The removal of ultrathin amorphous polymer films in contact with nonsolvent/solvent binary mixtures is addressed by means of neutron reflectometry and atomic force microscopy. The high resolution of neutron scattering makes it possible to resolve the distribution profiles of heavy water and benzyl alcohol inside Laropal (R) A81, often employed as a protective varnish layer for Culture Heritage in restoration of easel paintings. The swelling kinetics and distribution profiles were recorded as a function of time and increasing benzyl alcohol concentration in water. The varnish film swells by penetration of the good solvent. At higher concentrations water-filled cavities appear inside the varnish and grow with time. Contrary to homogeneous dissolution dewetting is observed at late stages of exposure to the liquid which leads to the Breakup of the film. The high resolution measurements are compared to bulk behaviour characterized by the ternary phase diagram and the Flory-Huggins interaction parameters are calculated and used to predict the swelling and solvent partition in the films. Distinct differences of the thin film to bulk behaviour are found. The expectations made previously for the behaviour of solvent/non-solvent mixtures on the removal of thin layers in the restoration of easel paintings should be revised in view of surface interactions.

Convective dispersion of solutes is inherent to flow in channels because of the nonuniformity of the velocity profile. When diffusion is negligible, for large particles for example, the trajectory of particles can be solely described by a kinematic approach. Here, we investigate such a phenomenon for micrometer-size beads flowing in a circular pipe. We show that the presence of large bubbles, namely in the case of a segmented flow, either prevents the convective dispersion or leads to the accumulation of particles at the rear of the bubble moving in front. The destabilization of the initially homogeneous suspension occurs when liquid inertia comes into play. Indeed, for moderate Reynolds number of the particles, particles move away from the wall, thus exploring different flow lines that finally impact the axial dispersion features. Moreover, since the bubbles impose an axial boundary condition of the mean velocity, a net flux of particles directed along the flow direction is built up above a critical particle Reynolds number. This work is motivated by the understanding of the flow behavior of biological samples, and especially in the context of cell encapsulation.

Liquid core capsules having a hydrogel membrane are becoming a versatile tool for three- dimensional culture of micro-organisms and mammalian cells. Making sub-millimeter capsules at a high rate, via the breakup of a compound jet in air, opens the way to high throughput screen- ing applications. However, a control of the capsule size monodispersity, especially required for quantitative bioassays, was still lacking. Here, we report how the understanding of the underlying hydrodynamic instabilities that occur during the process can lead to calibrated core-shell biore- actors. This implies: i) to damp the shear layer instability that develops inside the injector arising from the co-annular flow configuration of liquid phases having contrasted viscoelastic properties; ii) to control the capillary instability of the compound jet by superposing a harmonic perturbation to the shell flow; iii) to avoid coalescence of drop during jet fragmentation as well as during drop flight towards the gelling bath, and iv) to ensure a proper engulfment of the compound drops into the gelling bath for building a closed hydrogel shell. We end up with the creation of numerous identical compartments in which cells are able to form mutlicellular aggregates, namely spheroids. In addition, we implement an intermediate composite hydrogel layer, composed of alginate and collagen, allowing cell adhesion and thus the formation of epithelia or monolayers of cells.
 

Liquid jets ultimately break up into droplets through an instability driven by surface tension. For highly viscous liquids, drops are connected by cylindrical liquid filaments whose radii linearly decrease with time, thus forming drops on a string structure. For a jet composed of two aqueous phases made in air by coaxial extrusion, we observe that, for moderate Weber and capillary numbers, drops slow down with different velocities, leading to drop coalescence. The origin of the traffic collision is linked to the spatial feature of the capillary instability where capillary and viscous forces acting on the drops evolve along the jet and ultimately amplify small velocity fluctuations. The emergence of such fluctuations is related to the unstable nature of the annular coflow of liquids having contrasting viscoelastic properties. From a practical point of view, flow and actuation conditions can be adjusted to inhibit drop collision and thus drop coalescence. These findings allow then the fabrication of monodisperse submillimeter core-shell objects based on the fragmentation of compound jets made of polymer solutions that find applications for three-dimensional cell culture.
 

L’objectif de cette theÌ€se est la formation de capsules submillimétriques aÌ€ cœur aqueux pos- sédant une membrane fine d’hydrogel d’alginate. Ces capsules, biocompatibles, peuvent trouver des applications dans les biotechnologies. Elles sont formées par fragmentation d’un jet composé d’un cœur aqueux et d’une coque d’alginate. L’étude du co-écoulement au sein de l’injecteur a montré l’existence d’une instabilité élastique en présence de cations, souvent présents dans les milieux de culture cellulaire. L’alginate est un polyélectrolyte et les cations modifient ses propriétés viscoélastiques. Nous avons montré que cette instabilité était aÌ€ l’ori- gine d’un battement du jet composé en sortie d’extrusion. En provoquant un décentrage du cœur, l’instabilité induit une relaxation asymétrique du profil de vitesse, ce qui crée un couple qui courbe le jet. Nous avons mis en évidence cet effet en produisant des jets courbés aÌ€ partir d’une pointe biseautée. La viscosité du fluide utilisé fixe alors le degré d’asymétrie. La frag- mentation du jet est ensuite controÌ‚lée graÌ‚ce aÌ€ un actuateur piézoélectrique. Nous mesurons une décroissance de la vitesse d’onde aÌ€ la surface du jet due aux faibles valeurs du nombre de Weber, ainsi qu’une dispersion de cette vitesse qui s’amplifie. La formation de chapelets de gouttes et de faibles variations de distances entre gouttes sont aÌ€ l’origine de cette amplification. L’ajout de tensioactifs peut exacerber cet effet en induisant une tension de surface dynamique aÌ€ la surface du jet. Finalement, nous parvenons aÌ€ produire des capsules submillimétriques de taille controÌ‚lée, monodisperses, possédant une membrane fine, avec un taux d’encapsulation supérieur aÌ€ 99%.

Deciphering the multifactorial determinants of tumor progression requires standardized high-throughput preparation of 3D in vitro cellular assays. We present a simple microfluidic method based on the encapsulation and growth of cells inside permeable, elastic, hollow microspheres. We show that this approach enables mass production of size-controlled multicellular spheroids. Due to their geometry and elasticity, these microcapsules can uniquely serve as quantitative mechanical sensors to measure the pressure exerted by the expanding spheroid. By monitoring the growth of individual encapsulated spheroids after confluence, we dissect the dynamics of pressure buildup toward a steady-state value, consistent with the concept of homeostatic pressure. In turn, these confining conditions are observed to increase the cellular density and affect the cellular organization of the spheroid. Postconfluent spheroids exhibit a necrotic core cemented by a blend of extracellular material and surrounded by a rim of proliferating hypermotile cells. By performing invasion assays in a collagen matrix, we report that peripheral cells readily escape preconfined spheroids and cell–cell cohesivity is maintained for freely growing spheroids, suggesting that mechanical cues from the surrounding microenvironment may trigger cell invasion from a growing tumor. Overall, our technology offers a unique avenue to produce in vitro cell-based assays useful for developing new anticancer therapies and to investigate the interplay between mechanics and growth in tumor evolution.

Liquid pearls designate liquid core capsules having a thin elastic shell.

We report an experimental investigation on the formation of liquid-core capsules having a thin hydrogel elastic membrane, named ‘liquid pearls’. These fish-egg-like structures are initially made of a millimetric liquid drop, aqueous or not, coated with an aqueous liquid film containing sodium alginate that gels once the double drop enters a calcium chloride bath. The creation of such pearls with a micrometer-thick membrane requires the suppression of mixing until gelling takes place. Here, we show that superimposing a two-dimensional surfactant precipitation at the interface confers a transient rigidity that can damp the shear-induced instability at impact. Based on this, pearls containing almost any type of liquid can be created. This opens the possibility to use such structures as a new tool for screening microorganism survival and growth in various three-dimensional environments.

La présente invention se rapporte à un procédé pour libérer de manière contrôlée un principe actif contenu dans une émulsion multiple de type eau dans huile dans eau caractérisé en ce que: ladite émulsion multiple comprend une émulsion inverse Ei avec une phase aqueuse A1 qui contient au moins un principe actif hydrophile, ladite émulsion Ei étant dispersée sous forme de gouttelettes d'émulsion directe Ed, dans une phase continue aqueuse A2, avec les deux émulsions Ed et Ei stabilisées par au moins un agent de surface, présent au niveau de leurs phases continues respectives et en ce que ladite émulsion multiple est mise en présence d'une quantité suffisante d'un agent, de manière à la transformer en une émulsion directe et à induire la libération du principe actif, contenu dans la phase aqueuse A1 de l'émulsion Ei, dans la phase aqueuse A2.