protocol results in a more disordered solid structure with significantly less cracks. To interpret these observations, this paper conjectures that a longer drying protocol favors the formation of aggregates. It is further argued that the number and size of the aggregates increase as the drying rate decreases. This results in the formation of a more disordered, porous film from the viewpoint of the particle packing, and a more resistant film, i.e. less cracks, from the macroscale viewpoint

Reshaping and Capturing Leidenfrost drops with a magnet

Liquid oxygen, which is paramagnetic, also undergoes Leidenfrost effect at room temperature. In this article, we first study the deformation of oxygen drops in a magnetic field and show that it can be described via an effective capillary length, which includes the magnetic force. In a second part, we describe how these ultra-mobile drops passing above a magnet significantly slow down and can even be trapped. The critical velocity below which a drop is captured is determined from the deformation induced by the field.Comment: Published in Physics of Fluids (vol. 25, 032108, 2013) http://pof.aip.org/resource/1/phfle6/v25/i3/p032108_s1?isAuthorized=n

Dynamiques spéciales de gouttes non-mouillantes

This work, based on various experiments, tackles the dynamics of non-wetting drops in situations where gravity does not play a role, but other " special " forces are involved. In the first part, we look at drops of liquid oxygen, which undergo Leidenfrost effect on a substrate at room temperature. These drops are also susceptible to the presence of a magnetic field. We study the force exerted by a magnet on those ultra-mobile drops and we show that they can be deflected, slowed down, deformed, captured and even accelerated by the presence of a magnetic field. In the second part, we study a reverse situation, where the goal is to propel a non-wetting oil drop that is initially at rest. The drop is in a capillary tube and we show that a gradient in surfactant concentration leads to a spontaneous movement and allows the drop to escape from the tube. This experiment can be considered as a model situation of detergency. Non-trivial dynamics has been identified in this system : the movement is either continuous or intermittent, depending on the experimental parameters.Dans cette thèse, nous étudions à l'aide de plusieurs expériences la dynamique de gouttes non-mouillantes dans des situations où la gravité n'intervient pas, mais où d'autres forces, moins communes, sont à l'oeuvre. La première partie porte sur l'étude de gouttes d'oxygène liquide qui, en plus d'être en caléfaction sur un support à température ambiante, ont la particularité d'être susceptibles à la présence d'un champ magnétique. Nous étudions la force magnétique exercée sur ces gouttes ultra-mobiles et nous montrons qu'elles peuvent être déviées, ralenties, déformées, capturées et même parfois accélérées à l'aide d'un aimant. Dans la deuxième partie de ce travail, nous avons étudié une situation inverse, où nous avons cherché à mettre en mouvement une goutte non-mouillante initialement au repos. La goutte est cette fois faite d'huile se trouvant dans un tube capillaire rempli d'eau, et nous avons montré qu'un gradient de concentration en tensioactif provoque un mouvement spontané et permet à la goutte d'huile de s'échapper du tube. Cette expérience réalise ainsi une situation modèle de détergence. Une dynamique très particulière est mise en évidence à temps long : le mouvement est continu ou intermittent selon les paramètres de l'expérience

Dynamiques spéciales de gouttes non-mouillantes

Dans cette thèse, nous étudions à l'aide de plusieurs expériences la dynamique de gouttes non-mouillantes dans des situations où la gravité n'intervient pas, mais où d'autres forces, moins communes, sont à l'oeuvre. La première partie porte sur l'étude de gouttes d'oxygène liquide qui, en plus d'être en caléfaction sur un support à température ambiante, ont la particularité d'être susceptibles à la présence d'un champ magnétique. Nous étudions la force magnétique exercée sur ces gouttes ultra-mobiles et nous montrons qu'elles peuvent être déviées, ralenties, déformées, capturées et même parfois accélérées à l'aide d'un aimant. Dans la deuxième partie de ce travail, nous avons étudié une situation inverse, où nous avons cherché à mettre en mouvement une goutte non-mouillante initialement au repos. La goutte est cette fois faite d'huile se trouvant dans un tube capillaire rempli d'eau, et nous avons montré qu'un gradient de concentration en tensioactif provoque un mouvement spontané et permet à la goutte d'huile de s'échapper du tube. Cette expérience réalise ainsi une situation modèle de détergence. Une dynamique très particulière est mise en évidence à temps long : le mouvement est continu ou intermittent selon les paramètres de l'expérienceThis work, based on various experiments, tackles the dynamics of non-wetting drops in situations where gravity does not play a role, but other " special " forces are involved. In the first part, we look at drops of liquid oxygen, which undergo Leidenfrost effect on a substrate at room temperature. These drops are also susceptible to the presence of a magnetic field. We study the force exerted by a magnet on those ultra-mobile drops and we show that they can be deflected, slowed down, deformed, captured and even accelerated by the presence of a magnetic field. In the second part, we study a reverse situation, where the goal is to propel a non-wetting oil drop that is initially at rest. The drop is in a capillary tube and we show that a gradient in surfactant concentration leads to a spontaneous movement and allows the drop to escape from the tube. This experiment can be considered as a model situation of detergency. Non-trivial dynamics has been identified in this system : the movement is either continuous or intermittent, depending on the experimental parameterPALAISEAU-Polytechnique (914772301) / SudocSudocFranceF

Capillary Flow of Oil in a Single Foam Microchannel

International audienceWhen using appropriate surfactants, oil and aqueous foam can be intimately mixed without the foam being destroyed. In this Letter, we show that a foam, initially free of oil, can draw an oil drop under the action of capillary forces and stretch it through the aqueous network. We focus on the suction of oil by a single horizontal foam channel, known as a Plateau border. In such confined channels, imbibition dynamics are governed by a balance between capillarity and viscosity. Yet, the scaling law for our system differs from that of classical imbibition in porous media such as aqueous foam. This is due to the particular geometry of the liquid channels: Plateau borders filled with foaming solution are always concave whereas they can be convex or flat when filled with oil. Finally, the oil slug, confined in the Plateau border, fragments into droplets following a film breakup

Detergency in a tube

International audienceWe study the motion of a drop of oil inside a capillary tube induced by a gradient of surfactant concentration. We first show that the wetting condition selects whether the drop moves towards or away from the surfactant. We then focus on the non-wetting case, for which the drop eventually escapes from the tube, and explore the different dynamics of propulsion. On a long time scale, we observe two regimes: either the drop reaches a constant velocity, or it moves intermittently, by successive jumps and stops. As a paradoxical consequence, viscous drops can be as mobile or even faster than inviscid ones. © 2011 The Royal Society of Chemistry