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)

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

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

Determination of chemical irritation potential using a defined gene signature set on tissue-engineered human skin equivalents

There are no physical or visual manifestations that define skin sensitivity or irritation; a subjective diagnosis is made based on the evaluation of clinical presentations including burning, prickling, erythema and itching. Adverse skin reaction in response to topically applied products is common and can limit the use of dermatological or cosmetic products. The purpose of this study was to evaluate the use of human skin equivalents (HSE) based on immortalised skin keratinocytes and evaluate the potential of a 22-gene panel in combination with multivariate analysis to discriminate between chemicals known to act as irritants and those that do not. Test compounds were applied topically to full thickness HSE or human ex vivo skin and gene signatures determined for known irritants and non-irritants. Principle component analysis showed the discriminatory potential of the 22-gene panel. Linear discrimination analysis, performed to further refine the gene set for a more high-throughput analysis, identified a putative seven-gene panel (IL-6, PTGS2, ATF3, TRPV3, MAP3K8, HMGB2 and MMP-3) that could distinguish potential irritants from non-irritants. These data offer promise as an in vitro prediction tool, although analysis of a large chemical test set is required to further evaluate the system

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