Stability of clay particle-coated microbubbles in alkanes against dissolution induced by heating

We investigated the dissolution and morphological dynamics of air bubbles in alkanes stabilized by fluorinated colloidal clay particles when subjected to temperature changes. A quasi-steady model for bubble dissolution with time-dependent temperature reveals that increasing the temperature enhances the bubble dissolution rate in alkanes, opposite to the behavior in water, due to the differing trends in gas solubility. Experimental results for uncoated air bubbles in decane and hexadecane confirm this prediction. Clay-coated bubbles in decane and hexadecane are shown to be stable in air-saturated oil at constant temperature, where dissolution is driven mainly by the Laplace pressure. When the temperature increases from ambient, the particle-coated bubbles are prone to dissolution as the oil phase becomes under-saturated. The interfacial layer of particles is observed to undergo buckling and crumpling, without shedding of clay particles. Increasing the concentration of particles is shown to enhance the bubble stability by providing a higher resistance to dissolution and buckling. When subjected to complex temperature cycles, the clay-coated bubbles can remain stable in conditions for which uncoated bubbles dissolve completely. These results underpin the design of ultra-stable oil foams stabilized by solid particles with improved shelf life under changing environmental conditions

Droplets in Microchannels: Dynamical Properties of the Lubrication Film

International audienceWe study the motion of droplets in a confined, micrometric geometry, by focusing on the lubrication film between droplet and wall. When capillary forces dominate, the lubrication film thickness evolves non linearly with the capillary number due to viscous dissipation between meniscus and wall. However, this film may become thin enough (tens of nanometers) that intermolecular forces come into play and affect classical scalings. Our experiments yield highly resolved topographies of the shape of the interface and allow us to bring new insights into droplet dynamics in microfluidics. We report the novel characterization of two dynamical regimes as the capillary number increases: (i) at low capillary numbers, the film thickness is constant and set by the disjoinging pressure, while (ii) above a critical capillary number, the interface behavior is well described by a viscous scenario. At a high surfactant concentration, structural effects lead to the formation of patterns on the interface , which can be used to trace the interface velocity that yield direct confirmation of boundary condition in viscous regime. The dynamics of a droplet confined between solid walls and pushed by a surrounding liquid is an old problem, however recent theories are still being developed to describe unexplored regimes and experimental characterizations are still lacking to shed light on these novel developments. A complete understanding of the droplet velocity calls for accurate knowledge of the dissipation mechanisms involved, particularly in the lubrication film. Our understanding of the lubrication properties of menisci travelling in confined geometries has been steadily refined since the pioneering work of Taylor & Saffman [1]. Notably, the influence of the lubrication film left along the wall by the moving meniscus was first taken into account by Bretherton, who investigated the motion of an inviscid bubble in a cylindrical tube [2]. Far from the meniscus, this dynamical film reaches a uniform thickness h ∞ , related to the bubble velocity through the capillary number Ca = µ f U d /γ, where U d is the bubble velocity , µ f the viscosity of the continuous phase, and γ the surface tension. When the capillary pressure dominates over the viscous stress, i.e. in the regime where Ca 1, the thickness of the film follows h Breth = 1.34 r Ca 2/3 , where r is the radius of the capillary tube. Besides bubbles , the case of viscous droplets remains however largely unexplored. A recent theoretical advance in the field by Hodges et al. [3] shows by numerical calculations of the whole flow pattern that significant corrections in the thickness of lubrication films can arise at very low Ca. Furthermore, the regime of the Bretherton theory is only valid for lubrication films thicker than the molecular sizes or than the range of interfacial interactions. The typical velocities and lengthscales involved in droplet-based microfluidics would lead to lubrication films h ∞ ∼

Migration de gouttes en microfluidique : caractérisation et applications

Digital microfluidics is a growing field of research. However, droplet dynamics remains largely unknown. As an example, a question as simple as predicting the droplet velocity while pushed by an external fluid at fixed velocity is still not answered. Understanding and thus modelizing it requires the identification of dissipation mechanisms in the droplet, in the dynamical meniscus and in the flat film. This thesis presents a study on the dynamical properties of lubrication films using an interferometric method (RICM) that has been adapted to microfluidics. We first show that, in a static case, we are able to measure nanometric film thicknesses with very accurate precision and that it is set by the disjoining pressure, especially the electrostatic part. Then the film is studied when the droplets flow. At low speeds, the film thickness is set by the disjoining pressure, while at higher capilarry numbers we identify a viscous model in agreement with our experimental results. For a micellar solution, we observe spinodal decomposition allowing us to recover interfacial properties (velocity, Marangoni stress). Finally, in a collaborative project, we were able to develop a lab on a chip allowing droplets manipulations taking advantage of micro-heaters integration.La microfluidique utilisant les gouttes a connu un essor remarquable ces dix dernières années. Pourtant, la dynamique de ces objets reste largement inexplorée et incomprise. Une question aussi simple que déterminer la vitesse d’une goutte poussée par une phase porteuse à vitesse imposée, ne possède à ce jour toujours pas de réponse. Comprendre et modéliser la vitesse d’une goutte nécessite dans un premier temps de caractériser les mécanismes de dissipation intervenant dans la goutte, dans le ménisque dynamique et dans le film de lubrification.Ce manuscrit présente une étude de la dynamique de films de lubrification en utilisant une technique interférométrique (RICM) qui a dû être adaptée à notre système. Nous montrons dans un premier temps que, dans un cas statique, cette outil permet de mesurer l’épaisseur de ce film nanométrique avec une très grande précision, et que celle-ci est fixée par la pression de disjonction dont la composante principale est électrostatique. Puis, le film de lubrification est étudié dans un cas dynamique. Aux faibles vitesses, l’épaisseur du film est fixée par la pression de disjonction, tandis qu’aux nombres capillaires plus élevés nous montrons qu’un modèle visqueux permet de reproduire nos résultats expérimentaux. Pour une solution micellaire,nous observons une décomposition spinodale permettant d’extraire des propriétés interfaciales (vitesse, contrainte de Marangoni). Enfin, nous avons pu dans un projet fédératif développer une laboratoire sur puce permettant des opérations de manipulation sur des gouttes en utilisant l’intégration de systèmes de chauffage au niveau micrométrique

Microfluidics droplet migration : characterization and applications

La microfluidique utilisant les gouttes a connu un essor remarquable ces dix dernières années. Pourtant, la dynamique de ces objets reste largement inexplorée et incomprise. Une question aussi simple que déterminer la vitesse d’une goutte poussée par une phase porteuse à vitesse imposée, ne possède à ce jour toujours pas de réponse. Comprendre et modéliser la vitesse d’une goutte nécessite dans un premier temps de caractériser les mécanismes de dissipation intervenant dans la goutte, dans le ménisque dynamique et dans le film de lubrification.Ce manuscrit présente une étude de la dynamique de films de lubrification en utilisant une technique interférométrique (RICM) qui a dû être adaptée à notre système. Nous montrons dans un premier temps que, dans un cas statique, cette outil permet de mesurer l’épaisseur de ce film nanométrique avec une très grande précision, et que celle-ci est fixée par la pression de disjonction dont la composante principale est électrostatique. Puis, le film de lubrification est étudié dans un cas dynamique. Aux faibles vitesses, l’épaisseur du film est fixée par la pression de disjonction, tandis qu’aux nombres capillaires plus élevés nous montrons qu’un modèle visqueux permet de reproduire nos résultats expérimentaux. Pour une solution micellaire,nous observons une décomposition spinodale permettant d’extraire des propriétés interfaciales (vitesse, contrainte de Marangoni). Enfin, nous avons pu dans un projet fédératif développer une laboratoire sur puce permettant des opérations de manipulation sur des gouttes en utilisant l’intégration de systèmes de chauffage au niveau micrométrique.Digital microfluidics is a growing field of research. However, droplet dynamics remains largely unknown. As an example, a question as simple as predicting the droplet velocity while pushed by an external fluid at fixed velocity is still not answered. Understanding and thus modelizing it requires the identification of dissipation mechanisms in the droplet, in the dynamical meniscus and in the flat film. This thesis presents a study on the dynamical properties of lubrication films using an interferometric method (RICM) that has been adapted to microfluidics. We first show that, in a static case, we are able to measure nanometric film thicknesses with very accurate precision and that it is set by the disjoining pressure, especially the electrostatic part. Then the film is studied when the droplets flow. At low speeds, the film thickness is set by the disjoining pressure, while at higher capilarry numbers we identify a viscous model in agreement with our experimental results. For a micellar solution, we observe spinodal decomposition allowing us to recover interfacial properties (velocity, Marangoni stress). Finally, in a collaborative project, we were able to develop a lab on a chip allowing droplets manipulations taking advantage of micro-heaters integration

Freezing a rivulet

International audienceWe investigate experimentally the formation of the particular ice structure obtained when a capillary trickle of water flows on a cold substrate. We show that after a few minutes the water ends up flowing on a tiny ice wall whose shape is permanent. We characterize and understand quantitatively the formation dynamics and the final thickness of this ice structure. In particular, we identify two growth regimes. First, a 1D solidification diffusive regime, where ice is building independently of the flowing water. And second, once the ice is thick enough, the heat flux in the water comes into play, breaking the 1D symmetry of the problem, and the ice ends up thickening linearly downward. This linear pattern is explained by considering the confinement of the thermal boundary layer in the water by the free surface. The partial freezing of Niagara falls and the cancellation of thousands of flights during the cold snap of winter 2019 are a few examples of the disturbances caused by extreme weather events. Indeed, the accretion of ice on superstructures such as planes [1-3], power-lines [4], bridge cables [5] or wind turbines [6] can have dramatic consequences. Nowadays, the main strategy to prevent most of these undesirable effects is to develop anti-icing surfaces [7, 8], but new paradigms could emerge from a better understanding of the freezing dynamics in complex configurations. When water flows on a cold surface for example, the resulting ice structure is reminiscent from the manifold interaction between the heat transport and the flow [9]. The presence of a free-surface is also determinant in these problems, resulting in the apparition of a tip on frozen sessile drops [10], or in the explosion of droplets cooled from the outside-in [11] in static conditions. Freezing of capillary flows, widely encountered in the previous examples, can consequently reveal a very rich behavior [12-15] as in the formation of icicles [16] or ice structures following drop impact on cold surfaces [17]. In this Letter, we investigate experimentally the freezing of a capillary water river, the so-called rivulet [18-20] (see Fig. 1), flowing over a cold substrate. We show for the first time that the growing ice structure reaches a static shape after few minutes. The water then flows on a tiny ice wall that thickens downward, an observation we quantitatively explain considering the confinement of a thermal boundary layer. These results bring new understanding of the ice crust formation in the presence of streaming water and improve the prediction of its shape. The experiment consists in flowing distilled water dyed with fluorescein at 0.5 g.L −1 along a cold aluminum block of 10 cm long, with an inclination of α = 30 • to the horizontal. The temperature of the injected water T in ranges from 8 to 35 • C, see Fig. 1(a). The water is injected through a needle (inner diameter 1.6 mm) at a flow-rate Q = 20 mL.min −1 , such that there is no meander at room temperature [19]. A straight water rivulet is then formed [18], with a typical width of 2 mm, a thickness of h w = 800 µm, and a characteristic velocity of the buoyant flow u 0 ≈ 10 cm.s −1. As the Reynolds number of the flow is sufficiently small (Re = u 0 h w /ν = 80), the flow is laminar and mass conservation implies that the liquid layer thickness h w is constant [18]. The temperature of the aluminum substrate T s is set by plunging the block in liquid nitrogen for a given amount of time so that it ranges from −9 to −44 • C. T s is measured during the experiment and remains constant (±1 • C). Experiments performed with substrate temperatures below −44 • C consistently lead to the fracture [21] or the self-peeling [22] of the ice and are not considered here. Upon contact with the cold substrate, the water freezes and an ice layer grows while the water continues to flow on top, as shown on the sequence of snapshots of inset in Fig. 1(a) and in the Sup. Mat. movie. During that process, the fluorescein concentrates between the ice dendrites, causing self-quenching and fluorescence dimming in the ice [23]. This allows us to clearly distinguish between the ice and the water layers under UV light. The ice layer thickness h i (x, t) is then measured using a Nikon D800 camera recording from the side at 30 fps. The setup is placed in a humidity control box to avoid frost formation (H r ≈ 5 − 10%). Figure 1(b) presents the ice layer profile along the direction of the flow (x = 0 at the needle) at different times for T in =10 • C and T s =-36 • C. The analysis is restricted to the middle of the plate (x ∈ [1,8] cm) to avoid input and output influences. At early times, the ice layer grows homogeneously along the plane and the successive profiles are parallel to the substrate. After that, the ice layer continues to grow but not uniformly: its thickness increases along the plane. Finally, the ice layer stops growing and the system reaches a permanent regime consisting of a static ice structure, of thickness h max , on top of which a water layer is flowing. The final shape of the ice can be well described by a line of slope β as illustrated by the dashed lines in Fig. 1(b): h max (x) = h i,0 + βx, with β varying in our experiments between 0 and 4 •. Th

Solidification of a rivulet: shape and temperature fields

International audienceThe freezing of a water rivulet begins with a water thread flowing over a very cold surface, is naturally followed by the growth of an ice layer and ends up with a water rivulet flowing on a static thin ice wall. The structure of this final ice layer presents a surprising linear shape that thickens with the distance. This paper presents a theoretical model and experimental characterisation of the ice growth dynamics, the final ice shape and the temperature fields. In a first part, we establish a two-dimensional model, based on the advection–diffusion heat equations, that allows us to predict the shape of the ice structure and the temperature fields in both the water and the ice. Then, we study experimentally the formation of the ice layer and we show that both the transient dynamics and the final shape are well captured by the model. In a last part, we characterise experimentally the temperature fields in the ice and in the water, using an infrared camera. The model shows an excellent agreement with the experimental fields. In particular, it predicts well the linear decrease of the water surface temperature observed along the plane, confirming that the final ice shape is a consequence of the interaction between the thermal boundary layer and the free surface

A Review of Heating and Temperature Control in Microfluidic Systems: Techniques and Applications

This review presents an overview of the different techniques developed over the last decade to regulate the temperature within microfluidic systems. A variety of different approaches has been adopted, from external heating sources to Joule heating, microwaves or the use of lasers to cite just a few examples. The scope of the technical solutions developed to date is impressive and encompasses for instance temperature ramp rates ranging from 0.1 to 2,000 °C/s leading to homogeneous temperatures from −3 °C to 120 °C, and constant gradients from 6 to 40 °C/mm with a fair degree of accuracy. We also examine some recent strategies developed for applications such as digital microfluidics, where integration of a heating source to generate a temperature gradient offers control of a key parameter, without necessarily requiring great accuracy. Conversely, Temperature Gradient Focusing requires high accuracy in order to control both the concentration and separation of charged species. In addition, the Polymerase Chain Reaction requires both accuracy (homogeneous temperature) and integration to carry out demanding heating cycles. The spectrum of applications requiring temperature regulation is growing rapidly with increasingly important implications for the physical, chemical and biotechnological sectors, depending on the relevant heating technique

Frozen rivulet

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Droplet velocity in a micrometric Hele-Shaw Cell

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Dynamics of a 2D droplet in a Hele-Shaw cell (presenter B. Reichert)

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