Interfacial rheology of microcapsules and dynamics in flow

A capsule is a drop bounded by a thin solid membrane providing specific mechanical properties. It is used to control the spatio-temporal delivery of substances in numerous processes and also as a model system of cells. Its dynamics under flow depends on its membrane characteristics. Moreover, the delivery of encapsulated drugs is controlled by its deformation. The interfacial rheology of microcapsules can be tuned according to their formulation. We will focus on cross-linked membrane made with human serum albumin and chitosan assembled with a surfactant via electrostatic interactions. The interfacial rheological properties of these soft microparticles are deduced from their dynamics of deformation in elongation and shear flows. In elongation flow, the surface shear modulus of the membrane is measured and related to the kind of biopolymer used and to the main parameters of the process of fabrication. In the regime of large deformations, the microcapsules can present a non-linear elastic response or plastic deformations. Non-linear elastic constitutive law is deduced by comparison of the evolution of the shape of the microcapsule in the two main planes of deformation of the capsule with numerical simulations. In shear flow, the rotation of the membrane, i.e. the tank-treading, is visualised and quantified by decorating the membrane of microcapsules with particles. The tracking of the distance between two close microparticles showed membrane contraction at the tips and stretching on the sides. This dynamic of deformation induce viscous dissipation inside the membrane. The order of magnitude of membrane viscosity is determined by comparison with numerical simulations. Wrinkling instability is observed in extensional flow and studied by varying the interfacial properties of the microcapsules. In this way, the phase diagram of wrinkle instability for microcapsules has been deduced as the scaling law between the wrinkles wave-length and the membrane thickness. Finally, we have developed a set of tools to characterize the interfacial viscoelasticity of microcapsules, their bending modulus and their non-linear elastic properties. We conclude the talk with some results on break-up of microcapsules in flow. Please click Additional Files below to see the full abstract

Tank-treading of microcapsules in shear flow

International audienceWe investigated experimentally the deformation of soft microcapsules and the dynamics of their membrane in simple shear flows. Firstly, the tank-treading motion, i.e. the rotation of the membrane, was visualized and quantified by tracking particles included in the membrane by a new protocol. The period of membrane rotation increased quadratically with the extension of the large axis. The tracking of the distance between two close micro-particles showed membrane contraction at the tips and stretching on the sides, a specific property of soft particles such as capsules. Present experimental results are discussed in regard to previous numerical simulations. This analysis showed that the variation of the tank-treading period with the Taylor parameter (deformation) cannot be explained by purely elastic membrane models. It suggests a strong effect of membrane viscosity whose order of magnitude is determined. Secondly, two distinct shapes of sheared microcapsules were observed. For moderate deformations, the shape was a steady ellipsoid in the shear plane. For larger deformations, the capsule became asymmetric and presented a S-like shape. When the viscous shear stress increased by three orders of magnitude, the small axis decreased by 70 % whereas the large axis increased by 100% before any break-up. The inclination angle decreased from 40 • to 8 • , almost aligned with the flow direction as expected by theory/numerics on capsules and experiments/theory/numerics on drops and vesicles. Whatever the microcapsule size and the concentration of proteins, the characteristic lengths of the shape, the Taylor parameter and the inclination angle satisfy master curves versus the longest axis or the normalized shear stress or the capillary number in agreement with theory for non negligible membrane viscosity in the regime of moderate deformations. Finally, we observed that very small deviation from sphericity gave rise to swinging motion, i.e. shape oscillations, in the small deformation regime. In conclusion, this study of tank-treading motion supports the role of membrane viscosity on the dynamics of microcapsules in shear flow by both independent methods which compare experimental data with numerical results in the regime of large deformations and with the theory in the regime of moderate deformations

A biomechanical model of swallowing for understanding the influence of saliva and food bolus viscosity on flavour release

International audienceAfter swallowing a liquid or a semi-liquid food product, a thin film responsible for the dynamic profile of aroma release coats the pharyngeal mucosa. The objective of the present article was to understand and quantify physical mechanisms explaining pharyngeal mucosa coating. An elastohydrodynamic model of swallowing was developed for Newtonian liquids that focused on the most occluded region of the pharyngeal peristaltic wave. The model took lubrication by a saliva film and mucosa deformability into account. Food bolus flow rate and generated load were predicted as functions of three dimensionless variables: the dimensionless saliva flow rate, the viscosity ratio between saliva and the food bolus, and the elasticity number. Considering physiological conditions, the results were applied to predict aroma release kinetics. Two sets of conditions were distinguished. The first one was obtained when the saliva film is thin, in which case food bolus viscosity has a strong impact on mucosa coating and on flavour release. More importantly, we demonstrated the existence of a second set of conditions. It was obtained when the saliva film is thick and the food bolus coating the mucosa is very diluted by saliva during the swallowing process and the impact of its viscosity on flavour release is weak. This last phenomenon explains physically in vivo observations for Newtonian food products found in the literature. Moreover, in this case, the predicted thickness of the mix of food bolus with saliva coating the mucosa is approximately of 20 µm; value in agreement with orders of magnitude found in the literature

Dynamics of Particle Migration in Confined ViscoElastic Poiseuille Flows

Particles migrate in the transverse direction of the flow due to the existence of normal stress anisotropy in weakly viscoelastic liquids. We test the ability of theoretical predictions to predict the transverse velocity migration of particles in a confined Poiseuille flow according to the viscoelastic constitutive parameters of dilute polymers solutions. Firstly, we carefully characterize the viscoelastic properties of two families of dilute polymer solutions at various concentrations using shear rheometry and capillary breakup experiments. Secondly, we develop a specific 3D particle tracking velocimetry method to measure with a high accuracy the dynamics of particles focusing in flow for Weissenberg numbers Wi ranging from $10^{-2}$ to $10^{-1}$ and particle confinement $\beta$ of 0.1 and 0.2. The results show unambiguously that the migration velocity scales as $\text{Wi}\beta^2$, as expected theoretically for weakly elastic flows of an Oldroyd-B liquid. We conclude that classic constitutive viscoelastic laws are relevant to predict particle migration in dilute polymer solutions whereas detailed analysis of our results reveals that theoretical models overestimate by a few tenth the efficiency of particle focusing.Comment: 24 pages, 7 figure

A lubrication analysis of pharyngeal peristalsis: application to flavour release.

International audienceAfter eating a liquid or a semi-liquid food product, a thin film responsible for the dynamic profile of aroma release coats the pharyngeal mucosa. The aim of this article was to analyse the fluid mechanics of pharyngeal peristalsis and to develop a simple biomechanical model in order to understand the role of saliva and food bolus viscosity on the coating of pharyngeal mucosa. We began by analysing the physiology and the biomechanics of swallowing in order to determine relevant model assumptions. This analysis of the literature clarified the types of mechanical solicitations applied on the food bolus. Moreover, we showed that the pharyngeal peristalsis in the most occluded region is equivalent to a forward roll coating process, the originality of which is lubrication by a film of saliva. A model based on the lubrication theory for Newtonian liquids was developed in dimensionless form. The parametric study showed the strong influence of relative saliva thickness on the food bolus coating. A specific experimental device was designed that confirms the model predictions. Two sets of conditions that depend on the relative thickness of saliva were distinguished. The first is characterised by a relatively thin film of saliva: food bolus viscosity has a strong impact on mucosa coating. These phenomena are well represented by the model developed here. The second is obtained when the saliva film is relatively thick: hydrodynamic mixing with saliva, interdiffusion or instabilities may govern mucosa coating. Finally, these results were extrapolated to determine the influence of food bolus viscosity on the dynamic profile of flavour release according to physiological parameters

Instabilité de rides sur des microcapsules en écoulement extensionnel

International audienceLes particules déformables telles que les cellules, les vésicules ou les microcapsules, ont une dynamique spatio-temporelle riche enécoulement. Un exemple marquant concerne les globules rouges qui présentent des oscillations de leur forme ainsi que des rotations dans desécoulements de cisaillement. De plus, d'autres phénomènes non-linéaires surgissent tels que le flambement de la membrane, observé sur des globules rouges rigidifiés ou bien encore des capsules [1,2] dans les années 70. Inspiré par les globules rouges, nousétudions l'émergence de telles instabilitésélastiques sur des microcapsules enécoulement extensionnel. Des rides très bien définies apparaissent sur la membrane de la capsule en extension lorsque la contrainte hydrodynamique dépasse un seuil. Au-delà de ce seuil, cette instabilité supercritique se développe le long de la capsule jusqu'à ce que la longueur des plis sature. La valeur critique de la contrainte hydrodynamique dépend du moduleélastique surfacique de cisaillement de la microcapsule mais correspond toujoursà une valeur fixée de la déformation. Abstract. Deformable particles such as cells, vesicles and microcapsules, have rich spatio-temporal dynamics of their shapes under flow. A striking example is the case of red blood cells (RBCs) in shear flow : tumbling, swinging oscillations of the shapes. Except that, other non-linear phenomena, for example buckling, are observed on the stiffened RBCs or artificial capsules [1,2]. Inspired by RBCs, we investigate emergence of such elastic instability on biomimetic microcapsules in extensional flow. Well-defined wrinkles are first observed on the membrane of stretched capsules when the hydrodynamic stress is above a threshold. Above the threshold, this supercritical instability expands along the microcapsule up to saturate. The critical hydrodynamic stress depends on the surfacic shear elastic modulus of the microcapsule's membrane, but always corresponds to the same value of the deformation

Concept on self-assembly and structure of globular protein fluids

Globular proteins are ubiquitous in our daily life. Not only they are naturally present in the biological matter, they also offer many possibilities to adjust the nutritional and flow properties of fluids or to design drug vehicles [1]. Globular protein systems interact through short range attractive forces; and the interaction between them may lead the system to form aggregates through self-assembling process. Since such biological monomers are complex systems, their aggregation process is most of the time out of control. The current main conceptual framework to describe that process is based on the idea that the monomers may self-assemble through a diffusion and reaction mechanism known as DLA for diffusion limited aggregation, and RLA for reaction limited aggregation respectively [2]. Beta-lactoglobulin (blg) solution gives, after heat-induced denaturation, a suspension of polydisperse aggregates as predicted by the random aggregation concept. Therefore, the transition from native blg to denatured blg aggregate suspension leads to complex correlation with the flow behavior [3]. Although the dependency of the aggregation process to physicochemical factors like, ionic strength, pH, temperature and concentration has been intensively investigated, it still remains much to do to control the aggregate polydispersity via self-assembling process. The composition of the raw product, thermal processing, pH and entropy instability during the aggregation process, are some of the factors influencing the polydispersity of the aggregates. We use different techniques such as SAXS/USAXS, LS, SEM, CSLM and image analysis methods to characterize thoroughly the structure of globular protein aggregates formed after heat-induced denaturation at different experimental conditions [4]. Whether these aggregates are in solution or entrapped by gelation, we do think that investigating their structure will provide us with relevant information to solve the issue related to their formation. Please click Additional Files below to see the full abstract

Compréhension et modélisation des phénomènes physiques régissant la libération des stimuli orosensoriels

Understanding and modeling phenomena governing stimuli release during food consumption make it possible to respect both nutritional and sensorial criteria during its formulation. A model of salt release during the course of mastication was developed for "solid" products. The breakdown was comprehended by the generation of the area of contact between the product and the saliva that governs the transfers of stimuli. The area of contact was written as the product of two functions. The first was related to the subject and was function of his masticatory performance. The second was related to the product and depended on its breakdown behavior that can be determined by in vitro tests. During the pharyngeal stage, the biomechanics of swallowing governs pharyngeal mucosa coating and aroma compounds present in this layer. These phenomena are due to a thin film flow, stationary in a soft elastohydrodynamic contact whose the kinematics is equivalent to a forward roll coating process lubricated by saliva. Two sets of conditions were distinguished. When the saliva film is thin, food bolus viscosity has a strong impact on mucosa coating and on flavour release. When the saliva film is thick, the food bolus coating the mucosa is very diluted by saliva during the swallowing process and the impact of product viscosity on flavour release is weak. This second set of condition allowed us to explain the physical origin of in vivo observations on flavour release.La compréhension et la modélisation des phénomènes régissant la libération des stimuli orosensoriels durant la consommation d'un aliment doit permettre de respecter des critères de conception à la fois nutritionnels et organoleptiques. Un modèle de libération du sel au cours de la mastication a été développé pour des produits " solides ". La déstructuration du produit a été appréhendée en terme de génération de surface de contact entre le produit et la salive qui gouverne les transferts de sel. La surface de contact a été considérée comme étant le produit de deux fonctions. La première est reliée au sujet et est fonction de son efficacité masticatoire. La seconde est reliée au produit et dépend de ses propriétés de déstructuration qui peuvent être déterminées par des tests in vitro. Durant la phase pharyngée, la biomécanique de la déglutition gouverne l'enduction des muqueuses par le bol alimentaire et ainsi la libération des composés d'arôme présents dans cette couche. Ces phénomènes sont régis par un écoulement en film mince, stationnaire, dans un contact élastohydrodynamique mou dont la cinématique équivaut à un processus d'enduction par des cylindres contrarotatifs lubrifiés par de la salive. Deux régimes ont été distingués. Lorsque le film de salive est fin, la viscosité du bol alimentaire a un grand rôle sur l'enduction et la libération des composés d'arôme. Lorsque le film de salive est épais, le bol est dilué par la salive durant le processus de déglutition et sa viscosité a un faible effet sur l'enduction et la libération des composés d'arôme. Ce second régime permet d'expliquer l'origine physique d'observations in vivo concernant la libération des composés d'arôme

Steady streaming flow induced by active biological microstructures; application to small intestine villi

International audiencePhysiological transport of fluid at small scales is often achieved by microscopic active finger-like structures. It is recognized that they have to move in a non-symmetric fashion in order to break the symmetry of creeping flow and to induce a net movement of the fluid. However, in the limit of low, but non-vanishing, Reynolds number, irreversible flow on long time scales could also be generated by symmetric oscillations of these microstructures.Inspired by small intestine villi, we reported three dimensional direct numerical simulations of the irreversible part of the flow, namely steady streaming flow (SSF), generated by an array of oscillating finger-like. In order to capture these second order flow phenomena, the algorithm was based on a combination of lattice-Boltzmann methods with two relaxation times and the smoothed profile method. SSF was confined inside a steady viscous boundary above the villi. Two steady vortices at the tip of the villi characterized this flow, which induced mass transfers between the bulk and the periphery. Strikingly, the spatial extension of these vortices was not solely governed by the Stokes boundary layer but also by the lateral confinement between the villi. Moreover, secondary vortices outside the steady boundary layer were also observed. These findings were rationalized in a state diagram showing three regimes of SSF. Lastly, orders of magnitude showed that SSF should contribute to the transport of particles, such as bacteria or nano-particles, on a layer of a few hundred micrometers above the villi and on time scales of few minutes