1) Droplet-based microfluidics in a compliant geometry Nature of the

1) Droplet-­based microfluidics in a compliant geometry Nature of the project: Experimental Research team: Microfluidics Lab Contact: Tristan Gilet ([email protected]) Microfluidics is the emergent branch of fluid dynamics concerned with handling liquid volumes smaller than 1μL. It is mostly motivated by the idea of shrinking the equipment needed for everyday chemistry, biology and biomedical procedures to fit onto a centimetric chip. In droplet-­‐based microfluidics, a carrying liquid (usually oil) conveys some nanoliter aqueous droplets through the chip. These droplets are used as containers in which the liquid of interest is confined by surface tension. Most current microfluidic operations rely on rigid microchannels, usually made by soft-­‐lithography. Conversely, compliant microchannels are often encountered in nature, e.g. the blood capillaries and the alveoli of the lungs. If the walls are sufficiently thin (about a few tens of microns) and soft, they can be substantially deformed by the surface tension of the liquid therein. The coupling between surface tension and flexible is still very poorly documented. In this project, the student will investigate the motion of droplets in compliant microchannels made of PDMS. He/she will adapt the soft-­‐lithography procedure to building these compliant membranes. He/she will then analyze the interactions between droplets traveling in parallel channels separated by a compliant membrane. This work may lead to new ways of synchronizing droplets for parallel operations in microfluidic devices. Bubble synchronization in connected parallel microchannels M. Prakash et al., Science 2007
2) Hairy dipping Nature of the project : Experimental, theoretical Research team: Microfluidics Lab Contact : Tristan Gilet ([email protected]) Nectar drinkers must feed quickly and efficiently due to the threat of predation. Most bees and some ants ingest nectar by dipping their tongue into, then extracting it from, the viscous nectar. The nectar intake relies on viscous entrainment by the outer surface of the tongue. This coating process is well characterized in the case of a smooth surface (Landau-­‐Levich theory). The proboscis is usually covered with micro-­‐hairs, which inclination generates directional adhesion: the contact line moves more easily in the direction of the hairs than in the other directions. These compliant microstructures surely affect the coating dynamics. In this project, the student will fabricate hairy microstructures on a substrate. The compliance of these structures will be quantified. Then the student will build an experimental setup to investigate dipping dynamics at the microscale and measure the corresponding intake rates. Theoretical models will be proposed to explain deviations from Landau-­‐Levich theory. Hairy proboscis of a bumblebee W. Kim et al., PNAS 2011
3) Numerical simulation of static capillary bridges Nature of the project : Numerical Research team: Microfluidics Lab Contact : Tristan Gilet ([email protected]) Collaboration: Eric Bechet A capillary bridge is a meniscus of liquid between two solid surfaces separated by a sub-­‐millimeter gap. This meniscus exerts significant forces on both solids, and may serve for gripping, aligning or sealing micro-­‐objects. Analytical descriptions of this meniscus (shape and stability) exist only in simplistic geometries. On the other hand, existing numerical solvers (incl. Surface Evolver) only give the shape of the meniscus in complex geometries, without necessarily addressing the stability, the coexistence of several equilibrium solutions and the contact line dynamics. In this master’s thesis, the student will determine the individual macroscopic behavior of these capillary bridges, starting from their shape in several static geometries (i.e. the solid parts are fixed relative to each other). Fixed and moving contact lines will be considered. For each equilibrium position, he/she will determine the forces and torques applied by the meniscus on the solid parts. Capillary bridge between two surfaces, calculated by Surface Evolver (from http://www.geom.uiuc.edu)
4) Patterning fibers Nature of the project : Experimental Research team: Microfluidics Lab Contact : Tristan Gilet ([email protected]) Microtechnology has inherited from the fabrication toolbox developed for microelectronics in the last century. Lithography, coating, deposition and etching are used to transfer a 2D pattern onto the surface of a silicon wafer. Fiber geometries are already used in many different applicative contexts (optical fibers, mechanical filters, paint brushes, electrical interconnects, sensors, digital microfluidics, water harvesting, etc.). In many of these applications, introducing localized micro-­‐heterogeneities at the fiber surface may be particularly interesting, especially to control the wetting properties. This experimental project consists in adapting coating and lithography techniques to the patterning of fibers. The student will focus on the local modification of static and dynamic wetting properties (hydrophobicity gradients, contact angle hysteresis). Droplet sliding on a fiber T. Gilet et al., EPJE 2010
5) Structures fibreuses dans les colonnes à empilage Nature du projet : Expérimental Research team: Microfluidics Lab Contact : Tristan Gilet ([email protected]) Collaboration : Dominique Toye Les colonnes à empilage sont largement utilisées dans l’industrie chimique pour faire réagir une phase liquide avec une phase gazeuse. En particulier, il est possible de les utiliser pour capturer le CO2 présent dans l’atmosphère. D’ordinaire, de nombreuses plaques sont agencées au sein de la colonne, sur lesquelles le liquide s’écoule lentement par gravité. La phase gazeuse est insufflée depuis le bas de la colonne. Ce travail de fin d’études consiste à remplacer la structure en plaques par une structure fibreuse. L’écoulement du liquide et son interaction avec le gaz diffèrent fortement sur une fibre et sur une plaque. Dans ce travail expérimental, l’étudiant proposera/réalisera une structure à base de fibres et en évaluera les performances hydrodynamiques (débit maximum, surface d’échange, etc.) en fonction des divers paramètres (diamètre des fibres, densité du réseau). Entre autre, l’étudiant développera une connaissance et un savoir-­‐faire dans le domaine émergent de la microfluidique (écoulements aux petites échelles). Il se familiarisera également avec un certain nombre de techniques expérimentales de pointe (tomographie, trajectographie). Gouttes sur un réseau de fibres T. Gilet et al., Appl. Phys. Lett. 2009
6) Electrowetting between two movable electrodes Nature of the project : Experimental, theoretical Research team: Microfluidics Lab Contact : Tristan Gilet ([email protected]) Electrowetting is the modification of the wetting properties of a surface with an applied electric field. It is now used in a wide range of applications, including adjustable lenses, electronic displays, switches for optical fibers and digital microfluidics. In a typical scenario, a water droplet is squeezed between two parallel electrodes covered with a dielectric material (EWOD). When a voltage is applied, the droplet spreads on the electrodes. With this technique, droplets can be moved at will on a micro-­‐array of electrodes. So far, the gap between the electrodes has always been kept constant; and most of the time, the electrodes are fixed relative to each other. What would happen if they were allowed to move? Is there any pull-­‐in effect, analogous to what is observed in capacitive actuators? Can we use droplets as micro-­‐bearings? In this project, the student will pattern electrodes on two substrates. He/she will then investigate how to move them relative to each other with intervening droplets and electrowetting. The dynamical behavior will be described in detail. Micro-­‐motor based on electrowetting Takei et al., Lab Chip 2010