Smart hydrogels are useful elements in microfluidic systems because they respond to environmental stimuli and are capable of storing reagents. We present here a concept of using hydrogels (poly(N-isopropylacrylamide)) as an interface between continuous and discontinuous microfluidics. Their swelling and shrinking capabilities allow them to act as storage elements for reagents absorbed in the swelling process. When the swollen hydrogel collapses in an oil-filled channel, the incorporated water and molecules are expelled from the hydrogel and form a water reservoir. Water-in-oil droplets can be released from the reservoir generating different sized droplets depending on the flow regime at various oil flow rates (dispensing functionality). Different hydrogel sizes and microfluidic structures are discussed in terms of their storage and droplet formation capabilities. The time behaviour of the hydrogel element is investigated by dynamic swelling experiments and computational fluid dynamics simulations. By precise temperature control, the device acts as an active droplet generator and converts continuous to discontinuous flows.

Dans le contexte actuel d'une prise de conscience environnementale générale, concernant plus particulièrement la gestion des polymères en fin vie, la thèse se propose d'étudier la substitution d'une matrice non biodégradable (Polyéthylène) par une matrice biodégradable (Polybutylene Succinate) dans le cadre de la propriété singulière de thermorétraction mise à profit dans le secteur de l'emballage.Ce travail de thèse, cofinancé par un industriel du secteur de l'emballe et par l'Agence de l'Environnement et de la Maitrise de l'Energie (ADEME) a donc consisté à :- Mettre en évidence l'influence des branchements longs contenus dans certains polymères, ainsi que leurs mélanges avec des polymères linéaires sur le comportement rhéologique en cisaillement et en élongation,- Proposer une modélisation de la contrainte interne stockée lors de l'étape de mise en forme par extrusion de film à plat (extrusion dite « cast ») en vue de prédire la contrainte de thermorétraction,- Etablir le lien entre la structure des matériaux, ses conditions de mise en œuvre et la morphologie du film final afin d'établir une corrélation avec le taux de rétraction.Dans un premier temps, cette étude a été réalisée sur des matrices modèles de polyéthylènes et ensuite transposée à une matrice biodégradable de Polybutylène Succinate. / Due to environmental concerns growth including the management of the end of life of polymer wastes, the present work proposes to study the feasibility of a substitution of conventional non-biodegradable polyethylene by a biodegradable matrix of Polybutylene Succinate in order to produce heat shrinkable films with equivalent properties. This property is mostly used in the packaging industry to wrap product such as six packs bottles, cans or other food products together.This work half-financed by a packaging company based in France and by the French environment and energy management agency consisted on:- Highlight the impact of the macromolecular architectures of the polymers and their blends on the rheological behavior under shear and elongational flow,- Developing a modeling approach of the stress stored in the molten polymer during the cast film extrusion process in order to predict the shrinkage stress,- Correlate the structure of the materials with their processing conditions and the morphology(ies) of the final film in order to correlate them with the shrinkage rate.The first part of this work was dedicated to the understanding of the shrinkage mechanism and to develop an approach on polyethylenes matrices used as models and then to transpose this approach to a biodegradable matrices of Polybutylene Succinate and therefore conclude on the feasibility of the use of such a matrices for heat shrinkable films for packaging application.

Consider the following system model y = Ax + e, where x is n-dimensional sparse signal, y is the measurement vector in a much lower dimension m, A is the measurement matrix and e is the error in our measurements. The Dantzig selector estimates x by solving the following optimization problem minimize || x ||₁ subject to || A'(Ax - y) ||∞ ≤ ε, (DS). This is a convex program and can be recast into a linear program and solved using any modern optimization method e.g., interior point methods. We propose a fast and efficient scheme for solving the Dantzig Selector (DS), which we call "Primal-Dual pursuit". This algorithm can be thought of as a "primal-dual homotopy" approach to solve the Dantzig selector (DS). It computes the solution to (DS) for a range of successively relaxed problems, by starting with a large artificial ε and moving towards the desired value. Our algorithm iteratively updates the primal and dual supports as ε reduces to the desired value, which gives final solution. The homotopy path solution of (DS) takes with varying ε is piecewise linear. At some critical values of ε in this path, either some new elements enter the support of the signal or some existing elements leave the support. We derive the optimality and feasibility conditions which are used to update the solutions at these critical points. We also present a detailed analysis of primal-dual pursuit for sparse signals in noiseless case. We show that if our signal is S-sparse, then we can find all its S elements in exactly S steps using about "S² log n" random measurements, with very high probability.

Smart hydrogels are useful elements in microfluidic systems because they respond to environmental stimuli and are capable of storing reagents. We present here a concept of using hydrogels (poly(N-isopropylacrylamide)) as an interface between continuous and discontinuous microfluidics. Their swelling and shrinking capabilities allow them to act as storage elements for reagents absorbed in the swelling process. When the swollen hydrogel collapses in an oil-filled channel, the incorporated water and molecules are expelled from the hydrogel and form a water reservoir. Water-in-oil droplets can be released from the reservoir generating different sized droplets depending on the flow regime at various oil flow rates (dispensing functionality). Different hydrogel sizes and microfluidic structures are discussed in terms of their storage and droplet formation capabilities. The time behaviour of the hydrogel element is investigated by dynamic swelling experiments and computational fluid dynamics simulations. By precise temperature control, the device acts as an active droplet generator and converts continuous to discontinuous flows.