Transverse Instability of Avalanches in Granular Flows down Incline

Avalanche experiments on an erodible substrate are treated in the framework of ``partial fluidization'' model of dense granular flows. The model identifies a family of propagating soliton-like avalanches with shape and velocity controlled by the inclination angle and the depth of substrate. At high inclination angles the solitons display a transverse instability, followed by coarsening and fingering similar to recent experimental observation. A primary cause for the transverse instability is directly related to the dependence of soliton velocity on the granular mass trapped in the avalanche.Comment: 3 figures, 4 pages, submitted to Phys Rev Let

Designing Colloidal Molecules with Microfluidics

International audienceThe creation of new colloidal materials involves the design of functional building blocks. Here, a microfl uidic method for designing building blocks one by one, at high throughput, with a broad range of shapes is introduced. The method exploits a coupling between hydrodynamic interactions and depletion forces that controls the confi gurational dynamics of droplet clusters traveling in microfl uidic channels. Droplet clusters can be solidifi ed in situ with UV. By varying the fl ow parameters, clusters are prescribed a given size, geometry, chemical and/or magnetic heterogeneities enabling local bonding. Compact structures (chains, triangles, diamonds, tetrahedrons,...) and non-compact structures, such as crosses and T, diffi cult to obtain with current techniques are produced. Size dispersions are small (2%) and throughputs are high (30 000 h −1). The work opens a new pathway, based on microfl uidics, for designing colloidal building blocks with a potential to enable the creation of new materials

Magnetic Core Shell Nanoparticles Trapping in a Microdevice Generating High Magnetic Gradient

Magnetic core shell nanoparticles (MCSNPs) 30 nm diameter with a magnetic weight of 10% are usually much too small to be trapped in microfluidic systems using classical external magnets. Here, a simple microchip for efficient MCSNPs trapping and release is presented. It comprises a bed of micrometric iron beads (6–8 mm diameter) packed in a microchannel against a physical restriction and presenting a low dead volume of 0.8 nL. These beads of high magnetic permeability are used to focus magnetic field lines from an external permanent magnet and generate local high magnetic gradients. The nanoparticles magnetic trap has been characterised both by numerical simulations and fluorescent MCSNPs imaging. Numerical simulations have been performed to map both the magnetic flux density and the magnetic force, and showed that MCSNPs are preferentially trapped at the iron bead magnetic poles where the magnetic force is increased by 3 orders of magnitude. The trapping efficiency was experimentally determined using fluorescent MCSNPs for different flow rates, different iron beads and permanent magnet positions. At a flow rate of 100 mL h1, the nanoparticles trapping/release can be achieved within 20 s with a preconcentration factor of 4000

Self-rolled polymer film: A novel approach to microfluidic devices

International audienceWe present a new route for the fabrication of highly specialized micro-capillaries, based on the phenomenon of thin polymer films self-rolling. Before rolling, the surface can be patterned (chemically, topographically), permitting the fabrication of inexpensive fully functionalized capillaries. Spontaneous curvature is a well-known instability [1] which occurs in films with gradients of stress along the normal axis. Recently, this effect was used to fabricate tubes as a self-assembly process [2]. We focus on the application of those self-rolled microsystems to lab-on-chip technology. We propose methods to induce the spontaneous rolling of polymeric films, more precisely polydimethylsiloxane (PDMS). The advantage of such system is threefold: i-Those systems are inexpensive to design. ii-The inner surface of the capillary is accessible before rolling and can be properly functionalized and characterized. iii-The formation of the channel itself is not a lithographic process. The fabrication of patterned channel are done with only one lithographic step, which implies a great economy in terms of means and efforts. Spontaneous rolling occurs when there is a stress inhomogeneity in a thin film. One method is to harden the surface of a PDMS thin film, either by adding a capping of material or by hardening the surface by plasma oxidation [3]. The film is then exposed to solvents in gaseous phase. The flat configuration of the film is not an equilibrium anymore and spontaneous rolling occurs. The inner diameter of such systems can be controlled by changing the solvent, the nature of the top layer or the thickness of the whole system. In order to illustrate the potential of the method, we propose the geometrical patterning of surface before rolling obtained with simple embossing-like methods. We obtained capillaries as small as 70 microns with 13 microns deep patterns over the whole inner surface of the tube, which typically cannot be obtained with standard techniques. Other types of patterning such as chemical functionalization or electrode deposition are currently in development. To conclude spontaneous curvature effect can be used for the inexpensive auto-assembly of micro-capillaries. The inner surface of those is fully accessible before the rolling occurs and can be easily functionalized. The main remaining challenge of this technique is the integration of the rolled-up system in a larger microfluidic systems. Methods are developed to obtain soft lithographic / self-rolled hybrid systems in order to make use of the advantages of both processes. We believe in the potential of this method for the design of cutting edge microfluidic technology

Etudes expérimentales d'avalanches granulaires

Granular deposits are ubiquitous in nature, they display solid or fluid-like behavior as well as avalanches, mud flows and their catastrophic human and economical toll. The perspective of risk modeling of these phenomena is hindered by the lack of conceptual clarity since the rheology of the particulate flows is poorly understood. In this thesis, we investigate an experimental work on avalanches propagating on an erodible substrate. In the first part, the stability diagram of the layer deposited on an inclined plane is established, both in the air and in the water. The different steps of the formation of an avalanche front are explained. The rheology of steady uniform flows of sand is fund and compared to other results obtained in the literature. Experimental measurements, "sooted blade" and "multi-colored layers", show the existence of a static zone under the flow. The previous rheology is changed and the avalanches can be described in term of local rheology. Using this flow rule in the case of solitary erosion waves gives the erosion profile which takes place in the avalanche. The behavior is different for sand and for beads. The sand wave does not erode the substrate down the bottom and the beads wave erodes the entire layer. In the second part, the stability of those waves is studied. Above a given angle the wave become transversally unstable. After an initial wavelength, there is a coarsening process by fusion. At the final stage the coarsening is ended by fingering. The first instability is linear long wavelength instability. All the wavelengths measured collapse in the same curve when they are scaled by the size of the grains.- Les instabilités de dépôts granulaire sont omniprésentes dans la nature, elles présentent un comportement solide et liquide comme en témoigne les avalanches et les écoulements de boue qui sont a l'origine de nombreuses catastrophes humaines et économiques. Malgré son importance pratique, ces phénomènes sont encore loin d'être compris et bien décrit notamment en raison du manque de compréhension de leur rhéologie. Ce manuscrit présente une étude expérimentale d'avalanche se propageant sur une couche granulaire érodable. Dans la première partie, l'équilibre d'une couche granulaire sur un plan incline est étudiée, aussi bien dans l'air que dans l'eau. A partir de ce dépôt granulaire, la méthode de propagation d'avalanche est expliquée dans ces différentes phases. La rhéologie des écoulements stationnaires de sable est déduite et comparée aux résultats communément trouvés. Des mesures expérimentales, dites "méthode de la lame de suie" et "méthodes des feuillets colorées", montrent l'existence d'une couche statique prenant place sous l'écoulement. Dès lors la rhéologie précédemment établie est modifiée permettant ainsi de décrire les avalanches en termes de rhéologie locales. En appliquant cette rhéologie aux cas des ondes solitaires érosives, le profil d'érosion en est extrait. Pour le sable sur la feutrine il ressort que l'onde ne creuse pas la couche jusqu'au fond tandis que pour les ondes de billes la couche est entièrement érodée. Dans la dernière partie la stabilité de ces ondes est étudiée. Au delà d'un seuil les ondes se déstabilisent transversalement. Après une longueur d'onde initiale, un phénomène de coalescence par fusion est observé. Ce dernier est lui-même stoppé par la formation de doigts. Une étude expérimentale de stabilité a montré que c'est une instabilité linéaire à grande longueur d'onde et à nombre d'onde nul. Toutes les longueurs d'onde mesurées aussi bien dans l'air que dans l'eau se regroupent autour d'une même courbe dès lors qu'elles sont adimensionnées par la taille des grains

Etudes expérimentales d'avalanches granulaires

PARIS7-Bibliothèque centrale (751132105) / SudocSudocFranceF

Self-rolled polymer film: a route to microfluidic devices

International audienc

Self-rolled microsystems: A promising route toward fully functionalized and low cost micro-capillaries

International audienceWe present a new route for the fabrication of highly specialized miero-capillaries, based on the phenomenon of thin polymer films self-rolling. Using effects of solvent swelling or embedded stress, instability in polymer films are generated, leading to the spontaneous formation of capillaries. Before rolling, the surface can be patterned (chemically, topographically), permitting the fabrication of inexpensive fully functionalized capillaries

Replication of a Printed Volatile Mold: a novel microfabrication method for advanced microfluidic systems

International audienceA novel and simple method to fabricate microchannels is reported based on an inkjet printing of a volatile solid mold. A liquid ink -1,6 hexanediol- ejected from a piezoelectric nozzle is instantaneously frozen when touching a cooled substrate. The created mold is then poured with PDMS. Once the PDMS is crosslinked, the ink is sublimated and the device is ready. With this approach it is possible to make microchannels on diferent nature surfaces such as glass, paper, uncross-linked PDMS layer or non planar substrates. The versatility of this method is illustrated by printing channels directly on commercial electrodes and measuring the channel capacitance. Moreover, millimetric height microfuidic systems are easily produced (aspect ratio≥25) as well as 3D structures such as bridges. To demonstrate, we have fabricated a combinatorial microfuidic system which makes 6 mixtures from 4initial solutions without any stacking and tedious alignment procedure

Ultra-sensitive detection method with droplet based microfluidic device coupled to MALDI-TOF

International audienceWe present an automated microfluidic droplet generator/depositor for MALDI-TOF analysis allowing attomol detection of peptides from sub-nanomolar soltutions: a sensitivity which is a paramount importance for proteomics and diagnosis field. Droplet based microfluidic devices offer many advantages in integration of multiple biological or chemical processes, essential tools for biomarker/biological fluids analysis [1]. However the detection mainly relies on optical methods such as fluorescence which implies a labelling of target molecules but also suffers from a lack of sensitivity. At the opposite mass spectrometry is a highly sensitive technic based on a label free detection of analytes but interfacing droplet-based microfluidic and mass spectrometry is challenging and up to now there are few studies reported [2-3]. We focus on the integration of a droplet based microfluidic system with a MALDI-TOF. We develop methods which prevent the use of surfactants: a crippling factor since surfactants led to high background noise, and then are not compatible with high sensitivity detection method. To circumvent this effect, we designed a chip which prevents droplets coalescence by spacing each droplet with a controlled volume of oil (Figure 1). This simple geometry allows to finely tune the amount of spotted droplets and controlled the coalescence. The microfluidic chip outlet is then connected to a capillary, and the droplets are transferred to a MALDI plate mounted on a motorized xy-stage. To estimate the potential of the platform, we developed a full optimized method (sample composition, matrix composition, droplet generation, deposition and analysis) of relevant proteomics biomarker: a native peptide Angiotensin II involved in several diseases [4-5]. First, we compared our system with a standard MALDI deposition procedure i.e. manual pipetting. The spectra of Figure 2 show 500nl of a solution of 10fmol/µl Angiotensin II-left pipette deposited and-right platform deposited. In both cases, the 1046.2 m/z peak of Angiotensin II is visible but in the former case the intensity is increased by one order of magnitude. A closer look to the deposits shows a concentration effect with the platform deposition (see insets figure 2). In a second step, we made a sensitivity test by depositing different concentrations of analytes. The Figure 3 shows the average peak intensity as a function of n the quantity deposited in femtomole. We record good intensity signal down to 100 attomoles (see inset figure 3). Below this threshold the spectra are very noisy (Figure 4-left). In order to increase the sensitivity of the method, we developed a multi-spotting deposit. By repeated deposition of droplets, we increase drastically the amount of peptides on the spot, and can then reach very low level of detection. As a proof the Figure 4 shows the spectra of sub-nM solution before (left) and after the multideposit (right). We developed a method which allows to generate single droplet in a microchip coupled with a MALDI-TOF analysis. We control the volume of sample deposit on each spot, and by multi-spotting we reached a very high sensitivity compatible with physiological concentration of proteomic biomarkers