Modélisation de l'écoulement de fluides en loi puissance en milieux fibreux anisotropes

L'écoulement de fluide en loi puissance à travers des milieux fibreux anisotropes est étudié en s'appuyant sur les résultats théoriques obtenus par la méthode d'homogénéisation des structures périodiques. Afin de determiner la structure de la loi d'écoulement, des simulations numériques ont été réalisées sur des volumes élémentaires représentatifs d'un milieu fibreux modèle 2D constitué d'un arrangement carré de fibres parallèles de sections circulaires ou elliptiques. Enfin une méthodologie, basée sur l'étude de courbes d'iso-dissipation mécanique ainsi que la théorie de représentation des fonctions tensorielles, est proposée afin de formuler la loi d'écoulement macroscopique. Cette méthodologie est illustrée sur les résultats numériques obtenus

Rapid Prototyping of Reconfigurable Microfluidic Channels in Undercooled Metal Particle-Elastomer Composites

Conventional fabrication of microfluidic channels/devices are faced with challenges such as single use channels and/or significant time consumption. We propose a flexible platform for fabricating microfluidic channels simply through indentation on a smart composite—the so-called ST3R (Stiffness tuning through thermodynamic relaxation) composite. The application of ST3R composite allows rapid fabrication of microfluidic channels by hand or with a prefabricated stamp, and precise prototyping of complex designs using a 2D plotter. Indenter geometry, applied stress, filler loading, and number of repeated indentations affect channel dimensions and/or shape. These channels further exhibit; i) Substantial improvement against swelling by organic solvent, in part due to the high modulus of the solidified metal network. ii) Channel reconfigurability by heating the solidified undercooled metals. ST3R composite slabs have the potential to serve as microfluidic ‘breadboards’, from which complex channels can be integrated in a flexible manner

Détermination des propriétés microstructurales et physiques d'un matériau fibreux cellulosique à partir de microtomographies aux rayons X

Cette étude concerne la détermination des propriétés microstructurales et physiques d'un matériau fibreux cellulosique à partir de microtomographies aux rayons X. Les propriétés microstructurales telles que la porosité, la surface spécifique et l'anisotropie du matériau sont mesurées à l'aide d'outils d'analyse d'image. Les propriétés physiques, telles que la perméabilité et la conductivité thermique effective, sont estimées numériquement en s'appuyant sur les résultats de l'homogénéisation périodique. Ces estimations numériques sont comparées à des mesures expérimentales. L'influence de la microstructure sur les propriétés physiques étudiées est discutée

Electrically Activated Paper Actuators

This paper describes the design and fabrication of electrically controlled paper actuators that operate based on the dimensional changes that occur in paper when the moisture absorbed on the surface of the cellulose fibers changes. These actuators are called “Hygroexpansive Electrothermal Paper Actuators” (HEPAs). The actuators are made from paper, conducting polymer, and adhesive tape. They are lightweight, inexpensive, and can be fabricated using simple printing techniques. The central element of the HEPAs is a porous conducting path (used to provide electrothermal heating) that changes the moisture content of the paper and causes actuation. This conducting path is made by embedding a conducting polymer (PEDOT:PSS) within the paper, and thus making a paper/polymer composite that retains the porosity and hydrophilicity of paper. Different types of HEPAs (straight, precurved, and creased) achieved different types of motions (e.g., bending motion, accordion type motion). A theoretical model for their behavior is proposed. These actuators have been used for the manipulation of liquids and for the fabrication of an optical shutter.Chemistry and Chemical Biolog

Effect of surface morphologies and chemistry of paper on deposited collagen

Paper-based platforms for biological studies have received significant attention given that cellulose is ubiquitous, biocompatible, and can be readily organized into tunable fibrous structures. In the latter form, effect of complexity in surface morphologies (roughness, porosity and fiber organization) on cell-substrate interaction has not been thoroughly explored. We infer that altering the properties of a fibrous material should lead to significant changes in cellular microenvironment and direct the deposition of structurally analogous extracellular matrix (fiber-fiber templating) like collagen. Here, we elucidate the effect of varying paper roughness and surface chemistry on NIH/3T3 fibroblasts via organization of excreted collagen. Collagen intensity was found to increase linearly with paper porosity, indicating a 3D culture platform. The intensity, however, decays over time due to biodegradation of the substrate. Stability can be improved by introducing fluorinated alkyl silanes to yield hydrophobic paper. This process concomitantly transforms the substrate to a 2D-like scaffold where collagen is predominantly assembled on the surface, thus changing the cellular microenvironment. Altering surface energy also led to fluctuations in collagen intensity and organization over time for smooth (calendered) paper substrates. We infer that the increased roughness improves collagen adsorption through capillary driven petal effect. In general, the influence of the substrate simultaneously affects its ability to host collagen and guide orientation. These findings offer insights into the effects of secondary structures and chemistry of fibrous polymeric materials on cell culture, which we propose as vital parameters when using paper-based platforms

Paper-Based, Capacitive Touch Pads

Metallized paper is patterned to create touch pads of arrayed buttons that are sensitive to contact with both bare and gloved fingers. The paper-based keypad detects the change in capacitance associated with the touch of a finger to one of its buttons. Mounted on an alarmed cardboard box, the keypad requires the appropriate sequence of touches to disarm the system.Chemistry and Chemical Biolog

Fabrication of Low-Cost Paper-Based Microfluidic Devices by Embossing or Cut-and-Stack Methods

This communication describes the use of embossing, and “cut-and-stack” methods of assembly, to generate microfluidic devices from omniphobic paper, and demonstrates that fluid flowing through these devices behaves similarly to fluid in an open-channel microfluidic device. The porosity of the paper to gasses allows processes not possible in devices made using PDMS or other non-porous materials. Droplet generators and phase separators, for example, could be made by embossing “T”-shaped channels on paper. Vertical stacking of embossed or cut layers of omniphobic paper generated three-dimensional systems of microchannels. The gas permeability of the paper allowed fluid in the microchannel to contact and exchange with environmental or directed gases. An aqueous stream of water containing a pH-indicator, as one demonstration, changed color upon exposure to air containing HCl or NH3 gases.Chemistry and Chemical Biolog