Monodisperse microcapsules with controlled interfacial properties generated in microfluidic T-shape junction

Microcapsules are widely found in the nature (e.g. red blood cells and some bacteria), as well as in artificial products. They are generally well-considered as liquid drop bounded by an elastic membrane which is often used to protect the core materials from the external harsh environments. Capsules of biopolymers are exhibiting a large increase of promising applications in the encapsulation and release of medical drugs, food additives, and cosmetics[1-3]. Indeed, there is also a growing interest to model the dynamics of red blood cells (RBCs) motion in vessels or circulations using artificial microcapsules. Particularly, in some cases, it requires the homogeneous physic-chemical properties of capsules, such as uniform size, same shell structure and mechanical characteristics. Therefore, the most challenging work could be to develop a facile strategy to synthesis microcapsules with controlled properties-determined parameters. The preparation of monodisperse microcapsules involves emulsification of the disperse phase into the continuous phase which both are immiscible. There are several strategies been developed to fabricate capsules including batch methods (high-pressure valve homogenisers, static mixers, and rotor stator systems), electrospray techniques, and emulsification through membrane pores. These methods, however, require multistage emulsion processes, and capsules obtained with non-uniform properties and a largely wide distribution of sizes[4-5]. To overcome these problems, recently, microfluidic controlling techniques are introduced, by which monodisperse biopolymer capsules in micrometer size ranges are allowed to be generated in a single step. The main purpose of this study is to develop an approach of fabricating monodisperse biopolymer microcapsules with homogeneous properties on the base of microfluidic controlling components. Thereafter, the membrane properties of obtained capsules are proposed to be measured consisting of flowing a capsule suspension into an elongation flow. The deformation of capsules in the elongation flow can be divided into two regions: linear and non-linear zones. Surface shear elastic modulus of the shell in the linear region (small deformation) and membrane wrinkles instability or plastic deformation in the non-linear region are detected, respectively. Furthermore, thanks to the microfluidic techniques, the interfacial rheological properties of microcapsules are able to be modified via the synthetic procedures, such as the concentrations of chemicals and interfacial polymerization time. Results show that the physic-chemical properties of biopolymer capsules produced by the microfluidic route are very close for the same generating lot

Grandes déformations de microcapsules en écoulement élongationel et en cisaillement simple

Nous étudions expérimentalement des microcapsules dans toute la gamme de déformations accessibles avant leur rupture dans des écoulements élongationel plan et de cisaillement simple. La membrane élastique est faite de protéines réticulées et enferme une goutte de même viscosité que le fluide externe. En écoulement élongationel, nous avons développé un système micro-fluidique en croix permettant de visualiser les deux plans principaux de déformation de la capsule. L’analyse géométrique dans les deux vues permet d’identifier la loi de comportement et le coefficient de Poisson de la membrane. Pour des déformations modérées et en écoulement de cisaillement, la capsule a une forme ellipsoïdale. Au delà d’une valeur critique de déformation, la capsule devient asymétrique et présente une forme en ‘S’. Des microparticules attachées à la membrane nous permettent de quantifier localement le phénomène de ‘tank-treading’, c.à.d de rotation de la membrane. La période de rotation augmente avec la déformation. De plus, la vitesse de rotation instantanée est fortement dépendante de la position angulaire pour les grandes déformations ; les microparticules ralentissent aux pointes de la capsule

Stretching of capsules in an elongation flow, a route to constitutive law

International audienceSoft bio-microcapsules are drops bounded by a thin elastic shell made of cross-linked proteins. Their shapes and their dynamics in flow depend on their membrane constitutive law characterized by shearing and area-dilatation resistance. The deformations of such capsules are investigated experimentally in planar elongation flows and compared with numerical simulations for three bidimensional models: Skalak, neo-Hookean and generalized Hooke. An original cross-flow microfluidic set-up allows the visualization of the deformed shape in the two perpendicular main fields of view. Whatever the elongation rate, the three semi-axis lengths of the ellipsoid fitting the experimental shape are measured up to 180 % of stretching of the largest axis. The geometrical analysis in the two views is sufficient to determine the constitutive law and the Poisson ratio of the membrane without a preliminary knowledge of the shear elastic modulus Gs. We conclude that the membrane of human serum albumin capsules obeys the generalized Hooke law with a Poisson ratio of 0.4. The shear elastic modulus is then determined by the combination of numerical and experimental variations of the Taylor parameter with the capillary number

Effect of initial water flooding on the performance of polymer flooding for heavy oil production

In the domain of heavy to extra heavy oil production, viscous polymer may be injected after water injection (tertiary mode), or as an alternative (secondary mode) to improve the sweep efficiency and increase oil recovery. To prepare field implementation, nine polymer injection experiments in heavy oil have been performed at core scale, to assess key modelling parameters in both situations. Among this consistent set of experiments, two have been performed on reconstituted cylindrical sandpacks in field-like conditions, and seven on consolidated Bentheimer sandstone in laboratory conditions. All experiments target the same oil viscosity, between 2000 cP and 7000 cP, and the viscosity of Partially Hydrolyzed Polyacrylamide solutions (HPAM 3630) ranges from 60 cP to 80 cP. Water and polymer front propagation are studied using X-ray and tracer measurements. The new experimental results presented here for water flood and polymer flood experiments are compared with experiments described in previous papers. The effects of geometry, viscosity ratio, injection sequence on recoveries, and history match parameters are investigated. Relative permeabilities of the water flood experiment are in line with previous experiments in linear geometry. Initial water floods led to recoveries of 15–30% after one Pore Volume Injected (PVI), a variation influenced by boundary conditions, viscosity, and velocities. The secondary polymer flood in consolidated sandstone confirms less stable displacement than tertiary floods in same conditions. Comparison of secondary and tertiary polymer floods history matching parameters suggests two mechanisms. First, hysteresis effect during oil bank mobilization stabilizes the tertiary polymer front; secondly, the propagation of polymer at higher oil saturation leads to lower adsorption during secondary experiment, generating a lower Residual Resistance Factor (RRF), close to unity. Finally, this paper discusses the use of the relative permeabilities and polymer properties estimated using Darcy equation for field simulation, depending on water distribution at polymer injection start-up

Interfacial rheological properties of self-assembling biopolymer microcapsules

International audienceTuning the mechanical properties of microcapsules with cost-efficient route of fabrication is still a challenge. The traditional method of layer-by-layer assembly of microcapsules allows building a tailored composite multi-layer membrane but is technically complex as it requires numerous steps. The objective of this article is to characterize the interfacial rheological properties of self-assembling biopolymer microcapsules that were obtained in one single facile step. This thorough study provides new insights in the mechanics of these weakly cohesive membranes. Firstly, sus-pensions of water-in-oil microcapsules were formed in microfluidic junctions by self-assembling of two oppositely charged polyelectrolytes, namely chitosan (water soluble) and phosphatidic fatty acid (oil soluble). In this way, composite membranes of tunable thickness (between 40-900 nm measured by AFM) were formed at water / oil interfaces in a single step by changing the composition. Secondly, microcapsules were mechanically characterized by stretching them up to break-up in an extensional flow chamber which extends the relevance and convenience of the hydrodynamic method to weakly cohesive membranes. Finally, we show that the design of micro-capsules can be 'engineered' in a large way since they present a wealth of interfacial rheological properties in term of elasticity, plasticity and yield stress whose magnitudes can be controlled by the composition. These behaviors are explained by the variation of the membrane thickness with the physico-chemical parameters of the process

Characterization of the mechanical properties of cross-linked serum albumin microcapsules: effect of size and protein concentration

International audienceA microfluidic technique is used to characterize the mechanical behavior of capsules that are produced in a two-step process: first, an emulsification step to form droplets, followed by a cross-linking step to encapsulate the droplets within a thin membrane composed of cross-linked proteins. The objective is to study the influence of the capsule size and protein concentration on the membrane mechanical properties. The microcapsules are fabricated by cross-linking of human serum albumin (HSA) with concentrations from 15 to 35 % (w/v). A wide range of capsule radii (∼40–450 μm) is obtained by varying the stirring speed in the emulsification step. For each stirring speed, a low threshold value in protein concentration is found, below which no coherent capsules could be produced. The smaller the stirring speed, the lower the concentration can be. Increasing the concentration from the threshold value and considering capsules of a given size, we show that the surface shear modulus of the membrane increases with the concentration following a sigmoidal curve. The increase in mechanical resistance reveals a higher degree of cross-linking in the membrane. Varying the stirring speed, we find that the surface shear modulus strongly increases with the capsule radius: its increase is two orders of magnitude larger than the increase in size for the capsules under consideration. It demonstrates that the cross-linking reaction is a function of the emulsion size distribution and that capsules produced in batch through emulsification processes inherently have a distribution in mechanical resistance